Thread: Heat Pump heat mode temp differential in supply/return air

Does anyone know an answer to this question. In the heat mode what is the temp differential at the heat exchanger (supply/return) air answers are A.25-30degrees B.5-7 C.15F D.20-22F I will tell you what my training material stated the answer was after I hear what others have to say I am getting conflicting info. on this

Originally Posted by Gary Redmond

Does anyone know an answer to this question. In the heat mode what is the temp differential at the heat exchanger (supply/return) air answers are A.25-30degrees B.5-7 C.15F D.20-22F I will tell you what my training material stated the answer was after I hear what others have to say I am getting conflicting info. on this

It has been a while since I studied this but the 20-25 degrees is the temperature of the outdoor coil below ambient temperature as I remember. I do not remember ever seeing delta T figures given for the heating mode. The air leaving the indoor coil will be typically between 80 and 100 degrees. As the temperature outside drops so does the coefficient of performance (COP), making it more expensive but giving about the same heat. The problem comes with the balance point due to heat loss of the structure, a larger system will have a larger capacity to replace the lost heat.

I think you may have touched on a good point... "The problem comes with the balance point due to heat loss of the structure, a larger system will have a larger capacity to replace the lost heat." My training materials states 15-22 at the inside coil but my answer key says 25-30

Originally Posted by Gary Redmond

Does anyone know an answer to this question. In the heat mode what is the temp differential at the heat exchanger (supply/return) air answers are A.25-30degrees B.5-7 C.15F D.20-22F I will tell you what my training material stated the answer was after I hear what others have to say I am getting conflicting info. on this

E. All of the above. It depends upon the outside temperature and the heat pump. At a low ambient the delta T could be less than 10 degrees, but then the auxiliary heat would probably kick in. When it is about 60-65 degrees the delta T can be over 30 degrees.

Originally Posted by Mark Reinmiller

E. All of the above. It depends upon the outside temperature and the heat pump. At a low ambient the delta T could be less than 10 degrees, but then the auxiliary heat would probably kick in. When it is about 60-65 degrees the delta T can be over 30 degrees.

I would like to know where this range of numbers comes from. When in heating mode the heat pump controls the flow of refrigerant differently than when it is in cooling mode. When in cooling mode the temp at the evaporator (indoor coil) is designed to be close to 40 degrees. When in heating mode the evaporator (outdoor coil) can go below 0 degrees; changing the whole dynamics of the system. If you were to enter a vacant house that was at 30 degrees indoor temp would you be satisfied with 50 degree air at the register? Note: if you set the thermostat to less than a three degree rise the auxiliary heat should not come on. I expect 80 degree air within a few minutes, as soon as the ducts have warmed up.

The following information was posted on one of the home inspector message boards years ago. I do not know who posted it or where it came from. I have found that the information seems reasonably accurate on R-22 systems.

Jeff

For Heat Pump in Heat mode:

Starting at 32°F outside temperature, the indoor split (rise) should be 20°F. For each 3°F degrees rise in outside temperature, the split should rise 1°F.

Outside Rise across air:

Temperature: Handler should be:

32°F - 20°F
35°F - 21°F
38°F - 22°F
41°F - 23°F
44°F - 24°F
47°F - 25°F
50°F - 26°F
53°F - 27°F
56°F - 28°F
59°F - 29°F

(1) These are approximate values. Any manufacturer’s data takes precedence over these.

(2) Any supplemental heat strips should be shut off before running the heat pump and measuring the split.

(3) These data may not be applicable to the newest generation of refrigerants and units with high SEERs.

Many years ago I pulled some data from manufacturer's literature with the following numbers:

Ambient Delta T (degrees F)
0 12
10 15
20 17.5
30 21
40 23
50 25-27 (two different systems)
60 28-32 (two different systems)

I also measured temperatures of a few dozen or more systems. I did this around 1999 to 2000 and the hp data may have been old then. I suspect newer systems would have a much better delta T. I found pretty good agreement with the data above and what I measured at temperatures between about 20 to 30 degrees ambient. At higher temperatures my numbers tended to vary from a couple degrees better to as much as 5 degrees less.

Remember also that measured temperatures can vary significantly depending on airflow, etc. Also, it is hard to get a true reading because you need to run the system for quite a while without it shutting off or activating the auxiliary heat.

With heap pumps and ACs delta T is a general indicator of whether the system is working correctly, but I do not pay much attention to numbers unless they are significantly out of a "normal" range.

The temperature differential between the return air and the supply air (when any backup heating components are not operating) is one potential indicator of performance for a heat pump when it is operating in the heating mode. However, unless inspectors are performing calculations based on the size of the unit in tons, the elevation of the unit in feet above sea level, and a number of other factors, the temperature differential won?t be particularly useful in evaluating system performance.