Author: Kaleb Saleeby

Technician/Trainer/Content Creator Myrtle Beach, SC


Dielectric grease is an often misused and misunderstood product that could easily benefit HVAC/R technicians in a variety of ways. From food service to electrical connections, dielectric grease can help lubricate mechanical components and prevent corrosion on electrical connections. But we need to understand what it is to begin with, in order to properly apply it in the field.

Dielectric grease is silicone-based grease with insulating properties. Common uses for dielectric grease include electrical connections, spark plug wires, and mechanical connections. The most common misuse of dielectric grease relates to electrical connections.

I mentioned dielectric grease acts as an insulator, yet many technicians mistake silicone grease as conductive. For conductive grease, Conducto-Lube Silver or any carbon conductive grease will do. Conductive grease is for conducting electricity from one conductor through the grease to another conductor. 


To apply silicone dielectric grease properly to electrical connections, make sure the conductor mating surfaces are bonded before applying the grease. In coastal climates, low voltage wiring is particularly in danger of corrosion, especially right on the waterfront. 


To prevent corrosion and to protect the connections, make a solid connection with your exposed conductor wire with an appropriately sized wire nut. Then remove the wire nut and dip the exposed conductor into dielectric grease. Next, put the wire nut back on. If you really want to get crazy, you can then wrap the connection with electrical tape. For contactors and other connections, wire up the components as usual, then apply a dollop of Daisy…I mean Dielectric grease to the connection points.


Dielectric silicone grease can be used in a variety of mechanical applications, as well. The Refrigeration Technologies Silicone Grease is also food-grade and can be used in many refrigeration applications. 


Remember to always double-check your electrical and mechanical connections for the correct torque before applying the grease. If you’re not careful, things can get messy quick!



Sensors, Measurements, and Physics

As HVAC/R Technicians, we use tools and instruments to make measurements every day. In fact, 90% of our job could not be done efficiently without some kind of measurement. 

“How do we measure?”

“With what instruments?”

“How accurate are these measurements?”

These are all questions a thoughtful technician should ask before spending money on a tool or implementing solutions to solve a problem. 


Tools are our primary resource for measurement. Measuring tapes, scales, pressure transducers, thermocouples; the list goes on. But how exact are these measurements? How precise must our measurements be for us to use them to make decisions regarding the mechanical operation, occupant comfort, and occupant health? To answer these questions, we need a crash course in a little bit of physics. 

Don’t worry; we aren’t going into the rabbit hole too deep. I simply want to introduce you to a concept called the Heisenberg Uncertainty Principle in quantum mechanics. The Uncertainty Principle states that the more precisely you determine a particle’s position, the less precisely you can determine that particle’s momentum. Basically, the Uncertainty Principle limits our ability to measure things exactly. In modern physics, there simply is no such thing. There is only the agreed-upon accuracy and precision everyone is satisfied with to make practical decisions. For example, if you asked me how tall I was, my answer might be 5’11”. But is that 5ft. 11in. exactly? The line on the measuring tape with which I measured my height has a thickness, doesn’t it? Where within the thickness of that line do I fall? This principle is, of course, much more noticeable in the quantum (atomic) scale than on the macro scale. However, when measuring airflow and trying to solve occupant health concerns by measuring indoor air quality, accuracy, and precision matters. Our margin of uncertainty matters.

A technician doesn’t have to lose sleep over the fact nothing can be measured exactly. In our trade, and most of life, it’s not necessary. We can use this knowledge, however, to be more critical about the types of tools to choose to use. A duct traverse with a rotating vane anemometer can provide a quick and dirty idea for system airflow. Still, it’s not accurate nor precise enough to make complex troubleshooting decisions based on its results. The margin of uncertainty is too high. A flow hood or The Energy Conservatory’s TrueFlow Grid would be required for more accurate measurements upon which to base airflow balancing solutions. So how do we quality check for accuracy and precision? First, here’s a diagram showing the difference between the two and various combinations of accuracy and precision:

It’s important to understand the differences between these different combinations of accuracy and precision. Take any three brands of micron gauges and do a vacuum pump test. It helps if you have a digital gauge on the pump itself. Pull only on the pump first, and record the level of vacuum achieved (a good vacuum with fresh oil should be able to pull below 50 microns). Next, add the gauges one at a time and pull a vacuum in three more separate tests. You will very likely record three different micron levels for each gauge. In this experiment, the pump was our reference. In reality, the pump gauge itself would also need to be scrutinized for accuracy and precision, but as a demonstration, this test works well. Looking at the recorded micron levels from the gauges relative to the pump, where on the graph would your test results lie? Most are either accurate, but not precise, or precise, but not accurate. 

When thinking about buying a measurement tool, first consider what you are trying to accomplish with that measurement. If you aren’t planning on making accurate and precise capacity calculations, then you don’t really need the more accurate and precise hygrometers and airflow measurement tools. You may not need a digital manifold to charge a system to proper superheat and subcooling, because the margin of uncertainty is forgiving in that context. But a more accurate and precise sensor for a digital manifold or probe sure makes a pin-hole leak during a standing pressure test a lot easier notice.

Sensors are another factor to consider when choosing a measurement tool. Sensors are responsible for most of what makes a tool more expensive (not always, but most of the time). The product data for any instrument can be found either in the manuals or through a quick phone call to the manufacturer. In a recent podcast with Aeroqual’s Bernadette Shahin, we discuss some points to look for when researching the quality of a particular sensor.

  • Selectivity
    • A sensor that can measure the target parameter only (humidity, pressure, CO, microns, VOCs, etc.)
  • Sensitivity
    • A quality sensor should be able to have low cross-sensitivity to other parameters and should be stable in its ability to measure the target parameter over time.
  • Speed
    • A sensor has a published response time to change, but a manufacturer can confirm how long a sensor takes to reach an acceptable range of accuracy and precision expected from the sensor

Home automation is becoming very popular in the HVAC trade, and many are using Indoor Air Quality monitors to determine when and for long a mechanical system will operate, in order to maintain a comfortable and healthy home. To do this effectively, the accuracy and precision of the sensors in that monitor should be scrutinized. AQ-Spec is an excellent resource for this specific type of evaluation. Other manufacturers may have done their own third-party testing, and results can be released upon request. 

Keep in mind we aren’t referring to this concept as the “margin of error,” as many often call it. There is hardly ever any ill-intent when it comes to making a measurement, and manufacturers are not trying to create poor quality products. There are simply varying levels of uncertainty when we are using the tools at our disposal, and picking the right tool for the job is essential for quality solution implementation.

Another principle in physics, which all technicians should be aware of, is the Observer Effect. This theory states that the very act of observing a property’s state of being will inevitably change that property’s state of being altogether. In other words, by the time you have made a measurement, the state of whatever property you are measuring has changed, and no longer holds the same state of being it held before you started to measure it. A good example of this would be connecting hoses to a system. The mere act of attaching your hose has let out a little bit of pressure from the system; therefore, the pressure you will read is different from the pressure the system held the moment before you connected. This example might incentivize some to make the switch to probes, and ditch the hoses! Another example of the Observer Effect is checking electrical current on an indoor blower motor with the cabinet door removed.

One last point to make…accuracy and precision ≠ resolution. The resolution simply specifies to what decimal point the measurement is going to display. The resolution does not provide any direct information about accuracy, or precision. So next time you are comparing tools to see which is the best quality for the price, you may find a more accurate tool at a lower relative resolution. It is important to note, however, that precision and resolution are related. The higher the precision, the higher the resolution necessary to interpret the readings. It’s all about what you need the measurement to do for the application in which you use it.


Here are some links, in case you want to gain more insight into sensors, the Observer Effect, and the Heisenberg Uncertainty Principle:

This is the second article in a three-part series, where Advanced Psychrometrics are explored. The source material for each of the articles in this series is ACCA Manual P Sections 3, 4, and 5. This article is based on information found in Section 4.

If you followed the previous Advanced Psychrometrics article, you now know how to use a Psych Chart to plot a Room Sensible Heat Ratio (RSHR) Line, and how to calculate Design Room CFM. However, if you followed that exercise, you will note the absence of real-world variables, such as ventilation and bypass factors. Equipment Sensible Heat Ratios are almost never an exact match to the RSHR. This exercise will account for these variables, and walk you through how to plot these properties on a Psychrometric Chart. 

It is worth reminding you that this is an exercise to help illustrate the complexities of psychrometry in the real world. This may not always be a practical method utilized in the design process. 

When outdoor ventilation air is mixed with return air before the equipment coil, the equipment is exposed to latent and sensible loads beyond that of just the conditioned space. This characteristic causes the Coil Sensible Heat Ratio (CSHR) to alter from the RSHR. Remember, the Room Design Conditions will be met only when the supply air properties fall on the RSHR Line. With two different SHRs, we no longer have the luxury of choosing any supply condition we wish. The supply air must be able to cool and dehumidify the space. It also must now compensate for the additional load introduced by the ventilation air. Therefore, the only supply condition that will satisfy the Room Design Condition is the point at which both the RSHR Line and CSHR Line meet on the Psych Chart.

To plot the RSHR Line should be a breeze at this point. For a review on that process, and the first part of this article series, CLICK HERE.

The construction of the CSHR Line, however, is a bit more involved. There is a little trial and error in the construction of the CSHR Line. It’s not impossible, of course, and with practice, you get pretty good at nailing it on the first try. Here’s why a trial and error process is required in order to plot the CSHR Line:

  • The location of the CSHR Line is determined by the Mixed Air Condition (MAT) and CSHR
  • The CSHR and the MAT can’t be plotted without knowing the percentage of Outdoor Air (OA)
  • The percentage of OA can be calculated only when the supply CFM is known.
  • The supply CFM can be calculated only when the ΔT between the room return and supply is known, which is determined by the intersection of the CSHR and RSHR Lines
  • The CSHR Line is the line we are solving for; therefore, it is unavailable.

This is why a trial and error process is required. Simply put, we’re going to use an estimated guess as to what we think our supply air condition will be, then follow the process until we can determine if our selection actually results in the intersection of the CSHR and RSHR Lines. To help aid in the accuracy of your guess, keep in mind that, on average, a Direct Exchange Fan Coil can provide supply air temperatures which may fall between 14-25 degrees below the space temperature at typical relative humidities between 80% and 95%.

To begin this exercise, let’s start with some basic information, which will ALWAYS be available to you from a quality load calculation. This information can be plotted on the Psych Chart with complete certainty:

Room Sensible Heat: 21,700 BTUh

Room Latent Heat: 2,300 BTUh

Room Total Heat: 24,000 BTUh

RSHR: 0.90

Room Design Condition: 75℉ db / 50% RH

Outdoor Design Condition: 95℉ db / 75℉ wb

Ventilation Required: 245 CFM

In this scenario, a Room-to-Room load calculation has been done on a home. The RSHRs have all been averaged together for a mean room sensible heat ratio. We can go ahead and plot what we can on the chart:

Let’s select a 57℉ supply temperature at about 90% RH. Now we can determine the Supply CFM. Since the RSHR is the average of the entire home, the Supply CFM will equal the total system CFM.

CFM = Room Sensible Load ÷ (1.08 x ΔT)

21,700 ÷ (1.08 x 18) = 1,116 CFM

Now that we know the Supply CFM, we can calculate the percentage of ventilation air.

Ventilation =  245 CFM ÷ 1,116 CFM 

Ventilation = 22%

We have a good bit of information here now, but the math starts to get a little confusing without explanation. We now know that 22% of Outdoor Air (at 95℉ db / 75℉ wb) will be mixing with the remaining 78% Return Air (at 75℉ db / 50% RH). To calculate the Mixed Air Condition, complete the following equation:

MAT = (0.22 x 95℉) + (0.78 x 75℉)

MAT = 20.9℉ + 58.5℉

MAT = 79.4℉

We can now plot the Mixed Air Condition on the Psych Chart.

At this point, we have everything we need to construct the Coil Sensible Heat Ratio Line. If you notice on the Psych Chart, there is a list of helpful formulas to the left of the page. We need to solve for Total Coil Heat Load (Qt) if we are to determine Coil Sensible Heat Load (Qs) and CSHR. To do that, we need to figure out the change in enthalpy (ΔH). Enthalpy is heat energy in BTUs per pound of dry air.

ΔH = 30.6 – 23.4

ΔH = 7.2

Now let’s plug our ΔH into the Total Coil Heat Load calculation. (4.5 here is Air Density x Run Time in minutes. 0.075 x 60 = 4.5)

Qt = 4.5 x CFM x ΔH

Qt = 4.5 x 1,116 x 7.2

Qt = 36,158 BTUh

Solve for Coil Sensible Heat Load. To do this, make sure you are using the entering air condition the equipment will actually see: MAT.

Qs = 1.08 x CFM x ΔT

Qs = 1.08 x 1,116 x 22.5

Qs = 27,119 BTUh

We can finally solve for Coil Sensible Heat Ratio at this point:

CSHR = Coil Sensible Load ÷ Total Coil Load

CSHR = 27,119 BTUh ÷ 36,158 BTUh

CSHR = 0.75

We can now plot the CSHR Line on the Psych Chart.

If you look closely, you may be thinking, “Wait a second, the CSHR Line does not intersect with the RSHR Line.” You would be absolutely correct. This is why the trial and error solution is necessary. However, if you notice, the CSHR Line is extremely close to our selected supply temperature. The CSHR Line is just slightly above the RSHR Line.

What does this mean?

We can still use the design CFM and supply condition, and the equipment will satisfy the sensible load, but will maintain a slightly higher humidity level in the space than what was designed. Take a look at the actual grains of moisture for the Mixed Air Condition in comparison to the Supply Air off the coil at 57℉.

The equipment will be able to dehumidify from 73 grains of moisture/lb of dry air down to 63 grains of moisture, rather than the ideal 62 grains. We’re talking about a difference of 1 grain of moisture. This can be acceptable, and the difference likely unnoticeable. In cases where a coil selection will not match the latent load requirements of a space, a viable option would be to add supplemental dehumidification to deal with the remaining latent load. Ultra-Aire Ventilating Dehumidifiers are an excellent option, and will also help lessen the additional latent load from the ventilation air.

Lastly, let’s talk about Bypass Factor. Remember, the ideal supply temperature would be the apparatus (equipment) dew point. However, there is a small percentage of air that will bypass the coil and not transfer its heat to the coil. This can be calculated using the known apparatus dew point. The Bypass Factor formula is as follows:

 Bypass Factor = (Supply Air Temperature – Apparatus Dew Point) ÷ (Mixed Air Temperature – Apparatus Dew Point)

Bypass Factor = (57 – 53.5) ÷  (79.4 – 53.5)

Bypass Factor = 3.5 ÷ 25.9

Bypass Factor = 0.14

At this point, you would need to look up a manufacturer’s extended performance data for their equipment to ensure that the coil you select will meet a sensible capacity of 27,119 BTUh and a total capacity of 36,158 BTUh at 1,116 CFM, with an entering condition of 79.4℉ db / 65.6℉ wb and an outdoor condition of 95℉ db / 75℉ wb. Let me translate that to something you might actually see on a Performance Table:

Entering Air Condition: 80℉ db / 67℉ wb

Outdoor Air Conditions: 95℉ / 75℉ wb

Total Capacity: 36,000 BTUh

Sensible Capacity: 27,000 BTUh

Airflow: 1,100 CFM

If you can select a coil that will match these criteria, you will be able to maintain an indoor air condition that is nominally close to your design.

To see how this chart would look in another scenario (without going through the step-by-step process), here is a psych chart based on my house and ASHRAE Design Conditions:

Room Sensible Heat: 16,800 BTUh

Room Latent Heat: 7,200 BTUh

Room Total Heat: 24,000 BTUh

RSHR: 0.70

Room Design Conditions: 75℉ db / 50% RH

Outdoor Design Conditions: 90℉ db / 80℉ wb

Ventilation Requirement: 46 CFM

In this case, my selected supply air condition happened to fall perfectly at the intersection of the CSHR and RSHR Lines. The tricky part, however, is finding a coil that will meet the sensible and latent heat requirements under the design conditions. I would need to look for a coil with 18,000 BTUh sensible capacity and 29,000 BTUh total capacity. I’d have to settle for a 2.5 ton (30,000 BTUh) coil with a close CSHR under design conditions, and potentially add supplemental dehumidification. (A Carrier FB4C–030 would actually fit the bill quite nicely.) Remember, the equipment selection performance table will have actual capacities that differ from the nominal rating; thus, care must be taken when using manufacturer performance tables to select equipment.

If you’ve made it to the end of this exercise, congratulations: you are as nerdy as they come! I hope this helps illustrate the complexities of psychrometrics. If nothing else, the take away should be a new-found respect for psychrometrics, and its integration into a technician’s daily diagnostic toolbag.

Stay tuned for Part 3, where we will dive into ACCA Manual P, Section 5. There we will learn how to account for duct gains, and how reheat dehumidification looks on a Psych Chart.


–Kaleb Saleeby

This is the first of a three-part series of articles, which will dive deep into Advanced Psychrometrics. The source material for each of these articles may be found in ACCA Manual P Sections 3, 4, and 5. This article is based on information found in Section 3. 

Psychrometrics is the study of the physical and thermodynamic properties of gas-vapor mixtures. In HVAC/R, we are specifically interested in air-moisture mixtures, and how varying properties affect human comfort and equipment performance. The Psychrometric Chart is a tool used to describe all the possible combinations of gas-vapor mixtures, and can be used to calculate the sensible and latent loads associated with HVAC/R equipment.

Using a Psychrometric Chart can be a bit confusing at first, but with practice and familiarity of the formulas, a Psych Chart can be easily used for a wide variety of purposes. Basic Psychrometric education can be found in the Refrigeration and Air Conditioning Technologies Manual (RACT) and in the first two sections of ACCA Manual P. In this article, however, I’m going to show you how you can apply psychrometrics to calculating Design Room CFM and illustrate how psychrometry can be used to help a technician understand supply air properties. All of the information discussed here can be found in Section 3 of ACCA Manual P.

When selecting equipment for a home or building, it is recommended a Room-to-Room Heat Load Calculation be done as opposed to a Block Load Calculation (Wrightsoft is an excellent software for load calculations, just saying). Room-to-Room calculations result in a more accurate representation of the heat gains and losses per zone (room), and can greatly improve the accuracy and performance of system sizing and design. Assuming a Room-to-Room Load Calculation has been done on a building, the next step in utilizing the Psychrometric Chart would be to plot out the Room Sensible Heat Ratio Lines for each zone. Room Sensible Heat Ratio (RSHR) is the ratio of sensible heat to total heat (including latent) for a room (or zone). If, for example, a room had a total heat load of 2,500 BTUh and 1,800 BTUh sensible heat, the RSHR would be 0.72.

RSHR = Room Sensible Load ÷  Room Total Load

RSHR = 1,800 BTUh ÷ 2,500 BTUh

RSHR = 0.72

Now that we know the RSHR, it’s time to plot the RSHR Line on the Psych Chart. To do this, we need to find a “reference dot”.

80℉ db at 50% RH is considered the standard reference dot. Locate and mark the reference dot and then run a line through the reference dot using a straight edge that is lined up with the RSHR (0.72), which can be found on the far right-hand side of the chart. 

Now, locate the design conditions for the zone in question. Let’s say the design conditions (on a design day of 90℉) is 75℉ db at 50% RH. Plot that dot on the chart. Now, run a line straight through that dot heading to the left of the chart, making sure it is parallel to the reference line. This line is your RSHR Line. This line may now be used to select a supply air condition that will maintain the design room condition on a design day. However, the supply air condition must fall somewhere between the design room condition and dew point (which in this example is about 51.5℉). Theoretically, the lowest possible supply air condition would involve the evaporator coil in cool mode to be 51.5℉ (dew point), and the supply air leaving the register to be the same. However, this theory is in no way practical when you consider duct gains, air leakage, and bypass factors (let alone the fact no one wants a sweaty supply register). Practically, a supply condition falling somewhere between 80%-95% RH will result in good dehumidification, lower airflow, and low fan power consumption. 

Select a supply temperature condition. For this example, let’s choose 55℉ at 90% RH. The next step is to calculate the Design Room CFM. The equation for CFM is as follows:

CFM = Room Sensible Load ÷ (1.08 x ΔT)

Remember, the Sensible Load for this zone is 1,800 BTUh. The difference between the Room Condition and the Supply Air Condition is 20℉. 

CFM = 1,800 BTUh ÷ (1.08 x 20℉)

CFM = 83

The required volume of air given an hour of the runtime is 83 CFM for this room to maintain the design room air condition under design load.


But what if my ΔT is lower?

The required volume of air increases. The new supply air condition is 63℉ at 72% RH, giving us a ΔT of 12℉.

CFM = 1,800BTUh ÷ (1.08 x 12℉)

CFM = 139

Both of the different supply air selections will maintain the design room condition on a design day, because they each fall on the RSHR Line. But as the temperature difference between return and supply air decreases, the required CFM increases


What is 1.08 supposed to be?

That is the product of the following equation:

Runtime (minutes) x Isobaric Air Density x Isobaric Specific Heat of Air

60 x 0.075 x 0.24 = 1.08

Some caveats must be addressed regarding this formula, and I credit Alex Meaney with Wrightsoft and Genry Garcia with Comfort Dynamics, Inc. for helping me understand these complexities. Both gentlemen are brilliant-minded experts in their fields, and have contributed (and continue to contribute) to HVAC School.

 First, the runtime is specified in minutes, because we are solving for cubic feet per minute (CFM), but also using British Thermal Units per hour. Converting the hour of runtime to minutes gives us 60 minutes, and makes sure our units of measurement are compatible.

Second, you may notice the term isobaric. This refers to any property at a constant pressure. At sea level, atmospheric pressure is around 14.7 psia. At this presumed fixed pressure, the density of dry air is 0.075 lb/ft3, and the specific heat of dry air is 0.24 BTU/lb/℉.

In reality, atmospheric pressure is not fixed, and outdoor air is not always dry. While you may be able to correct for actual pressure and humidity, it may not always be practical. On the other hand, with the ability to use MeasureQuick (which corrects for air density and pressure in its calculations), the processes discussed in these articles may become more practical. It is important to note that manufacturers use isobaric air density and specific heat in their capacity ratings and airflow calculations. Therefore, the argument could also be made that even with this caveat, the end result will (on average) still land you nominally close to the actual air condition requirements. (Please note the wording used here) 😉

So how does this all circle back to practical application? It must be understood that a coil can operate in only one sensible heat ratio at a time, and it may not equate to any of the RSHRs calculated for any particular zone. In the case of a home with multiple zones, you may choose one of the following options when selecting a cooling coil to match the load conditions:

  1. If humidity control is critical to a specific zone, use the RSHR for that room to select a coil. All other rooms will vary in humidity, but the critical zone will be maintained.
  2. Average all the RSHRs together for a mean RSHR that can be used to select a coil. Each room will vary slightly from its individual RSHR, but it will be minimal and likely unnoticeable. 

And that, in a nutshell, is how you may use a Psychrometric Chart and data from a Load Calculation to determine Room Design CFM. This exercise, however, merely scratches the surface of the many factors that must be considered in an HVAC system. This exercise works only for a system that does not suffer from duct leakage, bypass factor, and has no ventilation whatsoever for the home/building. This exercise would fall short of providing any real-world insight into psychrometric properties involving an HVAC system. However, the skills learned here translate into the next phase of advanced psychrometrics! In the next two articles, I will detail how these variables can be accounted for (even solved for). In the end, I hope you will understand a little more about Psychrometrics in general, and how to add that knowledge to your ability to efficiently diagnose a system as a whole (including the envelope and people).

I’ll end this article with a quote from Alex Meaney, and I think it is important to keep this idea in mind throughout the rest of this series of articles:

“I’m of the opinion that local humidity is usually a[n] infiltration/ventilation/return problem, not a supply problem.”

–Alex Meaney

For access to the Testo Psych Chart I used for this article, click here.


— Kaleb

Many installers and service technicians know how to read and use a manufacturer fan table, but this is a quick review with a few extra tips for newer techs. It’s also a good reminder to senior technicians how this easy-to-use practice can also be easily abused.

At installation, it is imperative to the performance and longevity of the appliance to set up airflow properly. A practical way to do this is utilizing the manufacturer-supplied fan tables found in every installation manual. Here’s a review on how to set up airflow on a new system:

  1. Determine your target airflow (The national average is 400cfm/ton. However, in a dry climate, design airflow may be 450-500cfm/ton, and in a humid climate, airflow is typically designed at 350-300cfm/ton.)
  2. Set your fan speed (choose the speed tap, or set the dip switches)
  3. Verify the equipment and duct work is clean, and all packing materials are removed from inside the appliance (yes, this gets missed sometimes)
  4. Run the system in order to achieve the test conditions in which the Fan Table was created (Fan Table airflow readings are only valid if the field conditions match as closely to the lab conditions as possible; i.e. wet coil, dry coil, with or without heat strip kits, etc.)
  5. Measure Total External Static Pressure (see how to measure TESP below)
  6. On the fan table, find the model matching the equipment you have, and locate the speed tap being used
  7. Match the real-time static pressure with the fan table
  8. The point at which both the TESP column and Speed Tap row meet is the corresponding estimated airflow.
  9. Make any adjustments to ductwork or fan speed in order to achieve the target airflow (This is made easy if ductwork is slightly oversized and installed with manual dampers on the supply.)

For servicing, techs may use the fan table method as a quick and dirty way of verifying airflow without extensive and time-consuming testing. This can be acceptable, but only if the following conditions are met:

  1. The equipment and ductwork are clean (This includes making sure the filter has been replaced)
  2. The equipment has been benchmarked once before (Without a reference, the fan table cannot be relied upon as an accurate representation of estimated airflow.)
  3. The equipment is running as closely to the documented lab conditions as possible. (But even then, how wet is “wet”?)

Static pressure readings stand alone as a valuable measurement during a service call, and TESP can inform a technician whether more extensive testing is required. But if the equipment has never been worked on by you, or your company did not install the equipment, the fan tables will not be useful until a full-system commissioning has been completed. 

Carrier FB4CNF Installation Manual

Another important tip is to always keep the return static pressure below 0.4” w.c. According to many manufacturers’ literature, a return static pressure of 0.4’ w.c. or higher can potentially result in water from the primary drain pan being picked up and thrown around inside the cabinet area, and sometimes into the ductwork. 

It is important to understand static pressure measurement is NOT a measurement of airflow. This is where many technicians abuse this method. Static pressure is just that: a measurement of pressure in reference to the space outside the ductwork. Based on lab testing conditions, a manufacturer is able to determine the airflow of a system under a known resistance. Static pressure is used as a proxy to estimate airflow, but this method is only as good as the conditions in which it is applied. Static pressure readings are air density dependent, so zeroing a manometer in a cold, dry attic, then inserting the probes into a humidified, warm duct system will adversely affect the accuracy of your measurements. This method is also heavily dependent on how detailed the manufacturer fan table is. An example of a good fan table would be one that lists the equipment model, if the unit was tested under wet or dry conditions, if heat strips were installed during testing, and any corresponding wattage/rpm determinations under given conditions. 

Carrier FB4CNF Installation Manual

The difficulty with using Fan Tables as a way to measure airflow is realizing the resistance across the equipment is dynamic, and will likely change many times over the course of a test (the coil may get wetter as it is loaded with latent heat, the coil will become dirty over time, etc.) Measuring actual airflow is difficult to do, but static pressure measurements are still very valuable, and are a good way to determine if a problem exists and on which side of the ductwork it exists (supply or return). 

A great product for measuring airflow in the field is the TrueFlow Grid by The Energy Conservatory. For more information on Airflow and Airflow Measurements, TruTechTools has an entire section of literature and webinars on the topic. Here is a video we recorded for them in 2017 regarding Static Pressure and Fan Tables:

— Kaleb

     Newer technicians often get hung up and frustrated when searching for low voltage shorts. This is understandable due to the broad spectrum of possibilities for the location of the short. However, this doesn’t mean the process needs to be complex. The time it takes to find a low voltage short may vary greatly depending on where the short is located, what components are failed, and how tedious the equipment is to access. Regardless of these variables, there are a few common processes that can make the technician’s life a bit easier when diagnosing a low voltage short. [Quick note: this is a guide for diagnosing a dead short in the low voltage circuit. In other words, the fuse immediately blows upon return of power to the appliance]

     The first step is ALWAYS a visual inspection. You can save a lot of time and frustration by simply using good observation skills. Look for rub outs, loose connections at switches and coils, discoloration, wire splices, splits in wire insulation, etc. These can all give a technician a great starting point to searching for a short of any kind. I’ve done many visual inspections and found other issues unrelated to the short that may have gone unnoticed without thorough observation. This is why good observation skills and a thorough visual inspection is a great tool to use no matter what you’re diagnosing.

     The second recommended step would be to power down the appliance and install a resettable fuse. You can find this valuable tool for cheap at any supply house, or you could even make your own from an old transformer that utilized a resettable fuse. This prevents a technician from blowing through 20 fuses before the source of the problem is found. Be careful the resettable fuse product you choose, some of them don’t trip as quickly as the factory and we have seen transformers and boards fail due to this. We suggest going to a 3A version rather than 5A when possible for additional protection. 

     Step three: Rule out the transformer and thermostat. These components are rarely ever the issue, but the thermostat is also one of the first things newer techs will replace when panicked and trying to solve a low voltage problem. The first quick tests will help rule them out entirely. With your meter, check primary and secondary voltage against the rated voltage on the transformer. If the transformer secondary voltage is 24v, it is typical to see a range between 22v-28v. If you measure higher or lower than normal voltage from the transformer, it may be a good idea to disconnect the transformer from the circuit and ohm out the windings and check for low resistance, which would result in higher amperage. 

     Remove the thermostat from the wall, and unwire all the wires except Common. Then, using either a pair of jumpers or a wire nut, connect R, G, Y, O, W wires together. Now, re-energize the system. If the fuse pops, the thermostat is NOT the problem, because it isn’t even in the circuit and the fuse still popped. If the fuse holds, and the equipment is running perfectly fine without the thermostat in place, then you may start to suspect the thermostat. 

     Next, remove the jumpers or wire nut and isolate R, G, Y, O, W wires from each other. Reset the fuse, and one by one jump G, Y, O, W to R. Eventually, one of those combinations will pop the fuse, and it will be in that circuit the short is located. For example, let’s say the Y wire circuit pops the fuse when jumped to R.

     At this point, you’ve isolated the problem circuit, and you can begin testing everything related to that circuit. On a split system, the Y wire circuit will have the wire run from the thermostat to the indoor unit, from the indoor unit to the outdoor unit, from the outdoor unit to any defrost boards and switches, from those components to the compressor contactor. The best way to determine what is in the circuit is to read a wiring diagram, then follow the wire to verify the schematic. It is at this step a technician will repeat the visual inspection; this time more focused on a specific circuit.

     Now it’s time to test all circuit components (i.e. switches, relays, contactors, circuit boards, wire splices, etc.). Look for loose connections, burn markings, bare wire, rub out locations (like wire bending over sharp edges of the chassis) etc.

     If your testing leads you to suspect the wiring itself, you may isolate the wire by disconnecting the low voltage wire from the Outdoor unit completely. If the fuse still trips without any appliance connected to it (except the transformer power), then you can be certain the short is in the wire harness.

     The final step in the process is to make all necessary repairs. Don’t forget to remove your resettable fuse and install a new, appropriately sized fused for the appliance! This process is one of MANY processes senior technicians have developed, and you may find yourself using your mentor’s methods, instead, and that’s perfectly fine. Just remember to always diagnose the WHOLE system! Never know what else might be happening once the short is repaired, and you can operate the system again.

For another take on a low voltage short diagnostic that comes with a little entertainment, here’s #BERTLIFE Ep. 4


— Kaleb


At the time of the publication of this article, COVID-19 (coronavirus) is spreading across the world at an alarming rate, and many people have self-quarantined to help slow and/or stop the spread of the virus. These precautionary measures are prudent and responsible. However, with the increased amount of time people will now spend inside their homes, there is a hidden factor to be aware of, which many people won’t think about. The prolonged occupancy of homes with increased cooking, bathing, and cleaning time will significantly impact the indoor air/environmental quality of these homes. An issue like this may not be measurable, but it is inevitable. In a time when many technicians, companies, and manufacturers will use this health crisis as a way to promote the sale of IAQ products in ways that can only be judged as unethical, it is imperative to the honest and curious technician to understand how to do her part in educating customers, and keeping everyone healthy.

This article will stay away from talking about specific types of boxed devices out there that “purify” the air, because that’s a topic for another day. The focus here is on the three main processes available to technicians and homeowners to improve indoor environmental conditions. Taking these one by one, technicians should have a thorough crash-course understanding of each and its importance to indoor air quality (IAQ). Ventilation, Filtration, and Humidity Control.

The first step in understanding a healthy indoor environment is to recognize the villains one must fight against in order to keep an environment healthy. Particulate Matter (PM), Volatile Organic Compounds (VOCs), Humidity (high or low), Carbon Monoxide (CO), Carbon Dioxide (CO2), Ozone (O3), etc. are just a few. These are the elements that tend to concentrate themselves in tight indoor environments. Each of the “Holy Trinity of IAQ” is designed to deal with these undesirables in their own dedicated way.

Everyone should know what a bath fan is. If you don’t have a bath fan, you probably live in a house not updated since the 1970s, and you likely have other decor issues to deal with as well. Bath fans are the most common mechanical ventilation in homes today. They are a form of negative pressure ventilation. As the fan pulls air from the room and expels it (hopefully not in your attic), this creates a negative pressure on the building envelope, and air from outside is pulled in through the cracks and crevices around your windows, door frames, attics, and through Jerry’s mouse hole…which everyone has…right? This type of ventilation is by far one of the least desirable, because you exact zero control over the quality of air you are bringing into the home. The air could be high in humidity and temperature, or it could be passing through layers of blown-in insulation inside your attic; neither of which are ideal. Air from these places isn’t really fresh.

The general consensus is that positive or balanced pressure ventilation is best. Examples of positive pressure ventilation include Make-up air units (MAU), Dedicated Outdoor Air Systems (DOAS), and the use of a scuttle (a small duct run from outdoor air into the return ductwork for HVAC systems). Balanced pressure ventilation is accomplished through mechanical equipment like Energy Recovery Ventilators (ERV), Heat Recovery Ventilators (HRV), and Conditioning Energy Recovery Ventilators (CERV). Each of these technologies has their advantages and ideal applications. The reason positive/balanced ventilation is desirable is for its ability to control the fresh air. If you can control the air you breathe, you can keep it “fresh”. For all of these options, there are applications for which they can be used that actually improve upon the quality of the air entering the space. But why do we care about ventilation? What’s so important about it?

Houses used to be built loosely. This isn’t to say they were built poorly, but houses used to be loose enough to allow for tons of natural ventilation. The codes and standards have evolved, and we now construct assemblies more airtight than in the past. This is why the EPA has published that indoor environments are often 2-5 times higher concentrations of air pollutants than outdoor levels, and can reach upwards of 100 times worse! This is because as people bathe, clean, and cook, VOC concentrations, Particulate matter, and humidity levels increase dramatically. People thought bath fans were for bathroom odors, but really that’s just a nice side-effect. They are for removing water vapor during and after showers/baths. Ventilation is utilized to dilute VOCs, CO, CO2, and other chemicals in order to maintain a comfortable indoor environment. I know of people who grew up watching their mother open all the windows of the house for a couple hours a week in order to “flush” the house. Mechanical ventilation is just like that, except more controllable and technologically advanced.

Particulate Matter is another indoor environment characteristic, which can cause a variety of health concerns. Particulate matter is categorized by its size in diameter, which is measured in micrometers (or microns). A lot of buzz is generated around PM 2.5, which is particulate matter with a diameter of 2 and a half microns; that is due to PM 2.5’s ability to do major damage to the human respiratory system. To give you an idea of the size of PM 2.5, the EPA has published that PM 10 is considered inhalable. PM 2.5 is 75% smaller than that! This means PM 2.5 tends to stay in the air stream longer than larger, denser particles. However, PM 2.5 is not the smallest particulate matter that can potentially do harm. PM 1 and 0.5 are also in the air, and they can easily make their way to our lungs and bloodstream. In order to combat against these airborne particles, it is important to filter the air with a high-quality air filter. There are filters designed to trap PM 2.5 and smaller (MERV 11 up to HEPA), and they are a critical component to any air distribution system. The third edition (2018) of the EPA Technical Summary of Residential Air Cleaners states that a MERV 13 is recommended for every HVAC system, or as high a MERV rating as the system will allow. 

EPA Technical Summary: Residential Air Cleaners (2018)

It is important to note that Particulate Matter does not refer to just dust. Particulate matter can be made up of pollen, viruses, bacteria, fibers, fungal spores, vehicle exhaust, etc. This fact makes it clear that filtration is not only important for the HVAC system, but also for the incoming air to any mechanical ventilation system. Humans are constantly submerged in this fluid called air. We must give more thought to the quality of the air we breathe. 

The final head of our three-headed IAQ dragon is Humidity Control. This can refer to either high or low humidity levels. Either extreme is unhealthy and can create an environment prime for health risks. On one hand, high humidity can cause respiratory issues, encourage dust mite life, allow viruses and bacteria to increase, allow VOCs to become airborne, allow increased chemical reactions, and allow microbiological growth to take place. On the other hand, low humidity levels can also cause respiratory issues, irritate mucous membranes, allow viruses and bacteria to increase, and allow for the production of ozone. The happy medium is the generally accepted ideal humidity index, which falls between 35%-60% relative humidity.

In order to control humidity indoors, a technician must be aware of her climate zone, and whether she must work to increase or decrease humidity levels indoors in relation to outside levels. For arid climates, humidification is necessary, and options such as higher airflows and in-duct steam humidifiers are great solutions. For humid climates, running lower airflows and adding mechanical supplemental dehumidification is ideal. Some dehumidification systems allow for ventilation as an option, and they include a high MERV filter to cover all the bases. This option is an ideal solution for certain applications. Humidity must be controlled in an occupied space for that space to be comfortable. People are much more sensitive to humidity than temperature. 

Looking at these three paths to creating and maintaining healthy air inside a home, it is important to realize these are Indoor Air Quality solutions. To create and maintain a fully comfortable indoor environment, air leaks, insulation, and load matching are other issues that would need to be addressed. However, in addressing the current issues with air quality in homes, this “Holy Trinity” is all any technician needs to exert energy into in order to help keep occupant air clean. There is a mindset that humans are never more intimate with their surroundings than when they inhale the air into their bodies. Technicians must take action to educate consumers and recommend the most effective solutions for IAQ improvement. 

There are many companies and manufacturers using this health crisis to promote the sales of popular air “purifiers”, which use chemistry to “clean” the air in lieu of ventilation, humidity control, and filtration. The technology of these products will be discussed in a later article, but the most important take-away at this juncture is how important it is to maintain control over the humidity, the outdoor air coming into the space, and the concentration of particulate matter in the air stream. The methods of dealing with the issues mentioned in this article are the only methods that have been time and volume tested over decades, and they have standards in place to help ensure their effectiveness on IAQ. 

So what do technicians do right now? Many homeowners may not want to spend the money on advanced in-duct filtration, mechanical ventilation, and humidity control during this time of uncertainty. Joe Medosch from has shared a very ingenuitive and affordable solution for many people to effectively filter indoor air. 

This DIY method is a great way to help encourage homeowners to remain healthy as they spend more time inside their homes. This “box fan filter” may also make it more viable for sensitive people to open their windows and doors for longer periods of time during pollen season, as this enhances the circulation of air inside, and adds filtration throughout the home. Another recommendation for homeowners is to utilize the bath fans and kitchen exhausts as a way of ventilation. ALWAYS run a bath fan during bathing activity, and continue to run it 10-15 minutes afterward in order to prevent as much water vapor as possible from remaining inside the home. Portable dehumidifiers and humidifiers are also available. 

Another recommendation for every technician, business, and the homeowner is the use of IAQ monitors throughout the home. Real-time monitoring and translation of data over time allows people to see the effects of their activities on IAQ. For technicians and businesses, it is a great way to track the effectiveness of your work over time. Without measurements and testing, you can only guess!

As we work together to combat the spread of viruses in our communities and around the world, the HVAC/R industry has a large opportunity to help educate customers on how to create and maintain a healthy indoor environment. We must take care to avoid fear-mongering and sales tactics geared toward the exploitation of people’s vulnerability and miseducation. Practice integrity, do your research, and implement industry best practices always


– Kaleb


This is the tale of how I found myself stuck on a service call for over 12 hours on a weekend, due to my failure to re-diagnose an issue. I was working for a service company that had many accounts with local gas stations. These were large customers, and we did everything we could to keep them happy.

One Friday as I was gearing up for my on-call weekend, I was informed I must travel an hour and a half away early the next morning to a gas station where another technician had diagnosed a faulty X-13 blower motor. The technician didn’t have the right blower motor for the repair, so the system was still down. The catch was: no one knew what size blower motor was supposed to go in. No model numbers, no detailed notes, nothing. So I grab every size aftermarket X-13 motor I could find in the shop. I had up to ¾ HP. 

I arrived to find this location had two 5-ton package units mounted atop stands lifted 8 feet off the ground. After I setup the ladder and double-check the motor size, I realized it was 1 HP. I began calling all the parts houses in the area, hoping someone answered on a Saturday morning at 8am. No luck. I called parts houses in my local area and my co-workers to try and find a 1 HP X-13 motor that would work. Finally, I got in touch with one of my local suppliers. He had a motor that would fit the system I was working on, but he was an hour away from the supply house, and I was an additional hour and half away. Luckily, my employer at the time picked it up for me, and I had the part within a couple hours. I still had not re-diagnosed the system at that time.

Once I had the motor in hand, I quickly replaced it and had everything back together in a snap. I re-energize the system and….I curse loudly. The motor wouldn’t run. NOW I start re-diagnosing, a step I should have taken when I first arrived. Turns out, the original motor was just fine. The motor was not receiving 24v to the motor module, due to a faulty fan relay. I swapped out the 90-340 relay in the electrical compartment, restarted the system, and the blower ran beautifully. I hated myself.

I entertained the idea of packing up, walking away, and calling it complete, but I knew too well how that plays out. I began running complete system diagnostics, and found the system charge to be very low. I started leak searching the system with Big Blue from Refrigeration Technologies, and discovered a micro leak on the mechanical connection between the distributor tube and the TXV. No rubs outs were apparent, and it wasn’t a super loose connection, but it was clearly leaking. This was a package unit, remember, so I had to recover the entire system charge before I could make any repairs. 


Once recovered, I found the connection was just coming loose. A healthy dab of Nylogon the fitting connection and a torque wrench was all I needed to pass a nitrogen pressure test. Of course, the repair process was time-consuming, but eventually, I had the system evacuated, cleaned, recharged, and operating in peak condition under the current load.

I still would have needed to make the leak repair no matter what, but I could have easily saved 3.5 hours of time if i had re-diagnosed the system first when I arrived to the job. One could argue I was simply distracted by the chaos of the call, which would be true. However, a good technician should be able to follow the proper processes in spite of disorganization and frustration. I learned the importance of always checking behind yourself and others when you arrive to make a repair. Since, I have found the real causes for issues that were previously (either by me, or another technician) diagnosed as bad TXVs, reversing valves, motors, etc. 


ALWAYS double-check your work and other people’s work. You never know how valuable it is until you fail to do it, and it costs you time and money.

— Kaleb




Everyone in the HVAC/R trade uses some form of torch to braze or solder alloys together. So what is the proper way to handle an oxyacetylene torch? Turns out, there’s more than one right answer. Depending on which torch rig you use, the manufacturer’s manuals for operation may vary. 

Everyone (hopefully) knows a neutral flame on a torch tip is suitable for most applications. Sometimes a carburizing flame is useful for reducing oxidation. The only flame we all should avoid is the oxidizing flame. However, in order to achieve the correct flame, a technician must fully understand the type of torch tip they are using, and the application for which the torch is being used.

For example, a “rosebud” tip is a (often) a large high BTU tip, and may be too large for most residential applications. A lot of technicians will attempt to lower the fuel and oxygen pressures feeding the tip to reduce the temperature. However, the tip begins to starve due to a lack of adequate fuel/oxygen mixing, and the flame will back into the torch tip and coat the inside with a carbon coating, which can damage the tip and torch over time. On the other hand, a torch that is too small will never get hot enough for an application outside its design parameters.

So what tips are best? At what pressures must tips be set? There are many answers to these questions, and they all depend on the brand of equipment you use, and the application in which you work.

I had the opportunity to speak to Tim Thibodeaux from the Service Dept. at Victor Technologies, and with Matt Foster from Uniweld Products, Inc. Both confirmed that pressures are tip specific, and operating procedures are brand-dependent. For example, many technicians have been taught to shut off the FUEL first when shutting down the torch, but this contradicts manufacturer instructions.    

Uniweld states in their operation manual to shut off the OXYGEN first at the torch when following proper shutdown procedures. This is done to prevent flashback, or backfire. 

Uniweld Shut-down Prodedure

Victor Shut-Down Procedure

Victor, too, requires the operator to shut off the OXYGEN first at the tip, then the operator may shut off the fuel valve. The reasoning remains the same: to prevent backfire/flashback. So where does this “Shut off the fuel first” myth come from? Turns out, it’s been taught that way for decades, but not without reason.

I had the opportunity to speak with HVAC/R Training Legend Bill Johnson, one of the original authors of the Refrigeration and Air Conditioning Technologies (RACT) Manual, and we spoke extensively on the topic. The RACT Manual offers an alternative method of shutting down the torch rig. The textbook teaches to shut the FUEL off at the torch first

“Shut off the fuel gas (acetylene) valve at the torch first”
Refrigeration and Air Conditioning Technologies (8th Edition)

I asked Bill Johnson why that was, and he explained it was a way of protecting the technician over the tool. Starve the flame of its fuel first, and eliminate the flame right away. Also, this was the way he had been taught many years ago, and the first edition of the RACT Manual was published in 1987. Perhaps there was once a manufacturer operation manual that specified the “fuel off first ” method, but the procedures have since changed. This method is not without merit, as its intentions are pure.

With the proper PPE and setup procedures, following the manufacturers’ approved operation instructions should be standard across the trade. Some would argue that shutting the oxygen off first can cause the little carbon “bunnies” that are created when the acetylene pressure is low enough. This is easily rectified by changing the way you setup the torch to begin with keeping acetylene pressure at the tip specified level.

Uniweld Tip 17-1

Matt Foster from Uniweld mentioned several operating tips for torch tips and setting a flame. The most common torch tip he finds most technicians use is the Type 17-1, and is good for pipes with an inside diameter of up to 1”. The manufacturer’s design operating pressures for this tip is 5 acetylene/5 oxygen. Another common tip is the Rosebud Type 28-2; its operating pressures are 5-7acetylene/5-8 oxygen, and it is good for pipes with an inside diameter of up to 1-5/8”.    (Caveat: According to Matt, these published operating pressures may be even higher, as torch tip engineering changes over time, and the current catalog has not yet been updated. Therefore, when in doubt, take a look at the spec sheet that comes with the torch tip, or call the manufacturer to clear things up).

The Uniweld welding/brazing tip rated operating pressures can be found in their catalog here. The Victor welding/brazing tip rated operating pressure can be found here. As you can see, there is no one right answer when it comes to setting regulator pressure at the tanks. In schools, it is often taught to set fuel to 5psig at the regulator, and oxy at 10psig at the regulator. Some say the pressures should be the same at the regulator. The purpose for the pressure specifications is to ensure proper mixing of the gasses for the best quality flame, and to protect the torch rig from damage and compromised safety functions. So the answer to how to set your oxyacetylene regulator pressures is: it depends!

In other words…RTFM! (Read the FANTASTIC Manual) Hopefully, this clears up any confusion about torch rig operation and setup/shutdown procedures. Remember to ALWAYS wear proper PPE when dealing with any flame (eye protection, gloves, etc, and avoid polyester clothing), and follow industry best practices regarding safety.

— Kaleb

Pump down solenoid valves are commonplace for any refrigeration technician. They are energized with the compressor still running, shutting off flow in the liquid line so the refrigerant is pumped into the condenser and receiver. The compressor will then shut off once a low-pressure switch opens the circuit when the pressure falls below a set pressure. However, there are other applications for which liquid line solenoid valves are useful. Long line applications in HVAC incur a wide range of challenges a technician must evaluate. Among those challenges include oil return, refrigerant migration in off-cycle, compressor workload, efficiency and capacity losses, added refrigerant charge, and metering device selection.

Long line applications (for R410a straight AC and Heat Pumps with ⅜” liquid lines) are generally defined as any system with a line set longer than 80 ft in equivalent length. Equivalent length in this context means that all pressure drops (copper fittings, bends, diameter size changes) translate to a length equivalent to a run of straight copper. Manufacturer spec data for copper fittings will have printed the equivalent length of those fittings in its literature. The length to be exceeded before long line application procedures are used may vary depending on line set diameter size and on which plane the indoor and outdoor units are located, but 80 ft is the general rule for Residential AC and HPs. Any system with a 20 ft uninterrupted vertical rise in the line set should also be treated as a long line application, per Carrier’s Long Line Application Guideline, which will be linked here.


There are many ways manufacturers have sought to resolve the challenges with long line applications. Some of these solutions include crankcase heaters and txv metering devices. Most manufacturers will specify an OEM hard-start kit for the purposes of protecting compressor effectiveness against the added refrigerant charge. Some commercial applications require oil traps to aid in oil return. 


Liquid line solenoid valves are specifically utilized to prevent refrigerant migration in the off-cycle. The valve is positioned with the arrow printed on the valve body pointing toward the outdoor unit. For heat pumps, the valve must be biflow. It is important to note that the valve is normally closed in these long line applications. When energized with the contactor of the outdoor unit, the coil in the valve body will pull the valve open to allow flow. However, when closed, the valve only stops refrigerant from flowing in the direction of the arrow printed on the valve. With the system in the off-cycle, the solenoid valve will keep refrigerant liquid and vapor from migrating to the compressor down the liquid line. But don’t let the refrigerant tubing size fool you! Just because the liquid line is 3/8″ doesn’t mean any liquid line solenoid valve with 3/8″ sweat or flare connections will do. Care must be taken when selecting a solenoid valve. Choose valves to match the capacity of the system on which it will be installed (with a pressure drop of no more than 1 psi), then pay attention to refrigerant rating, THEN select by line set diameter size. 


Wiring a liquid line solenoid valve will generally tap in with the thermostat’s call for the compressor. The valve should be wired into the Y (outdoor unit contactor) and C (common) terminals on single-stage equipment. For two-stage equipment, make sure the valve opens with a call for the first stage of heating or cooling (Y1). This prevents the valve from remaining closed during compressor operation.

Solenoid valves are incredibly simple in design and operation, and troubleshooting for long line applications is also quite simple. Confirm the coil is receiving its rated applied voltage when the system is energized, and test temperature drop across the valve. A maximum of 3° difference is allowable. The valves are NC (normally closed), so if there is a temp drop across the valve body, but no applied voltage during system operation, confirm your wiring. 


Always make sure you are applying industry best practices when installing a solenoid valve. Remove the coil from the valve body before installation to prevent overheating. Use a heat absorption putty, spray, or wet rag on the valve body. Flow nitrogen while brazing, and install filter driers everytime (oversized if possible).


Long-line applications are few and far between in residential HVAC. But if you ever encounter a situation where you see a liquid line solenoid valve next to the outdoor unit, pay close attention to the way that system is setup and any other added accessories that may have been installed. You may refer to the Residential Long-Line Application Guideline at any time.


-Kaleb Saleeby

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