Author: Bryan Orr

Grounding is an area of many myths and legends in both the electrical and HVAC fields. This is a short article and we will briefly cover only a few common myths. For a more detailed explanation I advise subscribing to Mike Holt’s YouTube Channel HERE

Myth – Current Goes to Ground

Actually current (electrons) move according to a difference in charges/potential (Voltage). When a potential difference exists and a sufficient path exists there will be current. In a designed electrical system current is always returning to the source, the opposite side of a generator, transformer, battery, Inverter, alternator etc… current only goes to ground when an undesigned condition is present and ground (earth) can be a VERY POOR conductor at times. The only saving grace for the earth as a conductor is all of the parallel paths created with a ground rod because of all the surface area contact to earth.

Myth – To Be Safe, Add More Ground Rods

The ground can be an exceptionally poor conductor. The purpose of ground rods is to carry large spikes in current that comes down your electrical distribution lines away from the building. Adding more ground rods can actually EXPOSE the building to current from near ground strikes. Adding more rods isn’t always the solution and often does nothing useful.

Myth – Connecting Neutral and Ground Together In Multiple Places Is a Good Idea

Neutral and equipment ground should be connected in only one location at the main distribution panel to prevent the grounding conductors from carrying neutral current. If the equipment ground is carrying any current there is a problem.

Myth – Electricity (only) Takes The Path of Least Resistance 

If you have ever wired a parallel circuit you know that electrons travel down ALL available paths between to points of differing electrical charge.

Myth – Common, Ground and Neutral are the Same

Not even close. Common and neutral are terms used to describe the one side of a transformer. They are not grounded unless you ground them and when you do you are designating which side of the transformer will have an electrical potential that is equal with EQUIPMENT GROUND. The earth itself simply acts a really poor and erratic conductor between points of electrical potential that we designate and should not be confused with equipment ground.

Myth – Ground Rods Keep Us From Getting Shocked

Nope. Proper bonding connection between appliances, switches and outlets and equipment ground connected back to neutral at the main distribution panel in conjunction with properly sized circuit breakers and GFCI equipment keeps us safe. Grounds rods have little to nothing to do with protecting you from a ground fault.

Here is a great video on the topic and  you can find an article defining grounding and bonding terms HERE


— Bryan


What should the delta T (ΔT) be?

What do you mean!? 20 degrees of course!

We are referring to the air temperature drop across an air handling unit or evaporator coil in cooling mode.  Actually, depending on who you ask the answer will range between 18-22 degrees Fahrenheit. Heck, I’ve heard some insist that the ΔT across a properly operating air conditioner should always be 20 degrees, even if it was installed in the middle of a football field! No, really, quite literally a true story.

This is one of the most established and misleading rules of thumb in our industry. Yes, that’s right. I said misleading! Don’t get me wrong, getting an accurate measurement of what the ΔT is on an operating system has a lot of value. However, because it’s so simple to do and it doesn’t require a particularly expensive tool to measure it, there is a lot of abuse and misuse of it.

If while diagnosing a system we measure ΔT and it happens to be at a range that has been deemed acceptable by lack of information and bad habits, we’d be poised to repeatedly make avoidable mistakes. Creating like this some bad habits of our own.

When we measure ΔT we are only measuring how much sensible heat was removed from the return air. In providing human comfort, air conditioners are not only tasked with removing sensible heat, but there is also a good chunk of latent heat removal that has to occur – at least here in south Florida.

The rate of removal between sensible and latent is called Sensible Heat Ratio, you can read more about it HERE and please, take the time to read the two comments below the article (especially the first the one by Jim Bergmann) as it offers a simpler way to project the supply air temperature. – It would only, however, predict the dry bulb temperature. To add in the consideration for the latent heat removal and provide an intersecting point, the calculation has to go a little different, this way we would be working with the total capacity.

Ok, so what should the ΔT be then? Assuming a given airflow and proper refrigerant charge, it depends on the temperature and water vapor content of the air entering the evaporator coil. It does not depend on how many years of experience the person that you are asking has.

Because the purpose of this article it’s to emphasize the change in ΔT with varying return air conditions, we are going to assume proper system performance with 400 CFM per ton. Don’t worry, it could also be predicted with different airflows (I prefer to be closer to 350 CFM per ton myself) but to streamline the calculations we’re going to use a standard airflow that most are familiar with.

There are a few things we need to start:

  1. A psychrometric chart:
  2. A manufacturer’s extended performance data:
  3. The total capacity formula at standard air conditions:

BTU/H = CFM x 4.5 x (Eh – Lh)


CFM – Air volume being moved across the evaporator coil

4.5 – 60 (min. in an hour) multiplied by .075 (the density of air at standard conditions)

Eh – Entering air enthalpy

Lh – Leaving air enthalpy


3a. We need to solve for Lh:

Lh = Eh – BTUH / CFM x 4.5

We now need to pick which conditions we’re gonna use to run these examples. I’ve highlighted them in color for ease and convenience. See below.

Next, we need to identify in the psychrometric chart:

  1. The dry bulb temperature scale
  2. The sensible heat ratio scale
  3. The enthalpy scale
  4. The index point

Outlining the procedure.

For example one we are going to use our “blue” conditions. These are the colder and drier conditions of the three. At 72 degrees dry bulb and 59 degrees wet bulb, this system will produce a total capacity of 43,500 BTU/H of which 37,200 represent the sensible heat removal. This will yield an 85% SHR. (37,200 / 43,500 x 100 = 85%).

The steps to plot the lines we need on the psychrometric chart are:

  1. Find the point that marks the return air characteristics at the intersection of 72 degrees DB and 59 degrees WB
  2. Draw a straight line from our .85 SHR to the index point on the chart
  3. Plot out from the return air conditions intercept to the enthalpy scale to find the BTU content per pound of air. In this case the specific enthalpy of this air is 25.6 BTU/pound of dry air.
  4. From the re arranged total capacity formula calculate our leaving air (supply) enthalpy plot down diagonally from the left following the enthalpy lines. In this case it looks like this:

Lh = Eh – BTUH / CFM x 4.5

Lh = 25.6 – 43,500 / 1600 x 4.5

Lh = 25.6 – 43,500 / 7200

Lh = 25.6 – 6.04

Lh = 19.56 (round it up to 19.6)


  1. Draw a line that is parallel to the SHR line on step 2, but that runs across our entering air conditions and keep going until it intercepts the supply air enthalpy drawn on step 4. This is the process line for the current SHR, at this intercept find our supply air characteristics.
  2. Plot straight down from the supply air point to find the leaving air DB temperature.

It looks like this:

In this first example, the aforementioned 20 degrees ΔT rule of thumb holds under these conditions. That is with air entering the evaporator coil at 72 degrees DB and 45% RH.

Take 2

Let’s try our second set of conditions. With the “green” conditions we have our entering air at 75 degrees DB and 63 degrees WB and we are approximately at 75% SHR.

I’m going to spare you the tedious description of every single step again. But once we plot all our lines as we did in our first example this is what they look like on the chart:

Rule of thumb still strong! This is with 75 degrees of DB and 51% RH. As the case may be on most homes where the AC system was thoughtfully sized for the load and we might’ve found ourselves there doing a PM.

It got hot and humid!

But what happens when we plot our “red” set of conditions at 80 degrees DB, 71 WB and 56% SHR?

This is what it looks like:

Rule of thumb busted!

Unless you are apprentice running maintenance (nothing wrong with that), most of us find ourselves knocking on customers’ doors because their system isn’t cooling for whatever reason.

If we are responding to a service call where the AC system hasn’t been working for a number of hours or maybe a couple of days in the middle of the summer, is it unthinkable that by the time we get the system back up and running it is 80 degrees in the space with the relative humidity in the mid 60’s? I would say no. It’s actually very likely.

Same thing if we are doing a changeout. The house has had the whole day to warm up and build up some extra water vapor before the new system it’s ready to be turned on and commissioned. What do you think the conditions of the return air will be then?

But wait a second Mr. “I can draw colored lines on a psych chart”! I’ve been doing this for 45 years and I know I’ve measured ΔTs at 20 degrees under all kinds of conditions! Heck, I’ve gotta look for it but I’m pretty sure I’ve got a polaroid shot somewhere to prove it.

I believe it! What are the chances that ΔT readings have been and are still being taken on systems with PSC motors and questionable duct designs for the last 30 years? Or even before? I’d say pretty good. In fact, I have a strong suspicion that that’s where this misunderstood rule of thumb might’ve come from.

Needless to say, it is not practical to run all these calculations when diagnosing in the field. That’s what measureQuick is for. The larger point is – and to copy Jim Bermann’s argument – why are we checking the supply air temperature with a $14 thermometer if we don’t know what it should be really?

Without knowing what the water vapor content of the return air is, we can’t predict the supply air temperature. Ditch the thermometer and invest in a hygrometer. That way you can measure the moisture content of the air entering and leaving the unit, in addition to the ΔT if you still want to hang on to that.

By the way, if this summer you were to find yourself near the local YMCA and see an air conditioner installed in the middle of a football field, check the ΔT. Best case scenario it’ll be 10 degrees.


— Genry Garcia.






First a quick summary of the role of oil in the refrigerant circuit –

The compressor requires oil for lubrication of the moving parts in the compressor. If we could, we would keep 100% of the oil in the compressor but since that is generally unrealistic we need to utilize oils and oil strategies that will circulate the oil through the system and return it back to the compressor where it belongs on a regular and continuous basis.

There are components called oil separators that can strip most of the oil from the discharge gas and return the oil to the compressor, these are often used on larger systems and they are still less than 100% effective by themselves.

In VERY large systems such as chillers, we are beginning to see oilless technologies with magnetic bearings like TurboCor from Danfoss (shown above), but these are still pretty rare in the field.

So we are left with circulating oil through the system and returning it to the compressor as a regular part of A/C and refrigeration system operation under normal running conditions.

First, let’s cover the oil considerations a service tech can easily diagnose and impact.

Technician Considerations

We are tasked with preventing liquid refrigerant from entering the compressor which can cause more rapid and potentially catastrophic oil loss. This is called “flooding” and it can occur while the system is running when the refrigerant superheat is allowed to stay at zero as it enters the compressor which indicates the presence of liquid refrigerant mixed with the suction vapor.

A flooded start is flooding that occurs during startup when liquid refrigerant was allowed to collect in the compressor, in the suction line or even in the evaporator. Both of these conditions can cause oil loss from the compressor as well as oil dilution which can result in rapid compressor wear.

Preventing flooding is a significant part of oil management and involves setting superheat properly and using other strategies such as crankcase heaters, non-bleed expansion valves and pump down on the off cycle to help keep liquid refrigerant out of the compressor.

Another factor in lubrication is oil breakdown that can occur at high temperatures. We should consistently monitor discharge temperatures exiting the compressor to ensure it doesn’t exceed 225° which equates to around 300° at the compressor discharge valves (on a reciprocating compressor). This helps to ensure that the oil doesn’t break down and “carbonize”. Now, this does vary based on the compressor type, system and oil type, but is a generally accepted rule in the absence of a more detailed guideline. On a properly functioning compressor, the mass flow rate (amount of refrigerant moving through the compressor) and the suction gas temperature are the primary factors that impact compressor discharge temperature. Often high discharge temperatures occur when the suction pressure is low, superheat is low or compression ratio is high (high head pressure, low suction pressure) or some combination of these issues.

Oil Return 

Once the oil has left the compressor it must circulate through the system and return to the compressor crankcase and there are a few key factors that impact oil return-

  1.  Oil/refrigerant miscibility (how well the oil mixes and moves with the refrigerant)
  2. Oil Viscosity (oil thickness)
  3. Refrigerant velocity throughout the circuit

The oil being utilized should be suited to the refrigerant type and of the proper viscosity for the compressor and the temperature application. Refrigerant velocity should be maintained according to manufacturer recommendations and low velocity will primarily be an issue in evaporator coils and suction lines when the suction pressure is lower than design due to improper tubing sizing, low evaporator load, metering device underfeeding or undercharge.

Oil Quantity 

Keep in mind that the longer the refrigerant lines are, the larger the evaporator(s) the more oil will be out in the “circuit” and the more total oil the system will need to contain. Technicians who work in “built up” systems like market refrigeration are very aware of this and take an active approach to manage oil. Residential and light commercial HVAC techs may take the approach that it’s the “manufacturers job” to ship the system with the correct amount of oil but may fail to read long line guidelines that call for more oil to be added.

On the other hand, too much oil can also lead to compressor issues and poor system performance. This often occurs when a new compressor is installed with a new oil charge on a system that previously had oil return issues. The new compressor will only add more oil to the circuit making the situation worse and again leading to a reoccurring failure. This is why diagnosing fully and finding WHY the old compressor failed is a huge part of the process so that you can make some oil adjustments if an oil return issue was found and rectified.


Important Oil Terms

Miscibility – the ability of the oil to mix with and move with the refrigerant.

Viscosity – a measure of the oil’s flow resistance (how thick it is). Two units of measure are used with refrigeration oil. The older measure is Saybolt Universal Seconds (SUS), the newer is ISO viscosity grade number (ISO VG). In both cases a higher number is a thicker oil, just don’t mix up the two standards.

Hygroscopic – Many modern oils are hygroscopic which means they attract and hold moisture. It is very important to keep moisture away from hygroscopic oils to keep them from becoming contaminated.

Hydrolysis – decomposition due to a reaction with water. For example, POE oil decomposes into acids and alcohol in the presence of water which means that the once it decomposes it cannot be reconstituted with line driers or evacuation.

Oil Types  

Mineral – is a product of gasoline production. Naphthene based mineral oils are suitable for refrigeration systems using CFC or HCFC refrigerants and has been the standard oil used for generations. Mineral oil worked well with refrigerants that contained Chlorine but is not miscible with modern HFC and HFO refrigerants.

Alkylbenzene (AB) – a synthetic oil suitable for refrigeration systems using CFC or HCFC refrigerants. It is compatible with mineral oil and compared to mineral oil, it has improved refrigerant miscibility at low-temperature conditions which is why it was and is often used with HCFC refrigerants in commercial refrigeration.

Polyolester (POE) – The most common oil utilized in refrigeration and air conditioning systems using HFC / HFO refrigerants. It is also suitable for systems using CFC, HCFC refrigerants.

Polyvinyl Ether (PVE) – a synthetic oil that is being used as an alternative to POE oil and is very common in ductless and VRF. It is more hygroscopic than POE oil, but PVE oil does not undergo hydrolysis in the presence of water. This means that while PVE will grab water more easily it is capable or being dehydrated again unlike POE.

Polyalkylene Glycol (PAG) – a synthetic oil primarily used in automotive air conditioning systems. It is more hygroscopic that either POE or PVE oils, but like PVE it does not undergo hydrolysis in the presence of water.

Refrigerant Piping for Oil Return 

When oil does not return properly to the compressor it can cause compressor wear but it can also decrease system performance by coating the inside of the evaporator tubing walls and inhibiting heat transfer and can even cause restrictions.

Especially with mineral and AB oils it was very important to employ proper trapping strategies according to the manufacturers and industry guidelines such as THIS ONE FROM RSES 

with newer oils like POE and PVE these trapping and oil return strategies have become less critical in high and medium temperature applications due to the strong miscibility of the oil in the refrigerant. As always, read and follow manufacturers piping guidelines as the lower the velocity the more likely the oil is to have issues returning especially in retrofit applications where some Mineral or AB may still be present with the POE or PVE oils.

One good practice to use when running long runs of horizontal piping is to pitch it back towards the compressors. This is a common-sense practice no matter the oil, refrigerant or application whenever possible.

Oil Mixing 

When POE oils first gained common use it was widely rumored that mixing POE and mineral oil would result in “sludge”. This has been proven to be a myth, and to some extent manufacturers of retrofit, refrigerants have been suggesting adding small amounts of POE to mineral oil to help carry it through the system. It is always better to move to POE or PVE oils from mineral when retrofitting to an HFC refrigerant but small amounts of mineral oil have proven to be rather inconsequential in most cases.

New Oil Considerations

One thing that has become clear with the advent of POE oil is the importance of proper brazing practices (flowing nitrogen), proper deep evacuation and keeping the oil away from air and moisture during storage. Many poor practices that techs could get away with when CFC/HCFC and mineral oil were in common use can result in DISASTER with modern refrigerants and oils.

Keep the system clean and dry and use the correct oil in the correct amounts. Keep the oil from overheating and keep the compressor from “throwing” oil by preventing flooding. Maintain proper oil return through proper pipe sizing, pitching and trapping (as required) and by maintaining appropriate deisgn velocity of the refrigerant.


— Bryan





Just so you don’t get bored and quit reading let’s go straight to the point.

When the blower runs for more than a few minutes after the system has cycled off in cool mode the air may continue to be “cooler” (lower sensible temperature) coming out of the supply but the heat content of the air will remain unchanged. 

The only reason I say “may” be cooler instead of “will” be cooler is that we are assuming there is moisture on the coil and/or in the pan and the indoor RH is less than 100%.

Translation: When you run the blower once the system has gone off in cool mode you will continue to cool for a while, but that extended cooling comes from the evaporation of water out off of the coil and out of the pan. This results in sensible cooling and greater sensible efficiency but also increased indoor humidity.

Translation of the translation: It may feel cooler but there ain’t any less heat in the air by the time you figure for humidity.

Translation of the translation translation: If you live in a humid place run shorter off-cycle run times and think twice before running the fan in the “on” position. If you are in a dry place then let it blow until your heart is content.

Whenever cooling occurs by direct evaporation of a substance into an airstream (think a swamp cooler) it occurs at no net decrease to the heat content in the air. The heat is just going from sensible (what you can measure with a thermometer) to latent resulting in higher relative humidity air.

If you go below this line it is going to get nerdy… BEWARE

Now let’s talk about why, but first some terms.

Heat = Molecular energy or total molecular movement within a substance
Temperature = Molecular velocity, the speed that the molecules are moving
Adiabatic Process = A change in temperature without a change in heat content

Think of adiabatic process like this – You have a whole room full of ping pong balls bouncing around in a zero-gravity room. The balls are molecules, their total motion is the amount of heat and the speed they move is temperature. If you were to change the size of the room by bringing in one of the walls the balls the balls would bounce faster because the available space was decreased so the “temperature” would increase but the number of balls and the total motion would remain constant (this is what happens to refrigerant in a compressor by the way). If you were to move a wall outward and increase the size of the room the speed of the of the molecules would decrease, resulting in less speed and lower “temperature”. All the while the number of balls and the total motion remained constant (which is what occurs at the outlet of the metering device). In both of these examples temperature (Sensible heat) changes but the total heat content does not change, these are both examples of an adiabatic process due to compression and expansion of contained molecules.

An adiabatic process can also occur in uncontained systems like open airstreams, and evaporation of water is one such example.

Evaporation of water is a process where heat is absorbed into water molecules as they evaporate from liquid water and become entrained in the air as a vapor displacing some of the nitrogen and oxygen in the air. When that heat is absorbed from the air into the water it results in lower sensible temperature, but the water is still CONTAINED IN THE AIR. This means that while the air may be cooler it still has all the heat contained in it in the form of water vapor.

Now for the real shock..

Water vapor is NOT more dense than dry air at the same temperature it is actually less dense / lighter than dry air, however, is does contain more heat (enthalpy for you nerds like me). This means that when you run the blower after a cooling cycle the moisture on the coil and in the pan are evaporated back into the space and depending on the RH of the air it will lead to sensible cooling but latent gains. This means cooler but higher RH and this is due to the higher heat content of higher RH air at the same temperature.

Once again, depending on where you live this may be positive or negative.

In Arizona or Colorado? Run that blower after the cooling cycle.

Florida? May wanna shut it off right after the cycle or maybe 90 seconds at most and leaving the fan in the “on” position will likely result in a small increase of indoor RH.

— Bryan



P.S. – I also did a Facebook Live Video about it today

also… Here are some great videos on the subject by Jim Bergmann

Do you know how a solenoid valve works?




On the surface, I think we all understand how a solenoid valve works.  The Coil energizes creating an electromagnet.   That temporary magnetism lifts an iron plunger within the valve itself allowing refrigerant to flow.


But…  is it really that simple?


Turns out, the answer isn’t as straightforward as you’d expect.


The simplest type of solenoid valves are direct acting solenoid valves.   These are exactly what is described above.   The iron plunger directly controls the flow of refrigerant through the valve. Every single solenoid valve you see incorporates a direct acting valve, but there is more than what meets the eye.

Courtesy of Sporlan

Direct acting solenoid valves have an inherent limitation.   If the force created by the fluid flowing through the valve that is acting on the iron plunger is enough to lift that plunger, then it isn’t going to close regardless of what the electromagnetic coil tries to tell it to do.   What this means is that direct acting solenoid valves are limited in size, and that size is pretty small.



So, how can we control the fluid flow in larger lines with solenoid valves?


We start to use the pressure within the system to actually force the valve closed.


Say what???


These are called pilot operated or pilot actuated valves   The direct acting solenoid doesn’t try to control the entire flow, it only acts to control a small portion of the fluid which acts on a diaphragm or other device to open and close the valve.

Courtesy of Sporlan


Let’s see if we can start to understand how these valves work in practice.


First, a few basics.


  1. Solenoids, like most valves, are directional. If you install it backwards, it isn’t going to work correctly.    This is why.
  2. Solenoids must be sized properly. You can’t just go buy a ½” solenoid valve and expect it to work because your line is ½”.   This is to ensure a small pressure drop across the valve which is what actually makes the valve work.


Ok.   Refrigerant flowing through an energized solenoid.   Now, the coil de-energizes causing the iron plunger to drop and seal a tiny port.  What this does it stop a small amount of flow from inlet to outlet, preventing that small flow from leaving the valve body.    That small port being blocked causes pressure to build on top of the diaphragm or valve seat disc, forcing it down to seal the valve.    The small iron plunger and spring don’t have the force required to force the valve closed but, by utilizing system pressure, we have a much larger amount of force available.


In truth, the large majority of solenoid valves a technician sees are pilot operated valves.


— Jeremy Smith CM




I heard a great presentation by Ron Auvil on VAV systems and it got me thinking…

Can you size a commercial system / perform a block load by the number of occupants?


No, just kidding that’s crazy talk. There is way more too it than that.

However, in a commercial environment, while the perimeter of the building is affected by heat loss/heat gain to the outdoors, the internal zones are “cooling only” zones with the primary load usually being PEOPLE.

This is where the 500 btus per hour comes in. On average a sedentary worker in a building will add 500 btus per hour to ALL areas of the building whether it is hot or cold outside. This creates an issue in the winter when the perimeter of a building requires heating and the center of the building requires cooling.

Now, keep in mind, a sleeping person generates heat more in the neighborhood of 260 btu/ hr so if it’s a REALLY boring job where workers dose off at their computers it may be less.

Add in the internal electrical loads from lights, computers and other equipment and you start to realize that EXTERNAL loads are only part of the equation, especially in large commercial buildings with many occupants. In fact, in a busy commercial space the internal loads generally far outweigh the heat entering from the outside (external load).

This is where the concept of thermal diversity comes in. On a cold day there may be a need for heat at the perimeter of the building to offset heat losses to the outside while still requiring cooling in the center of the building to offset the internal loads.

In a good commercial design you must have some method of dealing with the thermal diversity between internal and perimeter zones along with maintaining appropriate ventilation / outdoor air.

Food for thought.

— Bryan

Knowing how to properly combine capacitors in series and parallel is a great, practical field skill to employ when you need to get a customer up and running and you don’t have the exact size.

Increasing in size is easy. Just connect in parallel and add the two sizes together. For example, if you needed a 70MFD capacitor you could easily connect a 50 and 20 in parallel will add up to 70MFD. Connecting in parallel is as easy as making two jumper wires with connectors and jumping one side of each capacitor to the other and then connecting one side like usual.

Series is a little more tricky, it goes like this

Total Capacitance is 1 ÷ (1÷C + 1÷C) = Total MFD When Wired in Series

The result is that the total capacitance will always be less than the smallest capacitor. Let’s imagine a real-world scenario where you need a 3MFD capacitor and all you have is 5 & 7.5 MFD on your van.

The math would be

1 ÷ (1÷5 + 1÷7.5) = Total MFD


1 ÷ (0.2 +.13) = Total MFD


1 ÷ (0.33) = Total MFD


3.03 = Total MFD

Definitely not something you will run into every day but a nice knowledge tool to have in the noggin toolbox

— Bryan


Duct and system design are two of the BIGGEST needs among technicians, salespeople and contractors. Matt Milton has generously agreed to teach a small online mastermind class on design, load calculation, the math of the trade and much more.

While this training may be at “no charge” it certainly isn’t FREE. It will require a lot of time and effort on your part to invest in yourself.

Here is the course summary

HVAC School – Residential Load Calculation

Residential Load Calculation is a 12 week online course to teach you the fundamentals of heating and cooling load calculations using ACCA Manual J (Abridged Edition).**WE WILL NOT COVER WRIGHTSOFT, COOLCALC OR SIMILAR IN THIS COURSE**

Topics covered include:
Print reading
Basic Construction Math
Construction Methods
Load Calculation

**Limited to the first 25 qualified responses received**

Tentative Schedule:
4/16/2019 -7/9/2019 (We will skip 4/23);
Online class from 7-11 PM EST each week (Most weeks will be 2-3 hrs max)
You should expect to spend 2-3 hours a week (average) on the homework project as well.

Week 1 – Introduction; Sections N & 1
Week 2 – Sections 2 & 3; Construction Math, Plan Reading
Week 3 – Section 4 – Heating; Worksheets A, B, D & J1
Week 4 – Section 4 – Heating; Worksheet E & J1
Week 5 – Section 4 – Heating; Worksheet G & J1
Week 6 – Test 1 – Heating Load Only & Full Heating Load Calc for Upper Floor Plan
Week 7 – Section 5 – Worksheet B, Table 3E-1 & J1
Week 8 – Section 5 – Worksheet D & J1
Week 9 – Section 5 – Worksheet E, G & J1
Week 10 – Section 5 – Internal Loads, Latent Loads & J1
Week 11 – Class Review & Full Cooling Load Calc for Upper Floor Plan
Week 12 – Test 2 – Heating and Cooling Load Calc.

Please see the details and sign up below.


I have spent most of my career being afraid of hard start kits, I heard too many horror stories of start caps exploding and sales technicians telling every customer they need one.

It dawned on me recently that it may be time for me to take a more mature look at start capacitors, potential relays, and hard start kits and find some best practices.

First, be aware that not everything commonly called a “hard start” is the same thing. The bottom of the barrel is called a PTCR which is essentially just a resistor that starts off at a low resistance when cool and changes to higher resistance when it gets hot. It creates a direct path from L2 (run side) through the start winding and as soon as it heats up, the higher resistance essentially removes it from the circuit. This is NOT the same technology as a start capacitor in any way and in my experience, they don’t work well and are prone to failure, at least in air conditioning systems.

There are also electronic and timer type “start kits” that utilize a capacitor but remove it from the circuit using a timer.

However, the most traditional and time tested method of start assisting a compressor in HVAC in the good old start capacitor and the potential relay.

Let’s start with how they work.

Photo Courtesy of Rectorseal

When a compressor first starts up, it requires a lot of torque to get from 0% up to 75% of running speed, especially when it has to start under pressure load (unequalized pressures). A start capacitor is designed to create the optimal phase shift for that first 75% of synchronous speed. A run capacitor is sized to create an optimal phase shift for a compressor that is running at full speed and at full design load because the run capacitor never comes out of the circuit.

Photo Courtesy of Rectorseal

While a run capacitor has heat dissipation capability for constant duty a start capacitor MUST be taken out of the circuit VERY quickly to avoid melting down as well as causing compressor damage.

The start capacitor is REMOVED from the circuit by a relay called the potential relay. The potential relay is normally closed and it OPENS when a sufficient PICKUP voltage is present between the 5 and 2 terminals on the relay. This pickup voltage is potential (voltage) that exists in the start winding when a motor gets above about 75% running speed and it is GENERATED in the start winding by the motor itself NOT the capacitor.

A capacitor DOES NOT boost the voltage when you see that increased voltage across the capacitor that is back EMF being generated by the motor, just like in a generator (pretty cool huh?).

Once the compressor shuts off the relay then DROPS OUT which closes the contacts again for the next time.

Some hard start manufactures wire the coil on the potential between start and common and some wire it between start and run. You will find that most OEM’s wire between start and common but this does not mean that wiring between start and run is bad… it just needs to be designed correctly for that purpose (Kickstart does it this way for example).

A properly sized start capacitor and potential relay are not BAD for a compressor, they just must be sized and installed correctly and there are some cases where they are more likely to be useful that others. When in doubt a factory start capacitor and potential relay is the best and safest bet.

Cases where they may be very useful useful

  • Long line set applications
  • Hard shut off expansion valves
  • More often on reciprocating compressors than scroll or rotary (but still OK on scroll and rotary when beneficial)
  • on 208V single phase applications

Things to consider

  • Mount the relay properly, there is a proper UP configuration on most potential relays
  • Use hard starts with REAL potential relays not timers, solid state or other relay types (in my experience)
  • Size the relay and capacitor according to manufacturers specs
  • Ensure that you have a good quality, properly sized run capacitor on any system with a hard start

For a complete write up on potential relays, you can read these articles HERE and HERE

Also, we have a podcast out with the technical manager for Rectorseal James Bowman HERE

— Bryan

It should be stated and restated that codes and code enforcement vary from location to location within the US. The IMC (International Mechanical Code) is one of the most widely utilized and referenced and the 2015 version of the IMC section 307 is what I will be referring to in this article.

Condensate Disposal 

The code as it relates to condensate disposal in the IMC is pretty vague. It says that it must be disposed of into an “approved location” and that it shouldn’t dump on walkways, streets or alleys as to “cause a nuisance”.

This leaves us a lot of wiggle room for interpretation and a lot of authority to the AHJ (authority having jurisdiction) and design professionals to establish what is and what isn’t an “approved location”. Here are a few good guidelines –

  • Don’t dump condensate in places that could cause people to slip
  • Don’t dump condensate around foundations, basements or other areas that could cause ponding, erosion and/or leakage
  • Don’t dump condensate on a roof
  • When discharging into a shared drain or sewer system ensure that it isn’t piped in such a way that waste fumes could enter the system or occupied space

Drain Sizing

IMC 307.2.2 tells us that an A/C condensate drain inside diameter should not be smaller than 3/4″ and should not be smaller than the drain pan outlet diameter. 3/4″ is sufficient for up to 20 tons according to the IMC unless the drain outlet size is larger than 3/4″.

Drain Pitch 

The IMC dictates a 1% minimum pitch of the drain which is equal to 1/8″ fall for every 12″ (foot) of horizontal run. In practice, it is safer to use 1/4″ of fall per foot to ensure proper drainage and provide some wiggle room for error.


Drains can be made out of many materials but PVC is by far the most common. When a drain line is PVC the IMC dictates that it should be supported every 4′ when horizontal (while maintaining proper pitch) and every 10′ vertically.


IMC 307.2.5 states that the condensate assembly must be installed in such a way that the drain line can be “cleared of blockages and maintained” without cutting the drain.

Traps & Vents 

The IMC states that condensate drains should be trapped according to manufactures specs HOWEVER, wording was added in IMC 307.2.4.1 that states that ductless systems must either have a check valve or a trap in the condensate line. While most manufacturers don’t specify this on this gravity ductless drains, it is something to look out for.

Venting after the trap (like shown on the EZ Trap above) is a really good idea in most applications because it helps prevent airlock that can occur due to double traps and shared drains as well as prevent siphoning. This vent is AFTER the trap and must remain open to be effective. The vent opening should always rise above the trip level of the condensate overflow switch when it is in the primary drain line or pan or above the secondary / aux overflow port on the primary drain pan. This helps ensure that if a backup occurs that the water properly trips the switch instead of overflowing out of the vent. While venting is a common best practice it isn’t part of the IMC code.

Drain Insulation 

The IMC code doesn’t directly state that the drain line must be insulated.  Many will point to the where the ICC energy efficiency code states

Mechanical system piping insulation.[/b] Mechanical system piping capable of carrying fluids above 105?F (40?C) or below 55?F (13?C) shall be insulated to a minimum of R-2. but this really isn’t talking about condensate drains when read in context.

Some municipalities do require that horizontal portions of drain inside the structure be insulated to prevent condensation and this standard makes sense to me. In Florida we always insulate horizontal portions of the drain because if we didn’t we would have consistent issues with growth and water damage due to the high dew points.

Condensate Switches 

IMC 307.2.3 states that all HVAC equipment that produces condensate must have either a secondary drain line or a condensate overflow switch, a secondary drain pan with a secondary drain line or condensate switch or some combination of these installations should be used to prevent overflow if the primary drain line blocks.

This includes rooftop units, ductless units and downflow units but the code does allow for the overflow prevention switch to be placed in the primary drain pan in these cases but NOT the primary drain line according to 307.2.3.1

— Bryan



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