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  1. #1
    Join Date
    Jan 2010
    Posts
    101

    Return vs. Supply Air Temperature

    Does the temperature of the air entering the return directly affect the supply temperature on a heat pump system?

    I'm asking because the finished room above our garage has the return (only 1) outside the room. Outside the room is a foyer and stairs, and there's only 1supply register in this space. It supposed to be conditioned space, but it's always very cold this time of year. I have to keep the room door closed all the time. The solid wood door is undercut for the return air. The VisionPro thermostat is located inside the room.

    Just wondering if the return should have been in the fully conditioned room and what affect it's having on heating/cooling the room. Based on how cold the foyer/stairs is, I would like to have an insulated exterior door that is weatherstripped, but I know I can't block the return air exiting this room. Or can I?

    Thanks for your suggestions.

  2. #2
    Join Date
    Jun 2009
    Location
    North Carolina
    Posts
    79
    Just a few questions to clear things up...
    Does the finished room above your garage have a dedicated unit conditioning only this room and the foyer/stairs?
    I assume you have another unit for downstairs?
    Do the stairs go into the house or into the garage below?
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  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,786
    You can block it. The unit won't heat or cool that room anymore though.

    The return air does effect supply air temp. And heat pump efficiency. With a 60 degree return air temp, the heat pump is more efficient then with a 70 degree return air temp.

    Perhaps a better question is: Why is the foyer so cold?
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  4. #4
    Join Date
    Dec 2009
    Location
    GA
    Posts
    10
    return air temp affects supply temp, 50 degree return will make supply lower than 70 degree return. A heat pump can struggle to produce good temp when its cold outside. the south has been colder than normal, I have seen many heat pumps struggle to get more than 15 degree temp rise with below 30 degree ambient temperature.

  5. #5
    Join Date
    Jun 2006
    Location
    Texas
    Posts
    497
    YES

  6. #6
    Join Date
    Jan 2010
    Posts
    101
    Thanks. Here are a few more details:

    The 1-1/2 ton heat pump/air handler are dedicated to this space (guest suite). The room is above a detached garage. The room door leads to small hallway which then leads to stairs to another small foyer/hallway like area at ground level. This little hallway has 2 doors. One door leads to the outside. The door opposite the exterior door leads to the garage. The air handler is located right outside the room in a cubby along with the return air.

    Along the stairs are 2 fairly large windows, and at ground level there are 2 more windows. This side of the garage/guest room never receives direct sunlight.

    For a variety of reasons there's no good way to add yet another door at the bottom of the stairs at ground level.

    I'm very surprised a heat pump is more efficient with a lower return air temperature. I was thinking the guest room supply air temp might be low because of the colder than room temperature air return.

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,786
    The greater the temp difference. the greater the heat transfer rate.

    So colder/cooler air absorbs heat better/faster then warmer air that is closer to the coils temp.

    Sounds like they though that hall and foyer area would warm up because its used for the return path.
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  8. #8
    Join Date
    Jun 2009
    Location
    North Carolina
    Posts
    79
    Not sure how big and well insulated the room in question is but there might be a design problem with the ductwork. Something to ask about next time you have it serviced.
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  9. #9
    Join Date
    Jan 2010
    Posts
    101
    Thanks everyone for the info. I'm very happy to know the location of the return air OK. As to the cold stairs and hallway, I would expect that it would be cold given there's only one supply in the ceiling for the entire space. I'm not sure why that happened and I wish I would have caught it during the build.

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