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  1. #1
    Join Date
    Oct 2007
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    Supply vs Return Temp Differential

    I'm preparing to have my home's HVAC system balanced. I've placed temperature sensors in the primary living spaces and have been recording the data. I had a couple of extra temperature sensors and for kicks, I put one in the return duct just before my air handler and another in the supply duct a few feet from the air handler. (The furnace is in my attic.)

    I've noticed that when my HVAC system isn't running, the temperature difference between the return air and the supply air about 10 degress F.
    (Right now with an outdoor temp of 40, the return is 51 and the supply is 61.)
    Since the temperature sensors are only about 8 feet apart, both in the insulated ducts in the attic, this temperature difference seems odd to me. I would expect the temperatures to be about the same.

    Could this temperature difference indicate a leak in the return ducts?

    When placed side-by-side, both temperature sensors report temps within half a degree of each other. When the system is running in heat mode, the return air peaks around 74, which is what I would expect based on my thermostat setting. (Supply peaks around 135.)

  2. #2
    Join Date
    Nov 2002
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    "Could this temperature difference indicate a leak in the return ducts?"

    Or could this delta indicate a chimney effect within the supply?

    Why not have a blower door test run on the structure (on a calm day) and see what delta that produces with the system off.

    Or see if the delta is a function of air speed across the structure (i.e. record wind speeds and correlate with the observed delta).

    Which professional journal do you hope to publish your study results in?

    Just a thought…

    God Bless…

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