The temperature of the refrigerant in the evaporator coil can really help us with diagnostics, but we need to know the targets we’re aiming for if we want to get the most out of it as a troubleshooting tool. If we take the return air temperature and subtract 35 degrees from that number, we get the design temperature difference (DTD).

A typical system would look like this: 

75°F return air – 35°F design temperature difference = 40°F evaporator coil temperature

But where did this equation come from, and what is it for? It turns out this equation is not some arbitrary number that just “happens to work;” it’s an actual engineering design with a well-thought-out purpose.

Design Temperature Difference (DTD)

Bryan wrote more in-depth about design temperature differences in this article. He says:

“In air conditioning applications, a 35°F DTD is a good guideline for systems that run 400 CFM (679.6 m3/h) of air per ton of cooling (12,000 BTU/hr). In refrigeration, the DTD is much lower than in air conditioning.

There are several reasons for this, but one big reason is the desire to maintain relatively high relative humidity levels in refrigeration to keep from drying out and damaging the product. Keep in mind that NOTHING is a substitute for manufacturer's data, but there are some good DTD guidelines for traditional/older refrigeration equipment below. Keep in mind that the trend is toward lower evaporator TD on newer equipment.

Walk-ins  10°F DTD +/- 3°F
Reach-ins  20°F DTD +/- 5°F
A/C 35°F DTD +/- 5°F

You then subtract the DTD from your box temperature/return temperature to calculate your target suction saturation. You can then use this target saturation/DTD and compare it to your actual measured saturation and DTD once the box is within 5°F–10°F (2.75K–5.5K) of its target temperature to help you set your charge, TXV, and EPR, as well as diagnose potential airflow issues when compared with suction superheat and subcooling/clear sight glass.”

The DTD in air conditioning is purpose-built to dehumidify. In other applications, this is not needed or even desired. 

Troubleshooting with DTD

You can use this information about what the coil temperature should be to troubleshoot a couple of different things. 

Low Airflow

If your evaporator coil temperature is really cold compared to the return air temperature (and you have low superheat), you can immediately flag that as a problem. For example, I recently changed the blower motor on a furnace. The motor was reversible (meaning it could spin clockwise or counterclockwise), and it seemed like it was spinning the right way. Air was blowing out of the supply vents. The house was about 85°F, and when I went out to check the refrigerant pressures, I had a 33°F evaporator coil temperature with 1 degree of superheat. 85 RA – 35 DTD = 50°F evaporator coil temperature. So, I immediately knew I had a problem. I switched the leads on the motor—problem solved. Here’s a great article on low airflow if you want to dive deeper.

Return Duct Leakage

If your evaporator coil temperature is really warm and the return air temperature measured in the conditioned space is normal, you can also immediately flag a problem. For example, I was recently at a friend’s house and had a 65°F evaporator coil temperature. The house was 85°F inside. So, my evaporator coil temperature should have been:

85°F RA – 35°F DTD = 50°F Evaporator Temperature

After some head-scratching, I finally found a large return duct pulling in a TON of hot attic air! What was my return air temperature? Probably around 100 degrees! No wonder it was warm in the house!

Final Thoughts

The nice thing about using DTD to troubleshoot is that you get to stand at the condenser and think about what could be wrong before you go crawling around in a hot attic looking for duct leakage or pulling panels off of evaporator coils looking for airflow restrictions. It can send up some warning signals and send you in the right direction with your troubleshooting.