Author: Bryan Orr

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

This is a quick, real-life diagnosis/find by Kalos Services senior commercial HVAC/R tech Eric Mele

Improper Defrost Timer installation

While servicing this walk-in cooler for an unrelated problem, I noticed the defrost timer installed in a way that will almost certainly cause early compressor failure.

Because this is a cooler (and not a freezer) it is set to defrost by simply shutting off the compressor four times per day while still running the evaporator fans. If you look at the timer you will see that the little pins are flipped four times per 24 hr period.

If you are not familiar with walk-in coolers, and freezers, they are designed to pump down into the condenser/receiver at the end of each cooling cycle by closing a liquid line solenoid and shutting off via low-pressure control. The reason for this is because the areas they control are kept so cold a good amount of liquid refrigerant would migrate to the evaporator in the off cycle.

The timer was installed at the condensing unit and would stop the condensing unit/compressor from running during the defrost event, but would not cause the unit to pump down during defrost. This will likely cause a flooded start at the end of each defrost when the liquid refrigerant rushes into the compressor on startup.

There are no interconnecting wires between the evaporator and condenser, which is common on remote walk-in systems. The easiest way to correct this problem is to relocate the defrost timer into the evaporator, and break power to the liquid line solenoid valve during defrost. 

The diagram below shows first how it is currently connected and then how we rectified the situation.


— Eric M.

Breakers are designed to trip anytime the circuit draws a current above the rating for a period of time. The time the breaker takes to trip is a function of how high the circuit amperage in comparison to the breaker rating.

The higher the amperage above the rating the faster the breaker will trip

Breakers can accomplish this either thermally, by tripping on increased temperature or inductively, by tripping on increased magnetic field when amperage increases.

The majority of residential circuit breakers are thermal which means they are more prone to trip during high ambient temperature than during low ambient temperature. This is one factor in why you will receive more nuisance or intermittent breaker tripping calls on a hot Summer day.

Many times breakers get replaced just for doing their job and tripping when they should.

There are five common causes of breaker tripping. Improper circuit design, Overload, ground fault, leg to leg short and breaker issues

Inappropriate Circuit Design

Improper circuit design can result in an overload condition when the circuit ampacity (amperage capacity) or the circuit breaker size is not correctly matched to the load, to begin with or someone added additional load to the circuit later on.

For HVAC equipment this means that the circuit size should be matched to MCA (Minimum Circuit Ampacity) and the circuit breaker or fuse should be matched to the MOCP (Maximum Overcurrent Protection)

If the conductor is smaller than the MCA rating or the breaker is lower than the MOCP rating. It can result in a tripping breaker.

You will also see cases where more than one system will be connected to one circuit breaker which is incorrect unless the systems have additional, independent overcurrent protection.

These issues usually cause an intermittent trip as it takes time under load to show up depending on the severity of the problem.


An overload condition occurs when the loads draw more current / do more work than they are designed for. Common overload conditions would be a compressors locking up, motor bearings binding, blower belts too tight or sheaves adjusted improperly. And overload generally occurs with inductive (magnetic) loads like motors in cases where the motor is either being placed under a greater torque load than it’s designed for or the motor itself is beginning to fail mechanically.

Overload conditions often don’t trip a breaker because the motor itself will usually have an overload that specifically protects the motor. This is why when a compressor is locked it is much more likely to shut off on thermal overload than it is to trip a breaker even though it will draw far higher amps than the breaker rating on startup. In these cases the thermal overload is designed to respond quicker than the breaker.

If a breaker is tripping because of an overload condition it will usually be after several seconds, minutes or even hours of operation. It will not be “instantaneous” unless someone installed the wrong breaker or fuse and used an “instantaneous trip” instead of a typical “slow blow” or slow acting type. This would be quite rare.

Ground Fault

A ground fault is a short circuit (no load path) between an energized circuit and equipment ground.

A ground fault is the most common cause of instantaneous breaker tripping

In most ground fault situations there will be very amperage, very quickly resulting in a breaker that trips right away.

Common cases would a shorted motor, such as a shorted compressor or a rubbed out wire.

A combination of visual inspection, isolation and ohm measurement to ground and megaohm / hi-pot tests or hot verification as needed is the best way to diagnose a short to ground (ground fault).

Leg to Leg Short (Bucking Phases)

When you have two legs of power that have different sine wave patterns such a 240V single phase or 3 phase power you must prevent the legs from coming into contact except through a load.

If they do come in contact there will be an enormous transfer of energy and a significant arc.

This can happen when two wires rub out, when switch gear becomes compromised or within a motor.

Many times techs will look for short circuits from “leg to leg” or “winding to winding” in a compressor or a motor without first measuring to ground.

This is not a good idea
Even when a motor does short “winding to winding” it is rare that it just stays shorted. Usually it will ALSO be shorted to ground or it will be open after the arc flash that resulted from the short.

Think of a circuit board. Circuit boards short out all the time and the result is a big black spot on the board and nothing works anymore (open). It rarely results in a continued short circuit because the arc from the short blew the connection apart.

The reason I encourage caution is because I have seen many junior techs condemn good compressors due to a “leg to leg” short just because the ohm reading between Run and Common appeared low to them.

The only way to know if a single phase compressor is shorted “leg to leg”  with an ohmmeter is to know what the windings should read in the first place.

On a three phase motor all three legs should read the same ohms leg to leg which makes it considerably easier.

When you do encounter leg to leg (only) short circuits it is more often on fan motors than on compressors.

Breaker Issues

Because most breakers trip due to heat, anything that causes the breaker to get hotter than normal can result in tripping.

This can be due to a poor connection inside the breaker itself, but often it is due to a poor wire connection on the breaker or a poor connection between the breaker and the bus bar.

Usually these types of breaker issues are caused by installation problems such as loose connection, wrong breaker type, failure to use anti-oxidation paste on alum to copper or excessive tripping / using the breaker as a switch.

Here are some tips for diagnosing a tripping breaker

Tripping instantly

  • Perform a visual inspection of all wires and connections. Look for signs of rubout, damage and arcing
  • Isolate components and ohm to ground
  • If you are unable to locate with an ohmmeter use a megohmmeter to ground (with caution especially on scroll compressors)
  • Finally, once you believe you have identified the cause, fully disconnnect the shorted component and power the unit back up and make sure everything else functions.

Tripping intermittently or after more than 3 seconds

  • Visually inspect all electrical connections and ensure they are clean and tight.
  • Inspect the breaker and bus bar connections
  • Check breaker and wiring size
  • Measure running voltage and ensure it is within +/- 10% rating
  • Measure for voltage drop during startup (less than 15%) as well as between the power source and right at the unit (less than 5% overall)
  • Measure component amperages while starting and running and compare to manufacturer specs
  • Measure motor and compressor temperatures and watch for temperature increase over time. Infrared and thermal imaging can assist with this
  • Watch for anything that can cause overload such as failing bearings, belts too tight, or sheaves adjusted for too much RPM
  • Measure current right at the breaker, if it remains below the breaker rating and the breaker STILL TRIPS, only then replace the breaker.

I also learned recently that AFCI (Arc Fault) breakers generate heat internally which mean that you will see a hot spot on them with a thermal imaging camera or IR thermometer.

Don’t replace a breaker unless you know it’s failed and don’t condemn a part as being shorted unless you can isolate it out of the circuit and every other component still functions (as possible)

— Bryan


Flowing nitrogen while brazing and pressuring with nitrogen are both great, but nitrogen in with the refrigerant? Not so much. Nitrogen is a “non-condensable” gas because it cannot be condensed (under normal conditions), but Nitrogen isn’t the only non-condensable.

First, let’s talk about what a non-condensable gas is.

Any gas that does not condense (change from vapor to liquid) under the normal compression refrigeration conditions is called a non-condensable gas or NCG. These would commonly be air, nitrogen, carbon dioxide, Argon and Oxygen.

Non-condensable in the system will result in high head pressure / condensing temperature and occasionally high side pressure fluctuations as well as decreased cooling capacity and efficiency due to higher compression ratios.

The only way to remove non-condensables COMPLETELY in a small air conditioning or refrigeration system is to recover the entire charge and recharge with virgin refrigerant. You can recover the charge, let it sit in the tank for a while and then recover the vapor off of the top into another tank and recharge with liquid only to remove most of the non-condensables but it’s a pretty inexact science.

You can’t remove non-condensables with a line drier and while you do remove air with a vacuum pump you only remove the air that entered the system once you open it. The vacuum does nothing for the refrigerant you already pumped down or recovered as the non-condensables remain mixed with the refrigerant unless you are dealing with large volumes where they can actually be separated and the NCGs removed.

Non-Condensibles Don’t Cause Restrictions 


Non-condensables is often a term used by techs to mean ANYTHING in the refrigerant that shouldn’t be there, such as moisture, solid contaminants and other refrigerants.

Carbon buildup from brazing is a solid contaminant, not a non-condensable. Moisture in the system is moisture in the system, not a non-condensable. A high glide refrigerant blend (such as R-407c) charged in a vapor instead of liquid is a fractionated charge…. not non-condensables

I think you get the point.

When we use a term like “non-condensable” as a replacement for “anything weird going on in the system we can’t explain” then it becomes a useless phrase, like saying a compressor is “bad” rather than explaining the actual fault.


Before we jump into the stuff that will make folks angry, let’s start with some common ground.

Can we agree that the desired result of education in the trades is –

Knowing what you are doing and doing it as safely, efficiently and correctly as possible

 If we can agree that we all have this common goal in mind, can we also agree that any way we can achieve this result in a faster, broader and more effective way would be a good thing?


Now what follows is admittedly one perspective on how we can better achieve these outcomes. This isn’t scientifically quantified, it certainly contains some confirmation bias, but I can state with all honesty that it comes from a desire to help the trades achieve these goals.


10 years ago when techs first started putting HVAC/R videos on YouTube there was a huge backlash. For any of you that were on HVAC-Talk back then, you remember all of the doom and gloom.

Homeowners were going to use the info and kill themselves, bad practices were going to take over the trades, guys were going to go to “YouTube University” and think they know it all.

A decade has passed and those prophecies just haven’t come to pass at any significant scale.

The reason for this (in my mind) is the people who actively seek answers to questions are far better off than those who simply swallow what they are told by their teacher or the old timer who trained them.

Out in the light of day ideas have a chance to either thrive or die on their own merit rather than festering in the cold damp corners of “that’s the way I was taught” or “it always worked for me”.

Sure… there have been some bad actors teaching some silly and dangerous stuff along the way, but there have also been some excellent resources that have started discussions and brought ideas to the forefront that could have NEVER spread so quickly without the free sharing of ideas.

“I have never let my schooling interfere with my education” ~ Mark Twain

Guess where some of the bad ideas that have persisted for generations came from before the YouTube and social media era?

In my experience, it was bad teachers and bad “senior” techs sharing poorly formulated ideas under the protection of intellectual isolation.

In other words, bad ideas formed and grew due to lack of scrutiny, or “peer review” if you prefer an academic term.

What are the gates and who are the Gatekeepers? 

They can be trade schools, manufacturers, traditional book publishers, universities, governing bodies, regulators, educators and the list goes on and on…

Anyone who intentionally places barriers in front of education is part of the problem in my worldview.

What I’m NOT saying –

  • Education should all be free
  • Formal education is worthless
  • The system is the problem
  • Poorly prepared workers should be thrown into the workplace

What I AM saying

  • Learning and progress should be heralded over certifications and degrees
  • What you know and can do is more important than how long you’ve been doing it
  • A lot of time and effort is wasted in bureaucracy and red tape rather than actually reinforcing learning and a passion for learning
  • Self-education is a lifelong skill that should be fostered at every opportunity

“Education is the kindling of a flame, not the filling of a vessel.” – Socrates

Self Education is Worth Promoting

Whether or not you are formally educated, self-education is paramount to success.

About a year ago I received an application for a service tech apprentice position with the following listed under the previous education field.

Self Study: EPA 608, R-410a Certification, PM Tech Certification, Refrigeration, and Air Conditioning Technology, Commercial Refrigeration: For Air Conditioning Technicians, Blue Collar Roots Network podcasts

When he came in for an interview he was polite and quiet, I asked him how he got the certifications if he didn’t go to a formal trade school, he replied: “I just found where I had to go and went out and got them”.

Do you think he ended up working out well? OF COURSE, HE DID!

He’s a self-starter, he doesn’t need a gatekeeper to tell him when or how to learn something he just went out and learned until he understood.

Does that mean we threw him in a truck right away? NO WAY! You can’t learn to ride a bike at a seminar and you can’t teach someone how to be an HVAC/R tech with a book, podcast or video.

He had to practice and apply what he had learned before the learning could manifest itself into skills but he came to the table with the proper mindset which led to the inevitable result of skill and mastery.

The “CYA” or Lawyer excuse

I sent out an email not long ago to a well known OEM seeking approval to use small portions of their bulletin content (with attribution) for some tech tips. Last I heard their lawyers were looking into it.

We get this a lot in the education side of the trade, a fear of “plagurizing” or saying the wrong thing so someone gets sued and then out of the OTHER side of their mouths comes complaints about the “skills gap” and difficulties in education.

I have a piece of advice on the lawyer and copyright stuff surrounding trade education…


Obviously, if someone is directly copying or republishing your content as their own then that’s a problem and needs to be dealt with. Other than that, WHAT IS THE PROBLEM!

If people are sharing excerpts from your manual or book or bulletin online, do you REALLY think that’s a risk to your brand or business?

Do you honestly believe that people who are excited enough about the trade to share or excerpt from something you made are a problem?

Are you HONESTLY concerned that overeducation of the general consumer is a valid problem to protect against in comparison with the growing skills gap in our trade?

Do you think that good quality traditional HVAC/R education is at risk of being replaced by people online sharing good training materials?

“Risk comes from not knowing what you’re doing” ~ Warren Buffet

Risk Aversion

I have 10 kids and only one broken bone among them over 17 years (by the grace of God).

My kids hang from trees, ride bikes around the yard and on the driveway (with no helmet at times), ride our gas powered golf cart (too fast at times) and work with tools on all sorts of things. They cut veggies with knives for dinner, climb ladders to the attic, walk on trusses (the older ones) and ride skateboards with no kneepads.

Does this upbringing sound familiar to you? It probably does because that is the way that many of us were raised and it was certainly the way the generation that went to the moon strapped to a rocket were raised.

The point is that we all learn how to do fairly risky things SAFELY by being allowed to do them in reasonable low risk environments.

But HEAVEN FORBID we allow a 16 year old to job shadow or climb a ladder or use a saw.

How did we get to be so risk averse, especially in a trade where we melt metal with fire, run explosive gasses into buildings and set it on fire, freeze things and make sparks regularly.

If we didn’t want to take risks we should have become a hotel concierge, not an HVAC/R professional.

Now there is no reason to be foolish and we should look for ways to do things as safely as practically possible… but, COME ON FOLKS! Let’s not kill training and education before it can begin by running everything through the lawyers. We are the experts, let peer review and some common sense solve the unwise risks associated with the trade, not a bunch of legal jargon and red tape.

It’s human nature that once we have a good thing going it’s easy to get comfortable and stop taking risks. I get it, but we can no longer rely on the certificates, degrees and processes of yesteryear to solve the staffing problems at our doorstep. We need to actively recruit, share, train, communicate and collaborate from contractors, schools, publishers, OEMs, reps, trade publications and industry bodies.

We need to try new things, be open to taking risks and stop defending our little piece of turf.

“I cannot teach anybody anything, I can only make them think.” ~ Socrates


Do you want the trade to get better? Is it your goal to see techs progress more quickly? Make a real difference?

Tear down the gates and focus on inspiring the spark of continuous learning in the trades 

That’s what’s on my mind today.

— Bryan




Humidifiers are a big part of HVAC systems in dry locations, especially in the winter. I have no experience with them personally because they aren’t prevalent at all in Florida.

I asked some of the HVAC School contributors to weigh in with some of their top humidifier tips – Thanks to Nathan Perney, Steve Domansky and Allen Pavolko for weighing in.

First, Nathan Perney gives us a detailed look at humidifier selection and sizing – 

We do a LOT of humidifiers here in dry old Denver.

My take on selection, like most things I do, is a little different than most.

If we look at section 27 in the eighth edition of Manual J there is a great methodology for determining winter humidification load.

It’s pretty straight forward.

First and for most get a good estimation of wintertime infiltration and exfiltration. That’s right! set up the blower door.   This step rarely happens.

Next, determine your target indoor humidity.  That part is a little tricky.  Manual J shows some calculations for determining winter humidity that will keep the building safe.  Safe means dry.  Dry means no hidden condensation.  Page 139 in the Builder Guide to Cold Climate Construction shows the same calculation.  I like Lstiburek’s approach over Rutkowski with regard to the humidity target. It’s the same math, I’ve just been a disciple of Lstiburek for longer.  Haha.

Once you get the target humitity, convert you CFM of air leakage to a mass air flow of air leakage.  Lets say we have a house leaking 100cfm at my air density of .0613 lb per cu/ft that would be 6.13 lbs of air per minute.

At this point get out your psycrometric chart.  ( Here is the one I use often, )

Now convert you mass air leakage to grains of air leakage.    6.13lb/min of air leakage at 50%rh at 70*f=66.6grains per minute x 60 minutesx 24 hours=95,904 grains per day. WOW! We know there are 7,000 grains per pound.  That give us 13.7 pounds of water a day, or about 1.7 gallons per day, or .07gallons per hour.

Alright! Now we can take the gallons per hour and look at the humidifier specifications.

Based on the Aprilaire data we might want the 400, 600, or 700 series.

The manufacturer alludes to this calculation in the performance information.

The really interesting thing is the air leakage component.  Older leaky houses need WAY more humidity that new tight houses. Just like the how the warmth from our heating systems leak out before can feel it.  In old leaky buildings, the humidity leaks out before you can sense it.  How ever in newer tighter houses we can have problems.  In fact, I have seen several new tight houses over humidifying and having serious condensation issues.  Water is amazing, it moves mountains. And it destroys buildings better than pretty much any thing aside from explosives. The good thing is cold air is dry air, so drying building in the winter is much more realistic than during the summer.

Wait, what did I just say drying a building during the winter. You better believe it.  Depending on what you do in your home,  how tight your home is, and what the building assemblies are comprised of,  you may well need to dry the building during the winter.  Just ask the Canadians.   As building codes are pushing building tightness tighter and tighter( this is a good thing) we will see more over humidification issues.   Over humidification of new buildings in cold climates is a very serious issue we will begin seeing more and more.

The biggest install failure is not getting the drainage correct.  Test the drains, or your asking for problems.  Test the operation, or your asking for problems.  Make sure they turn on and off when you ask them to.  I had a job recently where a stem humidifier was wired to create steam any time the unit had power.  The installer wired the relay incorrectly and we had 75*f 70% humidity in the house.  The windows were raining.  It was awesome.  Like anything read the dang manual.  Steam humidifiers are very temperamental, if you dont follow the manual you will have problems.  Steam humidifiers have very specific tolerances for where and how they can and can not be installed.  One thing to be vary of is you are setting up a steam humidifier to operate with constant fan.  Check the velocity of the air during fan only mode.  If the air is not moving fast enough you will get condensation in the duct work (this is bad).  Another common  install  error is the HUM terminal on the control board may 1200r 24 VAC.  All residential humidifiers that I  know of require 24VAC to control the unit.  If You don’t install a isolation relay you will fry the solenoid valve.

— Nathan Perney

Allen Pavolko shared his perspective from the Eastern US

Proper Selection

Choosing the right humidifier for your home depends on multiple things. These include, but are not limited to: size of the home, heating system, ductwork availability, and geographic location. Where I live (Southwestern NY, near Buffalo), most homes that have a forced air furnace have a bypass humidifier installed to help keep the relative humidity in the living space during those dry winter days. Due to cold air not being able to hold as much moisture, we tend to lose humidity in the winter time. This can lead to higher counts of airborne viruses staying alive in the air, static electricity build up, drier skin, improperly sealed/treated wood cracking, etc. Keeping the humidity between 30% and 50% in the living space year round can help battle all of those things.


The biggest thing to choosing a humidifier, in my opinion, is what ductwork you have available to attach a system to. If you have enough room to install a bypass humidifier, you can treat up to a 4,000 sq ft house with one, but you need ample room to mount the humidifier and pipe it to the other airstream. If you don’t have the room to do that, or have a bigger house, you can do a power humidifier. These have a fan built into them to push the air through the media and can treat up to 4,200 sq ft. The last option for ducted systems is a steam humidifier, which can be remote installed and piped into the airstream. These systems can treat up to 6,200 sq ft of living space and take significantly less space to install, and can be remote as I stated prior.

Note: Obviously Coverage size is specific to the humidity needs and design of the home as Nathan pointed out 

If there is no ducted system in the space, then you can get a non-ducted system which just adds water vapor to air and lets natural convection currents take over to distribute. Or you can use a steam humidifier paired with a fan pack to distribute the water vapor to a central location.

How to Install 

Best Scenario is to install the humidifier unit on the supply air plenum with it piped over to the return air drop. Our local rep for a well-known humidifier manufacturer advised to use hot water for the feed tube as often as possible, as it helps the water evaporate faster. Always utilize a flow restrictor on the water supply, as to not add way too much water to the evaporation pad. These things together make for the best, in my opinion, way to get humidified air into the living space. The hottest air passes over already warm water on the pad and evaporates as quickly as allowed. This goes into the airstream and passes back through to the house. If possible, install an outdoor air temperature sensor to automatically control humidity levels in the home.


Common Mistakes


The most common mistake I have seen in the field is failing to make sure the humidifier is level and plumb. This can cause the unit to leak!

Another common mistake is removing the fabric out of the distribution tray because it looks and feels gritty. This is there for a reason, and that is to allow the water to flow through each distribution channel evenly.


Common Service Failures 


I have often seen a solenoid valve misdiagnosed as being bad, due to water not flowing through it. I live in an area with high mineral content in the water, this, coupled with the small diameter water piping and small orifice, often leads to a plugged up orifice/flow restrictor.

— Allen Pavolko

Finally, Steve Domansky shared a link with some more info on humidifiers HERE

First, let’s cover the basics. X13 is a brand name for the Regal Beloit / Genteq brand of constant torque motors, there are other manufacturers who make them, but the term “X13” has become pretty much synonymous for the fractional horsepower HVAC constant torque motor.

Also, this article is specifically discussing the common residential/light commercial motors. There are other types of variable and constant torque motors and equipment not being addressed here.

Both variable speed and X13 motors are ECM or “Electronically Commutated Motors,” This means the DC power that drives them is electronically switched from positive to negative to spin the motor. Both are more efficient than the typical PSC motor with ECM motors commonly being about 80% efficient and PSC being about 60%.

Both X13 and variable speed motors are DC (or alternating DC if you prefer), 3 phase, permanent magnet rotor motors that use back EMF to determine motor torque and adjust to load conditions.

The primary difference is the type of inputs to the motor control. A variable speed motor is programmed for a specific piece of equipment to produce a set amount of airflow based on the particular static pressure profile of that system as well as based on the inputs from the air handler circuit board or system controller. In other words, a variable speed motor can ramp up down based on the static pressure as well as the staging of the equipment, pin/dip switch or controller settings for desired airflow output and “comfort profiles” that can be set up to allow the blower to ramp up or down for enhanced dehumidification and comfort.

An X13 motor is programmed to produce a set motor torque based on which input it is receiving 24v. This means that while an X13 motor is more efficient than a PSC motor and does a better job of ramping up to overcome static pressure increase it does not have the level of control that a variable speed has and it also does not produce an exact airflow output across the full range of static pressure.

This is why when you check the blower charts on a unit with a variable speed motor the CFM will remain the same over a wide range of static points, but when you look at an X13 system, the CFM will drop as the static pressure increases.

— Bryan

Let’s Start with the basics.

Water freezes at 32° Fahrenheit and 0° Celsius at sea level and atmospheric pressure. When any surface is below that temperature, and the air around it contains moisture, ice/frost will begin to form. In some situations, ice is to be expected, such as in refrigeration evaporators and exposed portions of the refrigeration suction line and the metering device outlet in refrigeration applications. Frost and freezing are also likely in heat pumps operating in heat mode. The outdoor coil on a heat pump becomes the evaporator coil, and during low outdoor ambient conditions, it is expected that the outdoor coil will eventually freeze and require defrost.

The reason that frost/ice is inevitable in some applications is just due to the laws of thermodynamics (moving heat). To get heat from one place to another you need to have a difference in temperature. So inside of a freezer where you hope to get the box temperature to 0°F  the coil temperature needs to be BELOW 0° to transfer heat out of the freezer and into the coil and then down the suction line to the compressor. If you have a freezer with a design coil temperature difference of 10° and a design superheat of 8° the coil will be at -10°F and the suction temperature at the evaporator outlet will be -2°F.

On a heat pump running in heat mode, you will commonly find an evaporator (outdoor coil) that runs 20° – 30° colder than the outdoor temperature. This means that if it is 30° outside the outdoor coil temperature could easily be 0°F. In these cases, ice is normal and periodic defrost is expected and required.

Some systems we work on and install will freeze. Air conditioning is not one of them.

In an air conditioning system, we must keep the evaporator temperature above 32°F. We can easily know the evaporator temperature by looking at our suction saturation temperature (suction gauge temperature for the particular refrigerant). For R22 32° is 57.73 PSIG at sea level, R410a is 101.58 PSIG, and R407C is 67.80 PSIG.  If we don’t keep our evaporator coils above these coil pressures/temperatures, the system will freeze. The rate at which it will freeze is a function of –

  • Time – The longer it runs at or below 32°, the more frost/ice will build
  • Moisture – The more humidity the air contains as it passes over the coil
  • Temperature Difference – The colder the coil, the faster ice will build
  • Air Velocity / Dwell Time – The faster the air moves over the coil, the slower ice will build, the slower it moves, the quicker it will build
  • Coil Design – Closer fins will freeze faster

The ice buildup always starts in the evaporator and works it’s way outside. If you have a frozen compressor, you have a frozen evaporator. When you find a frozen system, take your time and get it fully defrosted. Take care to manage the ice melt water and keep it away from motors and boards where it can cause damage and a shock hazard. Some towels and a shopvac are great to have handy when defrosting a unit. When possible allow it to defrost slowly and naturally to prevent damage.

So what circumstances can result in low coil temperature?

Low Evaporator Load 

Low load is often equated with low airflow… and it usually is low airflow, but there is a bit more to the story than that.

An air conditioning system has one final design result, one big end goal that we are shooting for. Matching the refrigeration effect to the evaporator load.

We must match the quantity of refrigerant moving through the evaporator coil to the amount of heat the evaporator coil is absorbing

That is our mission, and that is the primary reason we measure superheat. Superheat gives us a look at how well we are matching refrigerant flow to heat load. High superheat means underfeeding; low superheat means overfeeding.

There is an issue though, we could have a correct superheat and still have a coil temperature of under 32°, and this is not acceptable in an air conditioning system. When the coil absorbs less heat than designed the coil temperature and suction pressure drop. In cases where a TXV or EEV is controlling suction superheat the suction pressure will drop even further as the valve attempts to keep the superheat from plummeting.

This is why we must size a system, and it’s ductwork appropriately for one another as well as for the space, climate, and even altitude. If we install a system that requires 1200 CFM of airflow to properly balance the refrigeration effect to the load at 75° design indoor temperature and that system is only receiving 900 CFM of airflow, you run an excellent likelihood of freezing. This is especially true when the outdoor temperature drops or the customer decides to drop the thermostat lower than usual.

Low load is often due to low airflow, low indoor ambient conditions and equipment oversizing. Low load conditions will have symptoms of low suction pressure, low superheat, low head and high evaporator Delta T. Start by looking for the obvious, dirty coils, dirty filters, dirty blower wheels, blocked returns, mismatched equipment, improper blower settings, closed registers and undersized ducts. You can then move on to performing static pressure tests to locate more difficult issues.

Low load is the most common cause of persistent freezing and should be top of mind when a technician is diagnosing a freezing system

Low Refrigerant In the Evaporator

System undercharge or underfeeding due to restricted refrigerant flow (restricted filter driers, plugged screens, failed expansion valves or undersized pistons) can also result in freezing over time. Low refrigerant can result in fewer molecules of refrigerant in the evaporator coil which results in lower coil pressure because the coil contains both saturated liquid at the beginning of the coil as well as superheated vapor towards the end. This type of freezing requires time because less refrigerant in the coil equals less refrigeration (cooling) effect.

If the coil temperature is below 32° in an undercharged situation, the coil will simultaneously build frost as the beginning of the coil after the metering device AND underfeed the coil resulting in high superheat. Over time as the frost builds it will start to block the opening of the coil which blocks airflow and insulates that portion of the coil from airflow which reduces the coil load. Eventually, once the coil is blocked with frost almost all of the load is removed from the coil and you have a low refrigerant issue that LED to a low load issue that resulted in a complete freeze up.

Once you defrost the system and test you will find that low refrigerant charge conditions result in low suction, low subcool, high superheat and low head pressure. Refrigerant restrictions will be low suction; high superheat, high subcooling.

Often once you resolve the charge issue, you may also find another low load issue as well that contributed to the freezing. In many cases when low charge is the cause, the customer will notice the issue before the system is FROZEN SOLID.

Low refrigerant will often result in a partially frozen coil more than a full block of ice. Remember, low COIL refrigerant can be restriction related or low charge, but if it’s low charge you will have low subcooling if restriction it will have high subcooling.

Low Outdoor Ambient

When a cooling system is operated during low outdoor temperatures the condensing temperature and head pressure will drop. If the head pressure drops low enough the suction pressure will also drop resulting in freezing. The only way to resolve this cause is to install some type of head pressure control such as fan cycling or fan speed control to keep the head pressure from dropping significantly.

Blower Issues

If the indoor blower shuts off, the coil temperature will drop. Sometimes a blower motor will have internal issues or controls issues that cause it to shut off periodically. This can cause intermittent freezing that can be hard to diagnose. Checking controls, belts, blower amperage, bearings, and motor temperature can all help in diagnosing these issues. Sometimes leaving an amperage data logger on the motor along with a coil or supply air temperature sensor can give you the ammunition you need to pinpoint an intermittent issue.

When diagnosing a freezing situation don’t jump to conclusions, get all the ice defrosted before making a diagnosis and keep a sharp eye out for airflow and design issues. Freezing is often due to more than one issue combined that act to turn your customer’s air conditioner into an ice machine.

— Bryan


This is an internal guide we use at Kalos, and it works for our climate and the type of HVAC equipment we work on. Consult with your company leadership before implementing this or any process. Keep in mind that some of these guidelines are “made up” by me and are only useful in the absence of manufacturers data. The non-invasive test mode in the MeasureQuick app is a better method for testing many of these parameters.

Fieldpiece JobLink Rapid Rail Temperature Clamp


Checking The System (and Charge) Without Gauges


First, checking the charge without gauges is a balancing act, a trade-off. We get more accurate readings when we connect gauges but we also –


  1. Lose some refrigerant
  2. Risk contaminating the system with moisture and air
  3. Risk leaving a leak at the Schrader cores and caps


Like everything, your best INITIAL diagnosis tools are your hands, eyes, and ears. Look for dirt buildup, spot oil, listen for abnormal sounds, feel the lines and condenser discharge air when approaching the condenser, check for dirty blower wheels, evaporators, filters, and grilles when approaching the indoor unit. Look for wire and refrigerant tube rub-outs, look inside drain cleanout tees and in pans for gunk and buildup, look inside condensers for wires laying on the tubing, pay attention to disconnects that are loose, Belts and sheaves that are worn, high voltage connections that are getting discolored, capacitors that are bulging or leaking, electrical whips that are coming apart, stat wires that are nicked or bare, air handlers that are sagging or out of level, ducts full of mildew and broken or damaged line insulation. In refrigeration look for icicles hanging down, torn insulation on drains and suction lines, dirty EVERYTHING and damaged doors and door seals.



This means that we only connect gauges when there is a good reason to do so, such as –

  1. We have not touched the unit recently and want to make sure it is operating 100% (on air conditioning only, in small refrigeration you still don’t connect in this case)
  2. We made a significant repair that may impact the operation
  3. We need to “set” a charge because the system is newly started or we made a refrigerant circuit repair.
  4. Your readings or your gut tells you are out of range or a problem may exist.


On a system that has been appropriately commissioned you will have prior readings to go off of. Keep in mind that some benchmarks like DTD (Evaporator to return air design temperature difference), CTOA (Condensing Temperature Over Ambient), Subcool and Superheat on a TXV system and Static Pressures.


Readings like suction pressure, head pressure and superheat and subcooling on a fixed metering device system and air temperature split will vary with load conditions. If you have system historical data, you can often use it to learn about the system and its history before you begin taking readings.


When checking an air conditioning system without gauges do it in the following steps (these are subject to change and adjustment based on historical benchmarks, abnormal conditions, and manufacturer specs)  –


  1. Visually inspect the unit for all the above-listed items AND note if the metering device is a piston or a TXV
  2. Measure the outdoor temperature in the shade entering the condenser. This procedure will work best during outdoor temperatures of 70°F – 95°F
  3. ADD the CTOA (Condensing Temperature over ambient) based on the SEER rating and/or age  
  4. Subtract the nameplate subcooling or 10° if there is no nameplate
  5. Compare to the liquid line temperature. If +/- 3° on a TXV system or +/- 5° on a Piston the liquid temperature is in range
  6. You may also check the air temperature leaving the condenser fan, and it will usually be about ½ of the target CTOA (Condensing Temperature Over Ambient). So on an ancient system with a CTOA of 30°F the condenser discharge air will generally be 15°F +/- 3°F, and on a brand new high SEER unit with a CTOA of 15°F it will be 7.5°F +/- 3°F
  7. Also, note how much warmer the liquid line is than the outdoor temperature. It should be between 4° and 18° warmer than the outdoor temperature. If it is above or below that range, connect gauges.
  8. Measure the suction line temperature outside. If it is at or above 65° the compressor is in danger of overheating / oil breakdown. If the suction line is 40° or below the unit is at risk of freezing. Stop and connect gauges.
  9. Go inside and check the wet bulb and dry bulb temperature at the air handler/furnace inlet (return right before the inside unit or in the filter tray, cabinet make sure to keep the sensor out of “line of sight” from the evap coil) Indoor temperature should be between 70°DB and 80°DB for the best use of this method
  10. Take the return dry bulb (DB) and subtract 35°F (DTD), this is your target coil temperature difference.  
  11. If the system has a TXV add in 10° for superheat, if it is a fixed orifice (piston), then add in the target superheat based on a superheat chart or using the HVAC School app. This gives you a target suction line temperature at the evaporator.
  12. Compare the target suction line temperature to the actual suction line temperature at the evaporator is it is within +/- 5°F it is within range. Outside of that range connect gauges.
  13. Compare the indoor suction temp to the outdoor suction temp. 1°F of change per 20’ of lineset is allowable.
  14. Compare the indoor liquid line temp to the outdoor liquid line temp. 1°F change per 30’ of lineset is allowable.
  15. Check temp drop across all exposed line filter driers. Recommend replacement if there is a drop of 3°F or more across a filter drier and perform further testing if you get even 1°F of reduction with the same, accurate thermistor clamp.
  16. Use a Delta T chart to calculate target evaporator air temperature split like this one if the split is within +/- 3°F then it is within range. If higher then check for airflow issues and blower settings. If lower then connect gauges.
  17. When checking an RTU (Rooftop Unit) or residential package unit, you will often have easy access to the compressor, in this case, check the suction temp entering the compressor and the discharge temp leaving the compressor. The suction temperature should be above 35°F and below 65°F entering the compressor (Depending on indoor conditions) and the discharge line temperature should be below 220°F and above 150°F on a properly functioning RTU during typical indoor and outdoor conditions. NOTE: on an RTU make sure you are not attempting to measure liquid line temperature / CTOA rules when connecting to the DISCHARGE line. Also make sure that panels are in place for the condenser, blower and evaporator sections when run testing. When there is something that looks like a liquid line drier, but it is in the discharge line it is a muffler, not a filter/drier
  18. Check amps against manufacturer rating plates or part data plates if the compressor, blower or condensing fan motor are aftermarket
  19. Check capacitors, preferably while running
  20. Check the incoming voltage to the contactor and ensure it is within 5% of the rated voltage. In general, this means ensuring that voltage is over 198V from leg to leg on a 208V System and over 228V on a 240V system. This is based on the NEC 215-2(d) suggested guideline, not manufacturer specs, so it isn’t set in stone.
  21. Confirm that the voltage imbalance on a 3 phase system does not exceed 2%
  22. When applicable check TESP and Static Pressure Drop across coil and filters against benchmarks
  23. Confirm drainage/test and inspect float protection devices  


BEWARE of these common readings mistakes


  • Reading air temperatures in sunlight. If the sun is shining on a probe, it will always read too high
  • Reading air temperature in a place that is “line of sight” to a cold or hot surface like a coil, heat strips, heat exchanger, etc…  It is always best to have a probe in an area shielded from other hot or cold surfaces  
  • Reading line temps in an uneven or dirty area of the tubing. The sensor on your temp clamp must have full, flat, clean, tight contact to the line being measured
  • Trusting tools without testing tools. All tools require proper care and maintenance and must and can be tested. They can either be tested against other tools or a known constant (like the freezing temp of water), or they can be calibrated by a lab. Know your tools and learn how to test them.
  • Taking pressure readings without a fully depressed Schrader core. When checking refrigerant pressures or measuring vacuum with a micron gauge, the cores must be fully depressed (pushed in). If your hoses or couplers are not FULLY depressing the cores, you will see odd readings. When in doubt replace the Schrader with a core tool and try another hose.


This process is not theory or a diagnosis guideline; it is simply a practical process for verifying PROPER operation for a range of common air conditioning equipment. If you find readings that are outside of the guidelines listed, you will need to connect gauges and further diagnose the system. Before using this guideline, it is highly recommended that you read and understand the following training modules –


The Basic Refrigerant Circuit

Common refrigerant Circuit Terms

The 5 Pillars of Refrigerant System Diagnosis

Checking a Charge W/O Gauges Article by Jim Bergmann

Checking a Charge W/O Gauges Parts one and two

Charging an Air Conditioner by TruTech Tools

The Case for Checking The Charge Without Gauges

Air Conditioning Diagnosis Guide



There was a question in the Facebook group a few days ago about averaging sensors. There are two common configurations/methods used for averaging. The first is simply a setting in a thermostat or control where it reads separate sensors and then the thermostat itself averages out the readings using software.

For example, if the onboard sensor is being averaged with a remote sensor it could look like this:

Onboard Sensor = 78°
Remote Sensor = 82°

78° + 82° = 160

160 ÷ 2 = 80°

So the average temperature is 80° between the onboard sensor and the remote sensor. This could be handy if the remote sensor is in one room with a different solar or equipment load that the other but there is no automatic damper to separate the zones.

The other strategy is to simply wire sensors as averaging which has nothing to do with the thermostat or control and everything to do with Ohms law and the nature of parallel and series circuits.

A thermistor (temperature sensor) is a type of resistor that changes resistance based on temperature. There are many different types of thermistors but for this strategy to work, they all need to have EXACTLY the same thermistor properties.

You probably already know that when you connect resistors together in SERIES (Out of one into the next) that the resistance increases. So if you connect a 5,000-ohm resistor in series with another 5,000-ohm resistor they would have a resistance of 10,000 ohms.

What you may not know is that when you connect two resistors in PARALLEL you give the electrical current two paths which decreases the resistance. In fact, if you connect two 5,000 ohm resistors in parallel the total resistance would be half or 2,500 ohms.

This property of ohms law and parallel/series circuits means that we can easily average out thermistor temperatures so long as they are all the same and all the connections are good and we don’t have runs that are too long, as this will add in resistance and throw off the readings.

Take a look at the image at the top.

All you need to do is have the same # of sensors in parallel that you have in series and ohms law does the work. We don’t need to have the thermostat do the math because the series sensors add together and the parallel sensors divide.

This means you can have a few as 4 averaging sensors to as many as you want so long as there are the same # of series and parallel sensors. This means that the total # of sensors will always be a square of a whole #.

2×2 = 4
4×4 = 16
5×5 = 25

So on and so forth…

This can come in handy when conditioning a large room with a single zone but it is also somewhat troublesome because if any sensor fails the thermostat or control will read incorrectly.

— Bryan



We’ve been pretty spoiled in residential and light commercial in the USA because we haven’t needed to deal with glide much. R22 has no glide and R410a is a near-azeotropic blend which means it has almost no glide.

The days of being able to ignore glide are coming to an end.

Carrier has announced their replacement for R-410a will be R-454b which they will call “Puron Advanced” which still has very little glide (only 0.2°F), but many of the other options (like R-407c shown above) have a rather severe glide.

Glide comes down to the fact that some blended refrigerants boil and condense over a range of temperatures rather than at a single pressure/temperature point.

The point at which it is fully liquid before subcooling (or the point of the very first bubble in the liquid) we call bubble point and we use the bubble point to calculate subcooling.

The point when the mixture becomes fully vapor before superheating (or the first drop of liquid dew in a vapor) we call the dew point and we use it for calculating superheat.

Zeotropic blends (blends with glide) have several impacts on the system, but the one we notice most is in the evaporator. When blend with glide enters the evaporator coil, it will start by boiling at a lower temperature, and as it moves through the coil, the refrigerant temperature will increase until it hits the dew point before it starts to superheat. This means that neither the dew or the bubble temperature is REALLY the evaporator temperature, the true effective evaporator temperature is somewhere in the middle, we call this the mid-point.

Because some of the refrigerant flashes off right at the start of the evaporator the effective midpoint isn’t really the middle between the dew and bubble, it tilts more towards the dew and Emerson recommends a more accurate estimate would account for that “inlet quality.” So merely multiply bubble by 0.40, dew by
0.60 and add the two together to get a more accurate evaporator midpoint.

But let’s say you connect to a system that is off or connect gauges to a tank and want to know for sure that that refrigerant you think is in the tank or system is what you think it is?

Do you use bubble, dew or mid-point for static pressure?

The answer is you use bubble. Now I’ve not had anyone fully explain why to me but it stands to reason in my head that in the static state the majority of the refrigerant mass in the system (or tank) is in the liquid state and since it is neither in the process of boiling or condensing then it would be at the bubble point. That’s probably a very unscientific way of thinking about it, but it’s what I’ve got for now.

— Bryan

P.S. – Totally unrelated but my friend Andy Holt is putting on a Soft Skills training “camp out” seminar in Orlando starting on 4/1/19, and I will be stopping by to do some technical training as well. Follow THIS LINK to learn more.


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