Most HVAC technicians have grown up being taught that when checking air conditioning systems, supply air temperature should be measured.  Naturally, this seems like a perfectly logical step.  Since an air conditioning system is meant to supply cold air, measuring supply air temp is a good idea.

But what supply air temp should be considered acceptable?  When I pose this question to my students during air conditioning advanced troubleshooting classes, I get a variety of answers!  The most common of which is 55 deg. F.  When I ask if 55 deg F is good, lots of heads start nodding, and at least one student will want to throw up a “yeah, but” and insist that because of the altitude, 50 degrees is really the number we want to look for.

So, I will say, “What if the temp is higher than 55?”

“Must be low on refrigerant” is a popular response.

“What if the temp is lower than 55?”

At this, some folks will shrug and say, “Colder is better than warmer any day”.

All of these statements reveal a long line of air conditioning myths that have been handed down from generation to generation.  Let’s look into these a little further:

First, let’s start with 55 degrees.  The “well known fact” that 55 degrees is the preferred supply air temperature comes to us from the commercial world.  For generations, large commercial air conditioning systems have been designed around a constant supply air temperature setpoint of 55 degrees.  Even today, many modern control systems will default to a supply air temperature setpoint of 55 degrees, and this is also the setpoint used to control compressor staging on variable capacity and variable air volume systems.  If the rest of the system was designed around a 55 degree supply air temp, this works great!

Compared to the whole of air conditioning, variable capacity and variable air volume systems are relatively uncommon.  Most air conditioning systems in operation today are fixed capacity constant volume types.  On these systems, 55 degree supply air doesn’t necessarily mean anything!

On these systems, the ∆T is the value we are interested in.  The symbol “∆”, or “Delta” means “Change in Value”.  When combined with the “T”, the meaning is “Change in Temperature”.  Another familiar term would be “Temperature Drop”.  These terms refer to the amount the air drops in temperature as it passes over the evaporator coil.

For example:  If the air entering the evaporator is 80 degrees, and the air leaving the evaporator is 60 degrees, the difference in temperature, or the ∆T, is 20 degrees.

Most HVAC techs will agree that 20 degrees is a good ∆T.  The best techs when asked what the ideal ∆T for air conditioning should be will say, “It depends”.  Here is why:

In every operating air conditioning system, there are two types of heat transfer taking place between the air stream and the refrigerant.  Latent heat transfer and sensible heat transfer.  Sensible heat transfer will be measured on a thermometer while latent heat transfer will not.  Latent heat transfer takes place when moisture held in the air condenses on the surface of the evaporator.

For every pound or water that comes out of the condensate drain, 970 btu of heat is absorbed by the refrigerant in the evaporator.  These btus are the latent heat load on the evaporator.  The higher the relative humidity of the air, the greater the latent heat load on the evaporator.  The greater the latent heat load on the evaporator, there is less sensible heat transfer that can take place.  Therefore, the higher the relative humidity of the air entering the evaporator coil, the lower the sensible ∆T will be.

The following chart details the relative humidity and ∆T relationship:

Air Conditioning airflow chart
20% RH 26
25% RH 25
30% RH 25
35% RH 23
40% RH 22
45% RH 20
50% RH 19
55% RH 19
60% RH 18
65% RH 16
70% RH 15
*based on 400 cfm per ton of cooling
© 2014 Eric Shidell; HVAC Service Mentor

You will see that the “standard” ∆T of 20 degrees is only correct if the relative humidity of the air entering the evaporator is 45%.  As the RH changes to a higher value, the ∆T changes to a lower value.

The above chart is a great way to do a quick check on the condition of an operating air conditioning system.  First, measure the relative humidity of the air stream entering the evaporator coil using a digital psychrometer or similar device.  Next, accurately measure the temperature of the air entering the evaporator coil and the air leaving the evaporator coil.  Subtract the two to find the temperature difference or ∆T.

There are four major variables that contribute to the final ∆T in an operating air conditioning system.  They are:  Relative Humidity of the entering air, Refrigerant Charge, Air Flow Rate, and mechanical integrity of the system (compressor, metering device, coils, etc.)

In an operating system that is perfectly correct, the measured operating ∆T value should match the chart for the measured relative humidity within two degrees.  If the value does not match, something is wrong and further investigation is needed.

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