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.



This article was written by HVAC / Furnace technician Benoît Mongeau. Thank you Ben.



High efficiency (or 90%, or condensing) furnaces use a set of two heat exchangers in order to retrieve more heat from the combustion products than their mid-efficiency counterparts.  Because of this, they generate flue gases much colder than those of a mid-efficiency or natural draft unit.  This not only completely changes the way the furnace has to be vented (I will talk about venting specifically in a later tip) but also, and it’s what we’ll focus on, a lot of condensates is generated.  This water comes from two sources:  moisture which was already present in the combustion air, and the combustion process itself, as the hydrogen atoms from the natural gas molecules (methane, CH4) combine with oxygen to form water. Now as technicians you don’t need to know this part but if you’re a bit into chemistry, here’s the basic chemical equation:


CH4 + 2 O2 + heat = CO2 + 2 H2O


This means that in perfect combustion, for every molecule of CO2 you produce, there are also 2 water molecules produced. This adds up to a lot of water vapor.


In order for the furnace to work properly, that condensation needs to be drained out or else it would accumulate inside the heat exchanger, inducer and venting, impeding proper gas/combustion product flow.  Most furnaces will have at least 2 internal drains, typically one for the heat exchanger and one for the vent, usually at the inducer outlet or on the inducer housing.


The secondary heat exchanger outlet is sealed inside a plastic part called the collector box, which is designed to collect the condensate and drain it out.


All condensate drains go into a trap.  The condensate trap is absolutely mandatory for a high-efficiency gas furnace.  Since the drain taps into the exhaust system, leaving it open to the air would allow for a potential exhaust/flue gas leak in the living space, which is a big no-no.  Additionally, the inducer motor would suck air through the drain if it weren’t trapped, which could affect combustion, and would prevent proper drainage.  Keep that in mind, because if you ever add an extra drain (off a tee on the venting, for example), you will need to TRAP it, always.


The only downside to the trap is potential for blockage.  The trap needs to be cleaned out regularly, and that should be done every maintenance.  Rinse it out, make sure water flows through the trap properly from all its ports.  If there’s any poor flow, fill it up and blow through it a few times to get the dirt out.  Hotter water helps for stubborn blockages.  The need for regular cleaning also means that drains should be installed as much as possible in a way that allows for the trap to be easily removed.  I highly recommend using clamped flexible hoses for the drain, as close as possible to the trap.  Avoid hard-piping the whole drain, as it will be impossible to remove and clean out the trap.


To ensure proper drainage, here are the proper practices:

-Make sure every component that produces condensate is sloped towards the drain.  That means slope the venting down towards the furnace (typically a ¼’’ slope per foot of length, minimum), and also, slope the furnace itself!  Look in your install manual, most manufacturers will call for the furnace to be installed with a slight forward pitch to allow condensate to drain from the heat exchanger.

-Slope the drain line itself, obviously.  Avoid double trapping and vent the drain after the trap to prevent airlocks

-Avoid running the drain in an area where it could freeze.  That includes running it under the natural fresh air inlet if there is one.  


Finally, note that furnace condensate is acidic, and some states/provinces/countries may require the condensate to be neutralized prior to draining.

— Ben


Whenever there is a conversation where “code” is involved, it’s important to mention that codes can vary depending on the AHJ or authority having jurisdiction. It’s becoming more common that governments lean heavily on the ICC (International Code Council) and in the case of HVAC/R that is the IMC (International mechanical code) and in the case of fire protection and electrical codes the NFPA (National Fire Protection Association) has become the authority for codes and standards in the US.

So what is the purpose of duct smoke detectors?

NFPA 90A, 2012, A.6.4 makes this pretty clear by stating

“Protection provided by the installation of smoke detectors and other related requirements is intended to prevent the distribution of smoke through the supply air duct system and, preferably, to exhaust a significant quantity of smoke to the outside. Neither function, however, will guarantee either early detection of fire or the detection of smoke concentrations before dangerous smoke conditions if the smoke movement is other than through the supply air system.”

In other words, duct smoke detectors are there to keep units from circulating smoke in the space and when possible to send it outside. They aren’t there as a replacement for space smoke detectors.

When do they need to be installed? 

Both NFPA 90 and IMC 606.2.1 state similar things that can be summarized and paraphrased as “If the duct system is designed for more than 2000 CFM the system must have a duct smoke detector installed” and “If the duct system is designed for  more than 15,000 CFM one in the return and supply is required.”

NFPA 90 states that the smoke detector should be installed in the SUPPLY after 2000 CFM and IMC 606.2.1 says the RETURN. This means that it up to the AHJ to decide which standard they follow.

NFPA 90A also states “where an approved fire alarm system is installed in a building, the duct smoke detectors shall be connected to the fire alarm system.”

Now, these are summaries of more complicated texts with exceptions and lot’s of extras, so if you want to know all the details I would suggest you read the code for yourself but in general –

  • A duct smoke detector should shut off a typical blower and fresh air and turn on the exhaust
  • Duct detectors aren’t a replacement for room sensors
  • If the duct system is designed to carry more than 2000 CFM (5 tons nominal) of air you need one in the return if IMC is being followed and the supply if NFPA is being followed.
  • If the duct system is designed to carry more than 15000 CFM of air, you need one in the return and one in the supply
  • If a central fire monitoring system is in place a duct smoke detector is in use, it must be connected to the fire monitoring system

— Bryan



Take a look at the screenshots above. The one on the left is for Death Valley at 282′ below sea level and the one on the right is Denver, CO at 5,280′ above sea level.

Notice the barometric pressure, they are almost the same

This means that barometric pressure is corrected or “normalized” to sea level so that the weather can easily be compared from one place to another.  In other words, barometric pressure is there for weather forecasting not for calibrating tools or for calculating air density. In fact, If you own a simple dial barometer and you change its elevation you would need to recalibrate it to get it to match the forecast. Take a look at the chart below, you will notice that pressure drops by about 1″ for every 1000′ of elevation change. This is the way altimeters in aircraft work, they really aren’t measuring distance, they are measuring pressure and converting it into altitude.

If it wasn’t for this barometric normalization barometers in Denver would hover around 24″hg rather than the 30″ you will find in the forecast.

Of course, you can correct for this by subtracting from the forecast barometer pressure based on the altitude.

For the Denver reading of 30.24 you would do it like this –

5,280 ÷ 1000 = 5.28″ of mercury pressure below sea level 

30.24 – 5.28 = 24.96″hg (inches of mercury)

If you want to covert that to PSIA you divide by 2.036 to get PSI

24.96 ÷ 2.036 = 12.26 PSIA

Obviously, none of this is a problem when using tools that you simply “zero” to the atmospheric pressure, but in some cases, an instrument or calculation may require you to enter the pressure directly. This is when using barometric pressure can be an issue… unless you live at sea level like I do.

— Bryan


This is a basic overview of the refrigeration circuit and how it works. It isn’t a COMPLETE description by any means, but it is designed to assist a new technician or HVAC/R apprentice in understanding the fundamentals.

First, let’s address some areas of possible confusion 

  1. The Word “Condenser” Can Mean two Different Things Many in the industry will refer to the outside unit on a split air conditioner, heat pump or refrigeration unit as a “condenser” even though it will often contain the condenser, compressor, and other parts. It’s better to call the outside component the “condensing unit” or simply the “outside unit” to reduce confusion.
  2. Cold and Hot are Relative terms Cold and Hot are both an experience, a description, a comparison or an emotion. Cold is a way to describe the absence of heat in the same way that dark describes the absence of light. We will often use the words cold and hot to compare two things “Today is colder than yesterday” or to communicate comfort “It feels hot in here”. These are useful communication tools, but they are comparisons not measurements.
  3. Heat and Absolute Zero Can be Measured We can measure heat in BTUs and light in lumens, we cannot measure cold or dark. Absolute cold is the absence of all heat.  -460°F(-273.3°C) (cold) is known as absolute zero, -460°F(-273.3°C) is the temperature at which all molecular movement stops. Any temperature above that has a measurable level of heat. While this is a known point at which all molecular movement stops, it has not (and likely cannot) be achieved.
  4. Boiling Isn’t Always Hot When we say it’s “boiling outside” we mean it’s hot outside. This is because when we think of boiling we immediately think of water boiling in a pot at 212°F (100°C) at atmospheric pressure, which is 14.7 PSI (Pounds Per Square Inch)(1.01 bar) at sea level. Boiling is actually just a change of state from liquid to vapor, and the temperature that occurs varies greatly based on the substance being boiled and the pressure around the substance. In an air conditioner or a refrigeration system, refrigerant is designed to boil at a low temperature that corresponds to the design of the system. On an average air conditioning system running under normal conditions with a 75°(23.88°C) indoor temperature, the evaporator coil will contain refrigerant boiling at around 40°F(4.44°C). In air conditioning and refrigeration when we refer to “boiling”, “flashing” or “evaporating of refrigerant” we are talking about the process of absorbing heat, otherwise known as cooling.
  5. Cooling and Heating Cannot be “Created” We are not in the business of making heat or creating cool; it cannot be done. We simply move heat from one place to another or change it from one form to another. When we “cool” a room with an air conditioner, we are simply absorbing heat from the air into an evaporator and then moving that heat outside to the condenser where it is “rejected” or moved to the outdoors.
  6. Heat and Temperature Aren’t the Same  Imagine a shot glass of water boiling away at 212°F(100°C). Now imagine an entire lake sitting at 50°F(10°C). Which has a higher (hotter) temperature? That answer is obvious-I just told you the shot glass had 212°F(100°C water in it so it is CLEARLY hotter. But, which contains more heat?  The answer is the lake. You see, heat is simply energy and energy at its basic form is movement. When we measure heat we are measuring molecular movement; the movement of molecules–atoms stuck together to make water or oxygen or nitrogen. When molecules move FASTER they have a HIGHER temperature and when they move SLOWER they have a LOWER temperature. Temperature is the average speed (velocity) of molecules in a substance, while heat is the total amount of molecular movement in a substance. The lake has more heat because the lake has more water (molecules).
  7. Compressing Something Makes it Get Hotter (Rise in Temperature) When you take something and put pressure on it, it will begin to get hotter. As you pack those molecules that make up whatever you are compressing, they get closer together and they start moving faster. If you drop the pressure the molecules will have more space and will move slower causing the temperature to go down.
  8. Changing the State of Matter Moves Heat Without Changing Temperature  When you boil pure water at atmospheric pressure it will always boil at  212°F(100°C). You can add more heat by turning up the burner, but as long as it is changing state (boiling), it will stay at 212°F(100°C). The energy is changing the water from liquid (water) to vapor (steam) and the temperature remains the same. This pressure and temperature at which a substance changes state instead of changing temperature is called its “boiling point”, “condensing temperature” or more generally “saturation” point.
  9. Superheat, Subcool, Boiling, and Saturation Aren’t Complicated  If water is boiling at sea level it will be 212°F(100°C). If water is 211°F(99.44°C) at sea level we know it is fully liquid and it is 1°F(-17.22°C) subcooled. If water is 213°F(100.55°C) at sea level we know it is vapor and superheated. If something is fully liquid it will be subcooled, if it is fully vapor it will superheated, and if it is in the process of change (boiling or condensing) it is at saturation.

Where to start 

Take a look at the diagram at the top of this piece and start at the bottom left. Are you looking at the part at the bottom left? OK, now read this next line OUT LOUD:

Compressor > Discharge line > Condenser > Liquid Line > Metering Device > Expansion Line > Evaporator > Suction line and then back to the Compressor

When I first started in HVAC/R trade school this was the first thing my instructor forced me to LITERALLY memorize forward and backwards before he would allow me to proceed.

While I am not always a huge fan of rote memorization as a learning technique, in this case, I agree with committing this to memory in the proper order.

These four refrigerant components and four lines listed above make up the basic circuit that every compression refrigeration system follows. Many more parts and controls may be added, but these basics are the cornerstone on which everything else you will learn is based. Once you have these memorized we can move on to describing each.


The compressor is the heart of the refrigerant circuit. It is the only mechanical component in a basic refrigeration system. The compressor is like the heart that pumps the blood in the body or like the sun that provides the earth its energy. Without the compressor to move the refrigerant through compression, no work would be done and no heat would be moved.

The compressor creates a pressure differential, resulting in high pressure on the high side (discharge line, condenser & liquid line) and low pressure on the low side (suction line, evaporator and expansion line).

There are many different types of compressors, but you will most likely see Scroll and Reciprocating type compressors most often. A reciprocating type compressor uses pistons, valves, and a crankshaft. Reciprocating compressors operate much like car engines; pulling in suction vapor on the down-stroke and compressing that vapor on the up-stroke. A scroll compressor does not have any up-down motion like a reciprocating compressor. A scroll compressor uses an oscillating motion to compress the low-pressure vapor into high-pressure vapor.

The compressor pressurizes low-pressure vapor into high-pressure vapor, but it also causes the temperature of the gas to increase. As stated in the gas laws, an increase in pressure causes an increase in temperature and a decrease in volume. In the case of refrigerant cooled compressors, heat is also added to the refrigerant off of the kinetic (bearings, valves, pistons) and electrical (motor windings) mechanisms of the compressor. Compressors require lubrication; this is accomplished through oil that is in the compressor crankcase, as well as oil that is carried with the refrigerant. Liquid entering the compressor through the suction line is a very serious problem. It can cause liquid slugging, which is liquid refrigerant entering the compression portion of the compressor. Liquid slugging will most likely cause damage to the compressor instantly. Another problem is bearing washout or “flooding”. This occurs when liquid refrigerant dilutes the oil in the compressor crankcase and creates foaming, and it will greatly reduce the life of the compressor because it will not receive proper lubrication and too much oil will be carried out of the compressor and into other parts of the system. The compressor also (generally) relies on the cool suction gas from the evaporator to cool the compressor properly, so it’s a delicate balance to keep a compressor from being flooded and also keep it cool.


Condensers come in all different types, shapes, and sizes. Regardless, they all perform the same function: rejecting heat from the refrigerant. The refrigerant entering the condenser was just compressed by the compressor, and this process increased the temperature by packing the molecules together which added heat to the vapor refrigerant due to the motor and mechanical workings of the compressor. This process in the compressor also greatly increased the pressure from a low-pressure in the suction line entering the compressor, to a high-pressure vapor leaving the compressor.

The condenser has three jobs:

  1. Desuperheat the refrigerant (Drop the temperature down to the condensing temperature)
  2. Condense (saturate) the refrigerant (Reject heat until all the refrigerant turns to liquid)
  3. Subcool the refrigerant (Drop the temperature of the refrigerant below the condensing / saturation temperature)

The condenser’s job is to reject heat (drop the temperature) of the refrigerant to its condensing (saturation) temperature, then to further reject heat until the refrigerant fully turns to liquid. The reason it must fully turn to liquid is that, in order for the refrigerant to boil in the evaporator, it must first have liquid to boil.

The way in which the condenser removes the heat from the refrigerant varies. Most modern condensers flow air over the tubing where the refrigerant is flowing. The heat transfers out of the refrigerant and into the air. The cooling medium can also be water. In the case of a water source system, water is circulated across the refrigerant in a heat exchanger.

In either case, the condenser relies on the removal of heat to another substance (air, water, glycol etc..). For instance, if you turned off the condenser fan so that no air was flowing over the condenser coil, the condenser would get hotter and hotter. This would cause the pressures to get higher and higher. If it kept going that way it would trip the internal overload on the compressor or cause other damage.

The hot vapor from the compressor enters the condenser and the superheat  (temperature above condensing temperature) is then removed. The refrigerant then begins to change state from vapor to liquid (Condense). The refrigerant maintains a constant temperature until every molecule of vapor is condensed. The temperature of the liquid again starts to fall. This is known as subcooling. When we measure subcooling we are measuring degrees of temperature rejected once the refrigerant has turned completely to liquid.

Temperature above the saturation temperature is called superheat. Temperature below the saturation temperature is called subcool or subcooling. So when something is fully vapor (like the air around us) it will be superheated, and when it is fully liquid (like the water in a lake) it is subcooled. 

Metering Device

The metering device is a pressure differential device that creates a pressure drop to facilitate refrigerant boiling in the evaporator coil.

The metering device is located between the liquid line and the evaporator. The liquid line is full of high-pressure liquid refrigerant. When the high-pressure liquid hits the small restrictor in the metering device, the pressure is immediately reduced. This drops the pressure of the refrigerant to such a degree, that the saturation temperature is lower than the temperature of the air surrounding the tubing that the refrigerant is in. This causes the refrigerant to start changing from liquid to vapor. This is called “boiling” or “flashing”. This “flashing” brings the refrigerant down from the liquid line temperature to the boiling (saturation) temperature in the evaporator, and in this process a percentage of the refrigerant is immediately changed from liquid to vapor. The percentage of the refrigerant that changes during flashing depends on how great the difference is. A larger difference between the liquid line temperature and the evaporator boiling temperature results in more liquid lost to flashing and reduces the efficiency of operation.

There are a few different types of metering devices. The most common ones being the Thermostatic Expansion Valve (TXV/ TEV) and the Fixed Orifice (often called a piston)– as well as electronic expansion valves, capillary tubes, and others.


The evaporator is also known as the cooling coil, because the purpose of the evaporator is to absorb heat. It accomplishes this through the refrigerant changing from liquid to vapor (boiling). This boiling process begins as soon as the refrigerant leaves the metering device, and it continues until the refrigerant has absorbed enough heat to completely finish the change from liquid to vapor. As long as the refrigerant is boiling it will remain at a constant temperature; this temperature is referred to as saturation temperature or evaporator temperature. As soon as the refrigerant is done boiling, the temperature starts to rise. This temperature increase is known as superheat.

When the indoor air temperature or the air flow going over the coil is higher, the evaporator pressure and temperature will also be higher because more heat is being absorbed into the coil. When the air temperature or airflow over the coil is lower, it will have lower pressure and temperature in the coil due to less heat being absorbed in the coil.

The refrigerant leaves the evaporator, travels down the suction line and heads back to the compressor where the cycle starts all over again.

Refrigerant Lines 

Suction Line = Line Between the Evaporator and the Compressor

The suction line should contain low-pressure superheated suction vapor. Cool to the touch on an air conditioning system, and cold to the touch in refrigeration.

Discharge Line = Line Between the Compressor and the Condenser 

The discharge line should contain high temperature, high pressure superheated vapor

Liquid Line = Line between the Condenser and the Metering Device

The liquid line should be high pressure, slightly above outdoor temperature subcooled liquid

Expansion Line (When applicable) = Line Between the Metering Device and the Evaporator

On most systems, the metering device will be mounted directly to the evaporator making the expansion line a non-factor. Some ductless mini-split units will mount the metering device in the outside unit making the second, smaller line and expansion line. The expansion line is full of mixed vapor/liquid flash gas.

Yes, this was long, but more than anything else just keep repeating over and over: compressor>discharge line>condenser>liquid line>metering device>expansion line> evaporator>suction line and on and on and on…

Jim Bergmann did a great whiteboard video on the MeasureQuick YouTube page that explains the basic refrigerant circuit


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