Author: Bryan Orr

Bryan Orr is a lifelong learner, proud technician and advocate for the HVAC/R Trade

This is VERY in depth look at ACFM vs. SCFM and why it matters to airflow measurement from Steven Mazzoni… Thanks Steve!

Imagine your job is to figure out how fast baseballs were traveling before they hit a sheet-rock wall. The only method you have is to measure the depth of the dent left in the wall. Suppose at 60 mph, the ball leaves a ¼” deep dent. At 80 mph, it leaves a ½” dent, and so forth. No problem, all you have to do is measure the dents and you can derive the speed (velocity).

But it’s more complicated than that. You discover some of the balls are a bit lighter than others. Otherwise, they are all identical. What does this mean? The lighter balls leave behind a shallower dent than the heavy ones, even if they were traveling at the exact same velocity before hitting the wall. Obviously, more is needed than just the depth of the dents. The weight of the balls must also be factored in. Suppose you are able to weigh the balls in addition to measuring the depth of the dent they leave. You come up with an equation that factors in the ball’s weight and depth of the dent and solves for its velocity.

Something similar to the baseballs is happening when we measure airflow. To determine the airflow

(cfm, or ft3/min) in a duct, all we need to find out is its average velocity (ft/min) and the duct area (ft2). Measuring the air’s velocity (duct traverse) is the tricky part. A pitot tube & manometer measure the speed of the air flowing in a duct. At a faster speed, or velocity, more force is imparted to the column of water in the manometer. The pressure difference (velocity pressure or VP) is used to determine the air’s velocity, in feet/minute.

However, like the baseballs, air’s density isn’t always the same. Thus, the force it imparts to the column of water when traveling at a given velocity changes if it’s density changes. “Heavy” air will lift a column of water to a higher level (velocity pressure, in inches of water) on a manometer than “light” air will, even though moving at the exact same velocity. Thus, the velocity pressure and the air’s density must be factored in before we can determine it’s velocity.

What factors determine air’s density? Mainly its temperature and the barometric pressure. Warm air is lighter (less dense) than cold air. Air at higher barometric pressures near sea level is denser than air at lower pressures (high altitudes). Air’s moisture content also plays a minor role. Moist air (high humidity) at a given temperature is lighter than dry air at the same temperature.

The flow of air (volumetric) is usually expressed in cfm (ft3/min). To be more specific, actual cfm (ACFM) and standard cfm (SCFM) are used. ACFM & SCFM have been defined as follows:

Air is at “standard conditions” when it’s density is @ 0.075 lb/ft3. We can thus conclude a couple of key points. First, if the airflow measurement is taken at or near standard conditions, the ACFM and the SCFM will have the exact same value. Second, if the reading was taken on air at a significantly different density, ACFM and SCFM will have two different values.

Let’s work through an example duct traverse at a high elevation & temperature to show how to determine ACFM & SCFM. Suppose a 4-point duct traverse has been taken at the following conditions. A pitot tube was used to obtain velocity pressures (VP), but these have not yet been converted to velocity (ft/min). Let’s keep it simple and assume a 1.0 ft2 duct.

Elevation:4,000 ft
Barometric pressure:25.84”hg
Duct temperature:120 deg f
Duct area:12” x 12” = 1.0 ft2
Actual air density:0.059 lb/ft3
Standard air density:0.075 lb/ft3
Actual velocity pressure (VP) readings:0.020” wc

0.025” wc
 0.030” wc
 0.035” wc

Now, what do we do with these four velocity pressure readings? We need to convert them to velocity, using one of the equations below. The “4,005” equation is only valid for air at standard density. The “1,096” equation works at any density.

Here is where it gets interesting. Which density should we use to convert the VP readings to velocity, so we can then determine ACFM & SCFM? The actual density (0.059 lb/ft3), or standard density (0.075 lb/ft3)? We’ll explore 2 options.

  • Option 1: Calculate the actual average duct velocity using the actual density of the air measured.

Then multiply average velocity by the duct area in ft2. The result will be in ACFM.

Calculate ACFM Using Option 1:

0.020” wc =638 ft/min
0.025” wc =713 ft/min
0.030” wc =782 ft/min
0.035” wc =844 ft/min
Avg =744 ft/min

  • Determine SCFM for our example using one of these 2 methods:
    • Method A: Determine mass flow rate of the ACFM. From that, determine what volumetric flow at standard conditions would result in the same mass flow. The result will be in SCFM.
    • Method B: Multiply the ACFM by the ratio of the actual density to standard density. The result will be in SCFM.
      • Method A & B both result in @ 585 SCFM.
      • Option 2: Even though we realize the actual density at the traverse was not standard, calculate using standard density. Multiply by the area in ft2. Then take the result and apply a correction factor to determine ACFM & SCFM. o Calculate velocity & flow using the same VP’s from the non-standard density traverse, but using the standard density 4,005 formula:
0.020” wc =566 ft/min
0.025” wc =633 ft/min
0.030” wc =694 ft/min
0.035” wc =749 ft/min
Avg =661 ft/min

  • Is this 661 “cfm” the ACFM? No. Is it the SCFM? No. Obviously, it falls in between the 744 ACFM and 585 SCFM we calculated above. What is it then? It is a value that, when corrected, can get us to the true ACFM & SCFM.
    • Determine a unique correction factor for our example as follows. Notice the square root function:
    • Now what? Use this correction factor to convert the “uncorrected” 661 cfm to ACFM as follows:
    • Next, use the same correction factor to convert the “uncorrected” 661 cfm to SCFM as follows:

Conclusions: · Consider the type of instrument you are using to measure the differential pressure coming from a pitot tube. Velocity pressure readings from inclined manometers and simple differential pressure instruments will need the correct math applied. Electronic ones may be able to correct for local density and display the actual velocity.

  • Both Option 1 & 2 resulted in the same ACFM & SCFM values.
  • In Option 1, we used the actual local density to determine the actual average duct velocity and the ACFM. From the ACFM, we calculated the SCFM based on either the mass flow (Method A) or the ratio of actual density to standard density (Method B).
  • In Option 2, standard density was used to calculate a “reference cfm”. This reference cfm did not reflect reality, but was used to calculate ACFM & SCFM. A correction factor had to be calculated (square root of the ratio of the two densities) and used to convert the reference cfm to ACFM and SCFM. This method is similar to assuming all the baseballs are the heavy ones and calculating a reference speed based on that incorrect premise. Then the result must be corrected based on the actual weight of the baseball.
  • To avoid confusion, it seems best to use Option 1 along with Method B when working with air at non-standard conditions. At least then, the calculation gives you the ACFM directly, and SCFM can be calculated easily based on the ratio of the two densities. No other correction factors are needed.

Steven Mazzoni

HVAC/R Instructor

This article was written by Gary McCreadie from “HVAC know it all”. You can learn more about Gary and his tips and growing community on Facebook and on LinkedIn

What is an economizer?  Simply put, it is a mechanical device that is designed to reduce the consumption of energy, whether it be fuel, electricity, or other. According to Wikipedia, the first economizer was patented by Edward Green in 1845.  It was used to increase the efficiency of stationary steam boilers.

This article will revolve around air-side economizers.  You will typically see them as an accessory built into rooftop units used for the purpose of “free cooling”.  Free cooling is a funny term because it’s not actually “free”, the fan motor and economizer controls must be powered in order to operate, which consumes energy.  The term merely demonstrates the fact that less power consumption is taking place due to the fact we are utilizing outdoor air to cool a space rather than the use of a compressor or compressors.  Economizers also offer the added feature of providing fresh air to the building and its occupants.  A carbon dioxide sensor can be integrated into the setup.  As CO2 levels increase within the building, the outdoor air dampers are commanded to open, filling the space with fresh air.  As CO2 levels drop off, the dampers return to their minimum position.
The Guts of an Economizer
The economizer set up employs several parts in order to operate correctly.
1) A set of outdoor air dampers that are directly linked to the return air dampers are used to control air flow.  They move together as one, as the outdoor air dampers begin to open, the return air dampers begin to close and vice versa.
2) An outdoor air sensor.  This sensor is responsible for determining if the outdoor air is acceptable for free cooling.  In most cases, there will be an option between a sensible temperature sensor or an enthalpy sensor.
Sensible Temp Sensor – Measures dry bulb temperature of the air
Enthalpy Sensor –  Measures heat content within the air measured in btu/lb.  This sensor takes dry bulb temperature and wet bulb temperature into account for total heat content.
3) An indoor air sensor, this sensor reads sensible temperature and is responsible for maintaining mixed or discharge air temperature.  The damper assembly will modulate according to feed back from this sensor to maintain a pre-determined mixed or discharge air set point.  On newer economizer controls, like the Honeywell Jade for example, you are able to set the mixed or discharge air temperature as desired.
4) The damper actuator, which receives a signal from the economizer control board and moves to the assigned position to maintain the mixed air or discharge air set point.
5) When using free cooling you must remember that you are introducing fresh air, this added air into the space can cause positive pressure issues within a building.  To eleviate this problem economizers in most cases will have a built-in barometric relief damper or power exhaust system.
6) The control board is the heart and soul of the operation.  The control board receives sensor input signals, internally calculates the next step and relays the output signals to the damper actuator and power exhaust motor if utilized.
Order of operation
To keep it simple, the following example will be based on a single stage cooling rooftop unit complete with an economizer package.
On a call for cooling from the thermostat or BAS (building automation system), the Y1 terminal will be powered.  In most cases, the signal will first move through the rooftop control board and over to the econmizer control.  At that point, the econmizer control will then decide whether to proceed with free cooling or mechanical cooling based upon the outdoor air conditions either using sensible temperature of the air or the heat content of the air measured in enthalpy.  If the outdoor air is not suitable for free cooling, the control signal will be then relayed back to the main control board of the rooftop and initiate mechanical cooling (compressor operation).  If the outdoor air is suitable for free cooling, the outdoor air dampers will modulate from their minimum position (damper minimum position is set up during commissioning to maintain constant fresh air to the building and occupants) to maintain the mixed air or discharge air set point until the space temperature is reached.  Once the thermostat or BAS has been satisfied, the call for cooling will cease.
Most air side economizers in general, work as explained above.  It is best to contact the manufacturer of the equipment you are working on for technical advice or when issues pertaining to that system arise.
— Gary McCreadie –

When you start talking airflow, it can get pretty in-depth pretty quick. There is a big gap between what is useful for the average tech to apply every day and the whole story so let’s start with the simplest part to understand, Static Pressure.

Static pressure is simply the force exerted in all directions within any contained fluid, or in this case air. This means it’s not the directional force of air moving or blowing (that is called velocity pressure), it is simply to force pushing out on the positive side of the air system and pulling in on the negative side.

In other words, it’s energy exerted or inward in all directions instead of in one direction like velocity.

Measuring static pressure helps a tech know whether or not the system has excessive resistance to air flow overall or at a particular point.

Static pressure is measured in inches of water column (“WC) and is the amount of pressure needed to displace one inch of water in a water manometer.


A Magnehelic is a brand name for a high-quality Dwyer analog pressure gauge that comes in many different scales. Many techs will already have a high-quality digital differential manometer (like the  Fieldpiece SDMN5) for reading gas pressure, which makes getting a separate Magnehelic largely unnecessary.

When using a manometer or a Magnehelic, you will first zero it out to room pressure (for a Magnehelic make sure it is level). Next place the negative side probe in the return side of the unit after the filter but before the blower and place the positive probe in the supply duct. Keep the negative side probe away from the side of the blower and insert the probes in as straight and square as possible. It is advised to use a static pressure tip like the one shown below to prevent air velocity pressure or air currents from interfering with the static pressure reading.

With a static pressure tip point the tip against the direction of airflow (points opposite the airflow) in both the return and supply.

DO NOT confuse a static pressure tip with a pitot tube. A pitot tube is designed to measure velocity pressure or total pressure (velocity + static = total)  NOT static pressure, and it will have an open end and two connection points.

Total external static pressure is return plus supply, positive plus negative and in general, you would like to see it be 0.5″ or less…

If you see 0.8″ or higher that is when you start to see trouble on most newer residential systems, but as always, each piece of equipment is different depending mostly on motor design. Whenever possible design your equipment / duct system so the result is 0.4″ – 0.6″ of total static (Once again talking general residential / light commercial here).

If you do find it to be high, then read the return and supply separately to see which is higher which is just a matter of removing the hoses to your manometer or Magnehelic alternately. Whichever reads higher is the greater cause of the issue.

I could keep going on this, but instead, I will just link to some more in-depth articles if you want to do more reading.

— Bryan

Epic airflow write up from Dwyer 

Measuring Airflow from TruTech

Troubleshooting Ductwork by ACHR News


This is a piece about oversized air conditioners.

Though the symptoms and consequences of oversized heating equipment are similar to those of air conditioners, you’ll notice that the focus throughout the article will be on the cooling side. Specifically, from the perspective of climate zone 1 (hot and humid).
I’m gonna skip right through the lecturing about proper equipment sizing, selection, and duct design. There are trained professionals for that and I’m not one of them. Instead, we are going to riff from the perspective of a system that has already been installed and is doing damage.
We are gonna go over some of the symptoms, their characteristics and why making improvements to oversized HVAC it’s a slippery slope.

So, what does an oversized system looks like?
Like any other one, you’ve worked on. Except, these systems:

• Can’t keep the occupants comfortable throughout various rooms in the house.
• Comfort complaints are intensified at night.
• It short cycles periodically, but it specifically does so when it’s less than 94° outside and still feels warm inside. Even when the thermostat it’s showing 67° as the room temperature.
• The relative humidity is consistently high (over 55%) or, at best, goes through big swings throughout the day. These swings will normally track with the operation cycles.
• Light films of condensation might be visible on supply vents.
• Duct work sweating.
• Excessive noise from vents. Returns, supplies or both.
• The temperature feels (noticed I said feels, not reads) significantly warmer around the perimeter areas of the space (larger exposure to exterior walls) than on the interior ones (hallways and such).

If you pull up to a service call and any meaningful combination of these symptoms are the reason you’re there, put the gauges back in the truck. There is no need to worry about subcooling or superheat. I promise.

But why? What’s so wrong with oversized equipment anyways?

Run time is the obvious place to start. The lack thereof that is.
Oversized equipment will naturally result in larger and colder air volume being moved throughout the space. Invariably, the wall control will reach its setpoint faster and the system will cycle off before it had the chance to do its job.

What is its job exactly?

Let’s start with the mean radiant temperature. The linked article explains it very well but in short, human comfort has as much to do with the temperature of the surfaces around us as the one displayed by the thermostat.

Our body temperature is normally 98 degrees, our skin is closer to 94. So, if we were to stand by a wall with a surface temperature of 75 degrees our bodies will cool off by radiating heat to it at a more comfortable rate than if we were to stand by a wall at 85 degrees. And the same goes for couches, beds, kitchen counters, etc.

An AC system must run long enough to keep a cooler and consistent temperature on all the surfaces of a home. If the outdoor temperature it’s in the ’90s and yet, the system runs for only 10 to 15 minutes each cycle, this won’t be enough to keep the mean radiant temperature of the surfaces in your home under control. Therefore, you’ll be uncomfortable despite the thermostat reaching, and “maintaining” an indoor temperature in the 60’s. This phenomenon is worsened at night when the outdoor temperature drops and the AC runs even less.

Apparatus dewpoint (ADP) is next. ADP is the effective surface temperature of the cooling coil. Or as we call it, coil temperature. I will use these 3 terms interchangeably.

While a system is off, the evaporator coil will be at a temperature close to that of the return air path and its surrounding surfaces. This temperature is much higher than that of when the system is running. Once the system cycles on, the return air temperature will dictate the evaporator saturation temperature based on the DTD and it will reach the ADP.

But just because the refrigerant entering the evaporator is at 40 degrees doesn’t mean that all of the coil will immediately drop to this temperature. This process takes time. The cold refrigerant has to make several passes before it can first, absorb the heat from all of the evaporator’s body mass, for it then, come down to the design ADP.

If we are having average run cycles in the 10 to 15 minutes range, this won’t be enough to ensure that the whole evaporator surface reaches its design operating temperature, and dehumidifies the air before the system cycles off. Therefore, the dehumidification capacity of the system will be consistently and greatly compromised, resulting in poor relative humidity control in the space.

This phenomenon is seriously worsened when dealing with high-efficiency systems. To increase SEER ratings manufacturers have found ways to drop the compression ratio and therefore power consumption. To achieve this, they have increased the suction saturation temperature through the use of larger coils. So, not only does the evaporator starts out warmer, but now it has more surface to bring down to temperature. The shorter run times of oversized systems will accentuate the otherwise negligent consequences of having a larger and warmer cooling coil surface temperature.

Did you just say SEER?! At no other time, an AC system is more efficient than when is not running, right? Because is not using any energy. So, wouldn’t it make sense to provide the consumer with a system that cycles off more often then? Nope. To begin with, the upfront costs of having larger equipment installed are normally more than that one of smaller capacity.

Also, and more importantly, the single, highest point of energy consumption for an AC system is when it turns on. Once a system cycles on and off more times than necessary throughout the day, the presumed savings of not having it run for a given amount of time go out the door.
And to top it all off, the clients are ticked off! Not only did their electric bill not go down much if any, but now they are also uncomfortable.

So how can we fix it?

Well, to fix it we would have to replace the system with one of the appropriate capacity. But that’s probably not gonna happen right away is it? Not until the consumer has enough pain to motivate the expense anyways.

Before we invariably end up talking about extending runtime and/or lowering airflow I want to make a quick stop on static pressure.
When there is an oversized system connected to existing, older ductwork. As soon as you start diagnosing the issue, you’ll run into a high external static pressure reading. At this point, a light bulb will go off in your head “it’s the ductwork”!

You’ll carry on to quote duct improvement solutions that will drop the TESP, maybe even throw some return air path upgrades. Let’s say the customer agrees, and once the work is done you perform a complimentary (or not) test and balance and ultimately confirmed that the TESP is now within acceptable levels.

“I’m going to be a hero” you may say to yourself. Well, if you did in fact improved the duct system to a point where the equipment is now moving more air than before, then the problem just got worse.

I get that it’s a controversial stance but, next time you realize you are in front of one of these situations ask yourself:
More, colder air. Do I really want to make this oversized system run better?

About extending run time

If the envelope doesn’t change, then the alternative left would be extending runtime. There is a number of ways to achieve this:

• Strategically place remote temperature sensors on the warmest areas of the house that report to the thermostat and therefore will trick the system into running more. The thermostat may feature dehumidification specific algorithms.

• Purposely de-balance the airflow distribution throughout the house, so there is more air hitting the exterior surfaces and as little as possible on the interior areas where the wall control may be located – the ceiling on this strategy is pretty low in my experience.

• And all of the above plus reducing the airflow to its minimum possible setting to run a colder coil temperature and run a lower SHR. Therefore, the dry bulb temperature as sensed by the wall control won’t drop as fast…maybe.

Doesn’t sound that bad, does it? Except these will also result in colder supply air temperatures. This is the leading cause of sweating ducts and vents in these scenarios, but that’s not the worse part.

This will directly result in localized, colder surfaces throughout the envelope as well. Condensation on vents and ductwork you can notice fairly early, before they become a problem. But what about the condensation you can’t see? The one that had been forming on building materials for a while and wasn’t a problem until now that a coconut tree sprung out of one the walls. A “moisture” remediator gets called next and what follows it’s an unfortunate tale of lawsuits and bad reviews.

I am not saying that improvements to ductwork and runtime shouldn’t be made to an oversized system but…
Have you ever heard of the bull in a china shop metaphor?

The china shop is the house and the oversized HVAC is the bull.

Genry Garcia
Comfort Dynamics, Inc.

Keep in mind when reading ANY article about electrical theory or application is that it only scratches the surface of the topic. You can dedicate years of your life to understanding electrical theory and design the way many engineers do and still know just enough to be dangerous.  In HVAC we rarely need to have a DEEP understanding of electrical design but there are a few cases where a little understanding can go a long way to identifying issues before they cause trouble and that is the intent of this short article.

What is Three Phase Power?

Power is generated at the utility in three phases that are 120 degrees out of phase with one another at 60hz (Hertz). This simply means that 60 times per second each individual leg of power makes one peak and valley (a full circle), and all three of the phases together split the cycle into thirds (trisect).

This video is the best visual demonstration I have seen of three-phase power and how it works.

What Does an HVAC Tech Need to Know About Three Phase Power?

Three-phase motors don’t’ require a run capacitor because the 120-degree phase difference is ideal for efficiently spinning a motor so a “start” winding and phase shifting capacitor isn’t needed.

The biggest concern for the techs and installers with three phase is getting the phasing correct so that motors run in the correct direction. While this doesn’t matter for reciprocating compressors it is important for condenser fans and blowers and it is absolutely CRITICAL for scroll and screw compressors. Changing the direction of rotation is just as simple as swapping any two phases.

Keep in mind when installing replacement parts and equipment that if you keep the phases connected in the same way you will generally be in good shape. It is still a good practice to use a phase rotation indicator like the one above to confirm proper rotation. In most cases, clockwise phase rotation is what you are looking for but I’m sure there are exceptions to that. Alternatively, you can disconnect the compressor that could be damaged by improper phasing and start up the blower to see if it runs the correct direction before bringing the compressor(s) online. One caveat is that when motors use a VFD (variable frequency drive) the phase rotation will automatically correct making them an unreliable test in those cases.

Balancing Phases

Electricians are responsible for balancing the amperage of single-phase loads (both 120v single leg and 208v two leg loads typical on a wye three-phase system) both so the neutral doesn’t carry high amperage on the 120v loads and so the one leg of power doesn’t carry significantly more or less load than the other two. As the amperage load on a particular phase goes up, there is more opportunity for voltage drop depending on the size of the load, size of the transformer and service feeding the space and well as wire size and connection quality. This can become a challenge when there is a mix of single phase outlets, 208v appliances, and three-phase equipment.

Let’s say someone connects a bunch of space heaters on phase A, as well as a few smaller HVAC systems between phases A and B and almost nothing on phase C. If you have a large RTU that uses three-phases phase C will tend to have less load and therefore higher voltage while the load on phases A and B will fluctuate based on when the smaller systems and space heaters go on and off.

This can cause overheating of conductors and damage but it can also cause voltage imbalance which is a real cause for concern for an HVAC technician.

3 Phase Voltage Imbalance

Voltage imbalance is a motor killer. It causes poor motor performance and increased winding heat which leads to premature failure. In the case of HVAC blowers and compressors, this additional heat ends up in either the refrigerant or the air which must then be removed, further decreasing efficiency.

To test for three-phase imbalance always check from phase to phase not from phase to ground. You simply check the voltage from each of the three phases to one another and find the average (add all three and divide by three). Then compare the reading that furthest from the average and find the % of deviation. For most of you I know that sounds like a giant pain so we made this easy calculator for you.

The US Department of Energy recommends that the voltage imbalance be no more than 1% while other industry sources say up to 4% is acceptable. In general, you will want to make SURE the imbalance is below 4% and work to rectify anything over 1%.

What Can I Do About It?

You want to first look for the obvious. Melted wires, loose terminals and lugs, undersized wire, pitted contacts, poor disconnect fuse contact etc… Obviously, if you aren’t licensed or allowed to open a panel you won’t always be able to fully rectify the issue yourself but you can go a long way towards the diagnosis.

When checking voltage it is generally best to do it with the system running as close to the motor you are checking as possible. This is the actual voltage the motor is “seeing” and is what matters to the operation of the motor. You can then test back towards the distribution point, if you see a big increase in voltage as you test back towards the source you know you found a voltage drop and a cause or contributor to the issue.

From there the issues of amperage load imbalance in the panel, service size and utility issues must be considered once all the basics are covered. Most of all, if the imbalance is severe (over 4%) you don’t want to leave your motors running or you risk damage and expensive repairs.

— Bryan

There is a lot of misunderstanding about the HSO (hard shut off) or “non-bleed” TXV (thermostat expansion valve) and what makes it shut off, why it exists and how it “magically” opens.

Once you understand the forces inside the valve it is quite simple and obvious and sadly devoid of any magic.

The Three Forces 

The TXV has two primary closing forces, namely the evaporator pressure via the internal or external equalizer depending on the valve type and the spring pressure. It also has one opening force, the bulb / power head pressure.

The bulb is mounted on the suction line so the warmer the suction line the more wide open the valve will if the evaporator pressure remains the same and the colder the suction line the more the valve will close if the evaporator pressure remains the same.

Most modern valves are externally equalized with the suction/evaporator pressure at the outlet of the evaporator, so the higher the suction pressure the more the valve will close if the suction temperature remains the same and the lower the suction pressure the more the valve will open if the suction pressure remains the same.

Said more simply…

The bulb is the opening force and the spring and external equalizer are both closing forces

These two forces balance during operation with the spring adding some additional force so that the superheat can be set and maintained by the valve (to some degree). Without the pressure of the superheat spring assisting the external equalizer the pressures in the bulb and pressure in the evaporator will tend to exert the same force and therefore fail to modulate and control superheat… which by its very nature means that the temperature of the suction line needs to be greater than the saturation pressure in the coil.

This occurs while running because the refrigerant is dynamically moving and as the valve adjusts the coil pressures and temperature also change until a balance of the three forces is achieved and the valve settles in on it’s designed superheat.

What is the Superheat When The System is Off? 

This is not a trick question… what is the superheat of a system when it isn’t running? the answer is ZERO.

When the compressor goes off and the refrigerant becomes static (still) instead of dynamic (moving) the pressures in the evaporator coil begin to rise as the liquid and vapor in the coil reach equilibrium with the temperature until zero superheat (also called saturation) is reached.

So what does the valve do? It CLOSES.

It doesn’t need to be a SPECIAL hard shut off valve… it just does what it is designed to do and when you look at the forces that make it open and close it makes complete sense.

When the refrigerant stops flowing and the coil starts to warm up and the coil pressure starts to go up both the bulb and external equalizer forces rise equally but without refrigerant flow driven by the compressor pulling down the pressure in the suction line the valve goes shut due to the spring pressure.

In other words… unless a TXV is specifically DESIGNED not to close all the way it stands to reason that it will start closing as soon as the compressor goes off and eventually shut completely once the evaporator pressure rises to the point that the saturated pressure + spring pressure is greater than the bulb saturated temperature.

Clear as mud? Let me describe it in two practical ways you may have observed.

The Nitrogen Observation 

When I pressurize a new split system with nitrogen I like to feed it in the liquid line and watch the suction rise to make sure there are no restrictions in the system and to hopefully force any contaminants into the filter/drier or screen rather than the valve.

I always noticed that on a TXV system I could only pressurize it so far before the suction pressure stopped increasing. It always worked fine up to a certain point but once I got somewhere between 200 and 300 PSIG on an R410a system the suction would just stop rising….

Have you ever noticed that? Ever wonder why?

Take a look at a PT chart and you will notice that the pressure that it “shuts off” at will often be right around the saturation temperature of the refrigerant the valve is designed for at the current indoor temperature.

The example below shows the saturation pressure of R410a with an 80-degree coil… Notice the pressure is often about where the valve shuts off on you when pressurizing…

This happens because the system is off and the bulb pressure (opening force) is at approximately the indoor temperature when the suction line is at around the indoor temperature.

Initially, the valve is wide open when the coil has no pressure because there is very little closing force exerted by the external equalizer. As you add pressure to the coil the force exerted by the external equalizer increases to the point until you get near that saturation pressure and WHAM the valve shuts.

Vacuum on the Suction Side

When we first started advocating for pulling a vacuum with one hose on the suction side with the vacuum gauge on the liquid line there were many who thought it wouldn’t work because of “hard shut off TXVs”.

Now, this is true of closed solenoids or shut ELECTRONIC expansion valves but NOT of a non-bleed TXV… Why you might ask?

Think about the forces again.

When we pull a vacuum we are DROPPING pressure, in this case on the suction line. When we decrease suction line pressure we are also decreasing the external equalizer force which closes the valve. When you decrease a closing force you open the valve further so pulling a vacuum in this way actually drives the valve open and we have shown time and time again that for residential new installs and changeouts it is a very effective method of pulling a deep vacuum.

What is Hard Shut Off Good For? 

The advantage of a non-bleed TXV is simple, it helps reduce refrigerant migration and flooded starts. By closing soon after the system cycles off it keeps most of the refrigerant in the condenser coil which prevents it from gathering in the compressor or dumping down the suction when the system comes on. Since compressors can be easily damaged by flood back the non-bleed TXV is a good thing for that reason.

The only thing that causes trouble is the fact that some compressors struggle to start with more pressure on the discharge side and less on the suction side, this is why some manufacturers require hard start gear when an HSO / non-bleed valve is in place.

— Bryan

One trait I’ve seen with good technicians is that they take their jobs VERY seriously, but they learn not to take themselves too seriously. A few months ago I had someone tell me online that I must think I’m the A/C “god” because I’m always telling everyone the “right” way to do things. This got me thinking….

I don’t want to be an A/C god, too much pressure, and heaven knows I’ve broken all of these rules more than once. I’ll settle with being an A/C Moses, descending Mount Sinai with the oracles of truth from on high

The problems with this metaphor are many, but let’s roll with it. The truth is there are many “prophets” like Jim Bergmann, Dave Boyd, Dan Holohan, Jack Rise, John Tomczyk, Bill Johnson, Dick Wirz and Carter Stanfield that I have taken these “commands” from, and they likely learned these from those that came before them. Just DON’T build a golden calf to poor workmanship or we will smash the tablets and make a big mess… Ok here are the commands.

1. Thou Shalt Diagnose Completely

Don’t stop at the first diagnosis. Check everything in the system, visually first if possible, and then verify with measurements. Sometimes one repair must be made before other tests can be done, but often you can find the cause of the initial problem as well as other problems BEFORE making a repair which helps save time, provides better customer service, and creates a better result.

2. Thou Shalt Not Make Unto Thee Thine Own Reasons

Jim Bergmann often talks about how when techs don’t understand something, they start making up their own reasons that something is occurring, and then train other techs in these made-up reasons. If you don’t understand something, a bit of research and study goes a long way.

3. Thou Shalt Not Change Parts in Vain

In other words, DON’T BE A PARTS CHANGER. Never condemn a part on a guess or make a diagnosis out of frustration. Get to the bottom of the issue no matter how long it takes. This is better for the customer, the company, the manufacturer, and your development as a tech. If you aren’t confident, call someone who is fundamentally sound and get a second opinion BEFORE you leave the site. Better yet, send them a text with all the readings, model and serials, conditions, photos, type of compressor, type of controls, type of metering device, and what you have done BEFORE you call them. Get the diagnosis right the first time.

4. Remember the Airflow and Keep it Wholly

So much of HVAC/R system operation has to do with evaporator load, with LOW load being most commonly caused by LOW AIRFLOW, and low airflow being most commonly caused by dirt buildup. Keep blowers, fans, filters and coils clean and unobstructed. Check static pressure when duct issues are suspected in order to verify and properly setup blower CFM output to match the requirements of the space and outdoor environment.

5. Honor Thy Trainers and Mentors

New techs will often learn a few facts and cling to them as though they are the end all and be all of system diagnosis. I have met techs who get over-focused on everything from suction pressure (most common), to superheat, subcool, static pressure, delta T, and amp draw. A good tech continues learning from older and wiser techs and trainers who see the whole picture. When you are new, it’s hard to remember all of the factors that go into system diagnosis and performance. More experienced techs who have kept up on their learning develop a “6th sense” that can rub off on you if you “Stay Humble” (to quote the great philosopher Kendrick Lamar). Listen more than you talk, and learn the full range of diagnostic and mechanical skills.

6. Thou Shalt Not Murder The System by Failing to Clean

A good technician learns the importance of keeping a system clean early on and never forgets it. Condenser coils, base pans, drain pans, drains, evaporators, blower wheels, filters, return grilles, secondary heat exchangers and on and on… A system that is set up properly initially and cleaned regularly will last much longer, cool or heat better, and use less energy. In my experience, techs that don’t believe in maintenance don’t perform a proper maintenance themselves. Use your eyes, and clean what’s dirty.

7. Thou Shalt Not Commit Purgery without Vacuumy

Proper evacuation is one of the most overlooked disciplines of the trade. Jim Bergmann says again and again, a proper vacuum is performed with large diameter hoses connected to core removal tools. The cores are removed from the ports, the hoses have no core depressors, the hoses are connected directly to the pump (not through gauges). The vacuum (micron) gauge is connected on the side port of the core removal tool, not at the pump. The pump has clean vacuum pump oil and the pump is run until the system is pulled below 500 microns (exact depth depends on the system). The core tools are then valved off and the “decay” is monitored to ensure that the system is clean and tight.

Purging with dry nitrogen prior to deep vacuum helps with the speed of evacuation, and installing line driers assists in keeping the system clean and dry, but neither are a substitute for a proper deep vacuum and decay test.

8. Thou Shalt Not Steal (from the customer)

Good techs provide solutions for their customers to get a broken system working, as well as other repairs or upgrades that result in optimum performance. Most techs don’t INTEND to lie to a customer, but their lack of understanding on the products they are OFFERING, along with strong incentives to OFFER these upgrades can result in dishonest practices. A good, profitable technician has a deep understanding of all the repairs and upgrades they perform, as well as a sense of empathy for the customer.

9. Thou Shalt Not Bear False Witness Against Other Technicians

This all comes down to a witch’s brew of ego and insecurity all mixed together. You have either done this yourself, or you know of someone who has gone to a customer’s home or business and thrown the previous technician or company under the bus in front of the customer. In some cases it may be nothing but bravado, and in other cases it may have a measure of truth in it (or may be undisputed). Either way, talking negatively about other techs and companies does nothing but make you look petty and angry. Demonstrate your skill and knowledge by discussing the courses of action you intend to take, and if required, you can COMPARE these actions to previous actions taken; just stay away from personal attacks. Let the customer be the judge about the last guy.

10. Thou Shalt not Covet Thy Neighbor’s Job

Many good techs start to do poor quality work when they get burned out… and buddy let me tell you- I HAVE BEEN THERE. It is important to remember that every job from maintenance tech to business owner has good things and bad things about it. There are good days and bad days, great customers and total jerks, 16 hr days and 8 hr days. You may hit a spot where you decide to change jobs, and that is totally fine and may be a great decision. Just don’t make a rash decision because the grass looks a little greener. ALWAYS do quality work no matter where you work, or how bad it gets. Doing poor quality work because your job is getting you down is like a cancer that will grow and do harm to you and your career.

Take pride in your work, keep your eyes and ears open, learn something new every day and the HVAC/R gods will smile warmly upon you.

What commands would you add or remove?

— Bryan



The suction line accumulator is designed to keep liquid refrigerant from entering the compressor while still allowing for oil return.

The trouble is that if the oil return port/screen clogs, the accumulator can fill with oil and actually cause the compressor to fail. In addition to that, it can hold contaminated oil in a burnout.

As standard compressor replacement practice, you may want to consider removing the accumulator and dumping / properly disposing of excess oil to both remove contamination and check for excessive oil buildup as well as acid testing the oil.

In the case of a bad burnout, it may be best to replace the accumulator completely in addition to the other burnout protocol measures.

Here is a great video on accumulators from AC Service Tech

— Bryan

Ever since Nikola Tesla invented the modern induction motor we have been struggling with varying the speed of motors in an efficient and reliable way. The trouble in the HVAC industry is that there are several different types of technologies in play and they can easily get confused.

ECM (electronically commutated motor)

In residential and light commercial HVAC we have seen ECM (Variable Speed / X13) motors for years, primarily in blower motors but sometimes even in condenser fan motors. The first thing to know is that an ECM motor is “Brushless” DC motor. Most traditional DC motors require brushes to provide power to the motor rotor (spinning part). Brushes are notorious for wearing out over time making DC motors unreliable in constant duty applications. An ECM motor uses a permanent magnet rotor which eliminates the need for power to be fed to the rotor through brushes.

An ECM motor is a DC 3 phase motor with a permanent magnet rotor where the cycle rate is controlled by the motor module. Here is a great video on how they work.


VFD (Variable Frequency Drive) 

For existing A.C. (Alternating Current)  3 phase motors the only way to change the speed reliably and efficiently is the alter the “frequency” of the power applied to the motor to something other than 60 hz (60 cycles per second). A VFD intercepts the power applied to a motor, changes it to DC power with a bank of diodes (rectifier) also called a CONVERTER. It then smooths out the power using capacitors before feeding that power to a bank of transistors called an INVERTER which is constantly switching the power from DC back to a form of power called PWM (Pulse Width Modulation) which replicates frequency change to the motor. The drive needs to be able to provide this PWM power at the correct voltage and current in order to control a 3 phase motor properly.

Inverter / Inverter Drive

Many A/C systems are coming with Converters, Capacitor Smoothing (Intermediate Circuit) and then the Inverter all built in to the equipment itself to drive a compressor or compressors. This inverter technology is essentially an intelligent and specifically designed VFD built into the equipment itself. The Carrier Infinity system is one of many systems that utilize inverters.

These technologies are constantly evolving and changing and while they may be similar, the different names describe different types and applications of technology all designed with e end goal of making motors go more than one speed with the best efficiency and reliability.

— Bryan

This article is written by my good friend Neil Comparetto, a contractor and industry influencer who is helping to shape IAQ for the HVAC industry in the US for the better. Thanks Neil!

Indoor air quality (IAQ) monitors can tell you a lot about the air you are breathing. We find that the information is valuable for both contractors and clients. (Most monitors record temperature, humidity, CO2, volatile organic compounds (VOC), and particulate matter (PM). Carbon monoxide (CO) and radon can also be monitored, but typically in separate devices.) This graph shows PM 2.5 levels, and the differences with a poorly installed microwave range hood and a new properly installed range hood. This is what the EPA has to say about PM: “The size of particles is directly linked to their potential for causing health problems. Small particles less than 10 micrometers in diameter pose the greatest problems, because they can get deep into your lungs, and some may even get into your bloodstream. Exposure to such particles can affect both your lungs and your heart. Numerous scientific studies have linked particle pollution exposure to a variety of problems, including:

  • premature death in people with heart or lung disease
  • nonfatal heart attacks
  • irregular heartbeat
  • aggravated asthma
  • decreased lung function
  • increased respiratory symptoms, such as irritation of the airways, coughing or difficulty breathing.

People with heart or lung diseases, children, and older adults are the most likely to be affected by particle pollution exposure.” As shown in the graph, proper ventilation while cooking can drastically reduce PM2.5 levels. It’s recommended to use the range hood during all cooking tasks, from boiling water to using the toaster oven. One of the main issues with range hoods is that they’re loud, which makes them a nuisance to use. There are several factors involved making them loud; quality of the model, how much air they are moving, and most importantly how they are ducted. Even a normally quiet, high quality range hood that is poorly ducted will be loud. If you’re curious about your home’s IAQ I encourage you to get an IAQ monitor. (FYI, the one we install in our client’s homes is the IQAir AirVisual Pro.) Neil Comparetto,
Co-owner of Comparetto Comfort Solutions in Virginia

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