Have you ever been on a service call and everything seems to check out perfectly? The system charge is good, the filter is clean, the condenser and evaporator coils are clean, the blower speed is set properly, yet there is still a comfort problem in the home.

Here’s a trick you can pull out of your tool bag before you leave the problem unsolved.


All you need is a digital pocket thermometer, a pen, and a notepad. If you have a high-speed infrared or thermocouple device, even better.

Time to Get Busy

We normally measure the delta T, or the change in air temperature across the cooling coil, as part of the diagnostic process. This measurement is common and one of the first measurements we perform during our diagnostics. The temperature drop is affected most by the operation of the refrigeration cycle, airflow, and humidity. While important, the delta T, only helps identify issues with the mechanical system.

As a reference, the delta T over a cooling coil, in places with high humidity ranges from 16F to 19F. In less humid climates, like the southwest U.S., the temperature drop is around 19F to 22F.

If you only perform these temperature measurements on equipment that is working properly, you will never identify issues that affect the total system performance.

So, what to do if the equipment is operating properly?

The Total System

HVAC Ductwork System, never assume that it is working properly. In fact, ductwork is often overlooked when diagnosing system problems. Remember, ductwork loss or gain should not exceed 3F.

Now here is the part that will separate you from the average technician.

Start with the supply ducts.

  1. Measure and record the outlet temperature of the air handler at the supply plenum.
  2. Measure the temperature of the air leaving the supply duct furthest from the air handler along with several others.
  3. Calculate and record the average supply air temperature.
  4. Subtract and record the average supply duct temperature from the discharge temperature at the supply plenum.

*** Let the system run for 15 minutes prior to performing measurements


It is the middle of July in south Florida, 98F.. typical. You stick your head into the attic and it is at least 140F and you see a long run of supply duct (flex), condition unknown.

You measure:

  • The delta T across the coil – 19F
  • The outlet air temperature at the supply plenum – 60F
  • The average outlet temperature is – 68F

You have now discovered that there is a supply duct temperature loss of 8F… not good. Now let’s see what our complete system capacity is:

  • Delta T across the coil – 19F
  • Supply air temp loss: 8F

Divide the supply air temperature loss (8F) by the delta T (19F).

8F / 19F = 42%

This is a 42% loss in the entire system capacity!

To put this in perspective. If the system you’re checking has a capacity of 36,000 btu and is operating properly. The ductwork alone reduces the capacity down to 20,880 btu.

Finish with the return air duct

Perform the same measurements, this time on the return ducts. The calculations are not as important here. What you’ll see is an even greater loss of capacity because the return ducts suck hot air into them from the sweltering attic. Remember 3F is your benchmark for ductwork loss.

The supply and return air duct losses are big and have a huge impact on overall HVAC ductwork System performance. These simple and often overlooked components of those hard to diagnose comfort calls.

Ron Walker
Ron Walker

After retiring from the U.S. Marines and achieving his B.S. degree, Ron Walker entered the HVAC field. He has been an HVAC technician, service manager, and business owner. Working as a service manager, he spent many years training HVAC technicians to be more technically competent and really understand their trade. His passion for teaching and helping others resulted in the creation of HVAC Training Solutions, LLC.