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Ceiling Fan Dehumidifier

Danny GoughDanny Gough Posts: 185
edited July 2017 in Moisture & vapor control
Greetings, This situation is a real head scratcher. The attachment shows the 2nd floor of a new home which includes a 3 ton heat pump in the hot attic. The system has two zones, one for the bonus room and another for the remaining 2nd floor bedroom areas. Thermostats are Honeywell Vision Pro 8000 WIFI. The bonus room thermostat location is shown in the close up. Also noted is a ceiling fan centered on the windows in the shed dormer.

Here's the head scratching part. Vision Pro tstats read relative humidity. The bonus room thermostat reads 69 degrees F and 64% RH. The homeowner turns on the ceiling fan. Thermostat humidity reading immediately drops to 56%, 54%, 52% and eventually 50%. Turn the ceiling fan off and the RH reading goes right back up to +60%. Bonus room doors always remain open.

My theory is perhaps the inset corner forms a stagnant area and the humidity reading is obviously not representative of the room RH. But I was always under the impression that humidity in open air diffuses to equilibrium fairly easy. Moreover, where did the moisture come from that runs the RH up?


  • David ButlerDavid Butler Posts: 3,889
    Wow, that's really interesting.

    I know from my R&D days that RH sensors are very sensitive to local ambient as well as having good circulation. You're correct that vapor pressure will equalize without regard to room geometry. However, RH is by definition, relative to local ambient. Does the temperature reading change at all as the RH drops?

    I'm wondering if the Vision Pro might be poorly designed, allowing heat from internal components influence the RH reading when there's no air movement. Or heat from the interstitial attic space is bypassing into the hollow wall cavity behind the t'stat mounting plate. OTOH, if sensor gets warm when ceiling fan is off, I would think that would bias the reading low, not high. Hmm...
  • Danny GoughDanny Gough Posts: 185
    Follow up on this oddity. Yesterday, I experimented more with this house. Now we discover the RH in the bonus room drops not just with the ceiling fan running, but also when we closed the door leading into the bonus room. The behavior was consistent with or without the unit running. Although, the heat pump air handler does run continuously at very low flow. One more tidbit, is we did determine the bonus room is under substantial positive pressure with reference to the 2nd floor spaces. I didn't have my manometer. But it was + enough to move blow the door open unless it was latched. This is largely moot, since the homeowner seldom ever closes the door.