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  1. #1

    Return air temperature vs. register output temperature?

    First off, let me say that this is a great site. I was in the market recently for a new HVAC, and I read and learned a great deal about HVAC systems and a little about how they operate.
    Anywho, last week I had a local reputable installer install a new Heil 5 ton split system heat pump with a 2 stage compressor and a variable speed air handler to replace an old Goodman unit.
    When I turned the heat on to get the early morning chill out of the house for the first time since the new install, the system didn't seem like it was blowing warm air. Is there a standard rule of the temperature differential between the return air and the air coming from the registers. For example, if the return air temperature is 65 degrees, what should the temperatue be exiting the registers?

  2. #2
    Is this a forced air or Heat Pump or geo thermal

  3. #3
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    Varies with what the outside temp is.
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  4. #4
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    I put together a spreadsheet for my heat pump. Depending on outside temp the supply temp leaving the indoor coil could be from 85F to 105F ranging from 22F to 70F outdoor temperature and assuming a 69F return temp.

    My furnace by comparison has a supply temp of 107F in 1st stage and 119F in 2nd stage around in all conditions.

    But some heat will be lost in the ductwork. Moving air that less than about 98F, will always feel "cool" because it cools off you skin.. i.e. wind chill.

  5. #5
    Quote Originally Posted by Heat Exchanger View Post
    Is this a forced air or Heat Pump or geo thermal
    Heat Pump with emergency strips.

    Outside temperature was probably in the mid 50's to 60. Indoor temperature was probably 68 and the unit ran for about ten minutes with no change in the inside temperature, according to the Honeywell thermostat, before I shut it off to go to work.

  6. #6
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    Quote Originally Posted by t5killer View Post
    Heat Pump with emergency strips.
    The heat strips are for auxillary as well as emergency. They should start to come on during defrost, and when outdoor temps fall under around 25F... and run together WITH the heat pump. Maybe the heat strips aren't wired up correctly and aren't comming on as supplemental heat when needed.

    Or maybe the old heatpump was wired or setup wrong and the heat strips came on all the time...even when it was 50 or 60 outside.

  7. #7
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    Quote Originally Posted by motoguy128 View Post
    I put together a spreadsheet for my heat pump. Depending on outside temp the supply temp leaving the indoor coil could be from 85F to 105F ranging from 22F to 70F outdoor temperature and assuming a 29F return temp.

    My furnace by comparison has a supply temp of 107F in 1st stage and 119F in 2nd stage around in all conditions.

    But some heat will be lost in the ductwork. Moving air that less than about 98F, will always feel "cool" because it cools off you skin.. i.e. wind chill.
    Wow.

    You keep your place 29s? LOL
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  8. #8
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    Quote Originally Posted by t5killer View Post
    Is there a standard rule of the temperature differential between the return air and the air coming from the registers. For example, if the return air temperature is 65 degrees, what should the temperatue be exiting the registers?
    Temperature data for a 5-ton 14 SEER Goodman heat pump (your temps should be similar):

    Outdoor-----------Temp after indoor
    Temp--------------coil (based on 70F return)

    55F----------------100.8F
    45F----------------96.7
    35F----------------92.8
    25F----------------88.9
    15F----------------85.9

    Make sure that the condenser is not in a defrost cycle when checking temps.

    Good luck.

  9. #9
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    Quote Originally Posted by beenthere View Post
    Wow.

    You keep your place 29s? LOL
    I'm striving for optimum efficiency. Who says a 80% furnace can't be a condensing model.


    Fixed the typo... and I fired overseas subcontractor I used for proofreading.

  10. #10
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    Quote Originally Posted by motoguy128 View Post
    But some heat will be lost in the ductwork. Moving air that less than about 98F, will always feel "cool" because it cools off you skin.. i.e. wind chill.
    A point of interest for you: your skin surface temperature averages around 90F. Get an infrared thermometer sometime and check it out. Your body core temperature, on average, is around 98 degrees F.

    More points of interest: moisture is always evaporating from your skin. Place your arm against a cold window pane on a cold morning; the glass will fog adjacent to your arm. Air movement from a heat pump supply air discharge feels "cool", even with supply temps in the low nineties, because this air is also often very dry, and accelerates moisture evaporation from human skin. Additionally, by the time supply air reaches an occupant to where he/she feels the draft, that air is not the same temperature as it was when it emerged from the supply register. It has mixed with room temperature air and has cooled considerably.

    The above can be mitigated signficantly by careful supply register selection, placement, and aiming. This is always important regardless of forced air heating system selected, but is most crucial, from a comfort angle, for heat pump systems.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  11. #11
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    Quote Originally Posted by shophound View Post
    A point of interest for you: your skin surface temperature averages around 90F. Get an infrared thermometer sometime and check it out. Your body core temperature, on average, is around 98 degrees F.

    More points of interest: moisture is always evaporating from your skin. Place your arm against a cold window pane on a cold morning; the glass will fog adjacent to your arm. Air movement from a heat pump supply air discharge feels "cool", even with supply temps in the low nineties, because this air is also often very dry, and accelerates moisture evaporation from human skin. Additionally, by the time supply air reaches an occupant to where he/she feels the draft, that air is not the same temperature as it was when it emerged from the supply register. It has mixed with room temperature air and has cooled considerably.

    The above can be mitigated signficantly by careful supply register selection, placement, and aiming. This is always important regardless of forced air heating system selected, but is most crucial, from a comfort angle, for heat pump systems.
    You haven't measured the surface temp of my wife's skin, she's hot!!!

  12. #12
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    Quote Originally Posted by motoguy128 View Post
    You haven't measured the surface temp of my wife's skin, she's hot!!!
    You are setting yourself up for interesting replies with that post.

    I'll be nice (for now)

    Take care.

  13. #13
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    Quote Originally Posted by shophound View Post
    A point of interest for you: your skin surface temperature averages around 90F. Get an infrared thermometer sometime and check it out. Your body core temperature, on average, is around 98 degrees F.

    More points of interest: moisture is always evaporating from your skin. Place your arm against a cold window pane on a cold morning; the glass will fog adjacent to your arm. Air movement from a heat pump supply air discharge feels "cool", even with supply temps in the low nineties, because this air is also often very dry, and accelerates moisture evaporation from human skin. Additionally, by the time supply air reaches an occupant to where he/she feels the draft, that air is not the same temperature as it was when it emerged from the supply register. It has mixed with room temperature air and has cooled considerably.

    The above can be mitigated signficantly by careful supply register selection, placement, and aiming. This is always important regardless of forced air heating system selected, but is most crucial, from a comfort angle, for heat pump systems.
    Outstanding post.

    Thanx.

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