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  1. #1
    Join Date
    May 2009
    Posts
    51

    Let's Talk About Setbacks

    Say we have two days in a row, identical conditions (temp/humidity/thermostat set points, etc.) From purely an electrical consumption standpoint, which situation is better (assume "sleep" and "wake" was set at 73 degrees in both scenarios):

    1) have leave setback to 78 degrees begin at 5:30 am knowing that at x:xx time that afternoon, a/c will begin cycling on.

    2) have the leave setback to 78 degrees begin at 4:30 am knowing that at x:xx time that afternoon (presumably earlier than in case 1) a/c will begin cycling on.

    In case 2, I would assume the temperature difference between indoors and outdoors, although not greater at any identical time between cases, the a/c would run longer. In case 1, the a/c would likely short cycle more in the morning, making it less efficient.

    Is that correct thinking? Is it different for say a single stage a/c vs. a dual stage a/c? Could a dual stage a/c actually use less power in case 2 conditions than in case 1?

    Most importantly, where is my logic flawed?

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,347
    If I read your post correctly, the "leave" start time is only one hour apart. The outdoor temperature isn't likely to rise or fall all that much between four and five in the morning.

    Personally my thermostat does not go into "leave" mode until I leave the house in the morning. The house is often still occupied after my departure, but the occupants like it warmer than I, and the house does not rapidly warm up that time of day after the "leave" mode kicks in. Reason I delay the "leave" mode until my departure, in addition to hinting at personal comfort, is so morning moisture generation via showering, breakfast, etc. can be removed from the air prior to the "leave" mode kicking in, where the unit may not run for a few hours before the interior temp warms up enough to kick it back on. Longer than I want higher humidity hanging around uncontrolled.
    • 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.

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