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  1. #1
    Join Date
    Jul 2005
    Post Likes
    I have a window unit in an office. Humidity (RH) stays around 65%. It doesn't have an auto mode (I guess most don't) so it is always on. I have read on this forum that when the AC is "on" rather than "auto" the RH will be higher. Is there a way to retro-fit this unit with a thermostat that will have an auto mode? If not, can I possibly install a timer to turn the unit on and off at regular intervals? It's a 220 volt unit.
    specs: 11500 btu/h
    room size:
    280 sqft
    10 foot cieling
    all 4 sides to the exterior
    62 sqft of windows low E double pane
    well insulated
    I think it may be oversized for the space but I was limited because of cool/heat needs and I used some sort of btu calculator that took me to roughly that size based on the building being stand alone and the glass square footage, central texas.

  2. #2
    Join Date
    Jul 2005
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    Thread Starter
    Did some testing this morning and the RH is dramatically lower when I cycle the unit on and off my self. The RH stays around 48 to 50 rather than the 65 RH when I leave it on all the time. I'm going to research a simple timer that will give me the ability to set the frequency and duration of the on off cycle.

  3. #3
    Join Date
    Oct 2004
    Tyler, TX
    Post Likes
    Originally posted by wimtex
    It doesn't have an auto mode (I guess most don't) so it is always on.
    Quite a few have the auto mode. In fact, most do.

    It sounds like your unit is oversized.
    Turning the fan off won't solve that problem.
    RSES Certificate Member Specialist

    Southwest Regional Association of RSES Secretary, 2017

  4. #4
    Join Date
    Mar 2004
    Post Likes
    The reason that window units usually insist on running the fan all the time is that the thermostat is right next to the evaporator coil. They can only measure room temperature at that location if the fan is running; otherwise the cold coil confuses the temperature sensor.

    You'll get the better results using an external thermostat, rather than a timer. Check out

    I have a variant of this item that is for heating applications; I used it to remote-mount a thermostat for an electric space heater that was having the same problem (its thermostat was confused by its own heat output, instead of measuring the real room temp). This one requires hardwiring, though, but you'd probably need some hardwiring to get a timer to work with a 240V window unit anyway. In this application, set the window unit's thermostat to the coldest setting, so that it cools whenever the window unit is supplied power by the thermostat.

    I'm sure you can get these through a variety of industrial parts suppliers; this is just the one that I use.

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