Never had this...

I typically test the Delta T of the A/C at the return and just past the plenum. Today, I got a difference of 29°F which is way higher than anything I’ve gotten before.

The unit is 3 years old, outside temp was 75°F, overcast, RH about 40-50%. I let it run for about 20 minutes after running the heat for about 15 before I measured the temp. The temp past the A-coil was 41°F. I’m sure there are many factors to this and I don’t look at Delta T as the end-all of indicators, but I figured it’s worth discussing.

Is this high of a Delta T a concern? It seems odd to ask this question, but it’s just so out of the ordinary.


When I get a high Delta T during the AC test, I ask what can cause this?

  1. air speed. is the fan set at the right speed? I can’t test this
  2. Is the filter dirty? This can slow air speed through the unit allowing the air to become colder as it moves past the A-Coil more slowly
  3. As I have seen this happen before – If there is a humidifier on the HVAC system — the damper that is used to control air flow from the humidifier is open or missing and that is allowing colder air to flow directly back into the cold air return to join with the air coming from the living space and really cause a high Delta T. (this situation also caused some condensation as the duct was not insulated through a garage ceiling and water was dripping on the garage ceiling drywall). Once we blocked the air from the humidifier duct, the Delta T returned to normal.

Since this Delta T may be out of the ordinary (you didn’t say what the AC unit capacity is) then something may be amiss.

Hope this helps

My guess is they went from a R22 system to a R410 system, in addition to the great ideas above I would add overcharged system, or/and the air return is much smaller than what is needed, significantly decreasing the air intake.

Can not happen an over charge creates higher pressure which in turn creates higher temp suction and head pressure;-)

Super heat temp needs adjusted unit does not run the proper time between cycles to remove humidity

Cameron in this case I would this write up temperature differentials were not normal recommend a license heat and air contractor to to review and repair as necessary. It is not our job to analyze as you said many factors could be in play for this one. Overcharging would not cause this is Charlie stated

Thanks guys. That really helps, I appreciate you taking time to answer.