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  1. #1
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    Jul 2014
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    NM
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    Default 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

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  2. #2
    Join Date
    Mar 2008
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    Charlotte NC
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    Default Re: Heat Pump heat mode temp differential in supply/return air

    Quote Originally Posted by Gary Redmond View Post
    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.

    The beatings will continue until morale has improved. mgt.

  3. #3
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    Jul 2014
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    NM
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    22

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

    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


  4. #4
    Join Date
    Mar 2012
    Location
    Lansdale, PA
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    876

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

    Quote Originally Posted by Gary Redmond View Post
    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.


  5. #5
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    Charlotte NC
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    Default Re: Heat Pump heat mode temp differential in supply/return air

    Quote Originally Posted by Mark Reinmiller View Post
    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 beatings will continue until morale has improved. mgt.

  6. #6
    Join Date
    Mar 2007
    Location
    Peoria Arizona
    Posts
    76

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

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

    Outside Rise across air:

    Temperature: Handler should be:

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

    (1) These are approximate values. Any manufacturers 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.


  7. #7
    Join Date
    Mar 2012
    Location
    Lansdale, PA
    Posts
    876

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

    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.


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