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  1. #1
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,380

    Tempature/Humidity Thermostat Logic

    Are there any humidity sensing thermostats that can automatically raise/lower temperature in accordance with humidity? I know in an ideal world humidity and temperature would remain "fixed", but sometimes there isn't enough of a sensible load to cause the thermostat to cycle.

    I've found 2 possible solutions:
    1: Thermostat with humidity sensing that will cycle the A/C up to 3 degrees below setpoint to get to correct humidity level. (Honeywell)

    2: Thermostat that will raise setpoint based on run time of A/C. (White Rodgers "cool savings")

    I'd like a stat that would simply lower the temperature 1 degree for every 4% over humidity "setpoint" and raise 1 degree for every 4% under setpoint. Does such an stat exist?

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,358
    Quote Originally Posted by 54regcab View Post
    I'd like a stat that would simply lower the temperature 1 degree for every 4% over humidity "setpoint" and raise 1 degree for every 4% under setpoint. Does such an stat exist?
    Not only am I not aware of a thermostat that might employ this form of control strategy, I also do not think it's a good control strategy in terms of humidity and temperature regulation.

    This time of year presents humidity control challenges in residential settings because, as you've noted, there is not much of a sensible load but there is a considerable "latent" or moisture load. Using the a/c to dehumidify can bring the moisture levels down, but can also lead to overcooling and occupant discomfort.

    Until residential equipment evolves beyond "old school" configurations (and simultaneously achieves reliability and attractive price points concurrent with advances), you are stuck with a strategy that in shoulder season weather is out of its element in terms of humidity control. In the middle of summer, it's great. This time of year, and with many houses at night, not so great on the humidity control aspect.

    A whole house dehumidifier is one approach; another is installed equipment with variable capacity. Both are available and both ain't cheap. You asked about a thermostat that can make your existing equipment more conscious of humidity levels. First thing I'd want to do as a homeowner is assess how my structure is gaining moisture, and if there is any low hanging fruit I could go after in order to reduce the moisture load. After that assessment and actions taken I'd want to monitor how the existing equipment then handles normal daily moisture generation from occupants and indoor activities, coupled with remaining fresh air ventilation requirements. Afterward, if humidity was still not under control I'd begin looking at control or equipment configuration options.

    Personally, I'd like to create a dew point temperature control strategy for my house system. If dew point levels climbed above 55 degrees, the a/c would cycle on in a way that optimizes latent heat removal and minimizes sensible. Once the dew point came down below setpoint the system cycles off, and overcooling is likely avoided.
    • 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.

  3. #3
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Several thermostats will overcool by several degrees to dehumidify. This can serve the same purpose. I assume to goal is to maintain a constant comfort level.

    This is what both of my thermostats do at home. I set them for a fairly high temeprature, then set the relative humidity such that it maintains a acceptable comfort level. As the cooling load increases, the system more easily maintains the humidity setpoint and therefore no longer overcools.

    I also have it set-up to slow the blower on a call for dehumidificaiton as well.

    The only downside is that the system does short cycle a little on cooling calls for dehumidification. But that's more a function of my equipment being oversized (thanks ot the PO's). If it was properly sized, it would run a lot longer and overall use less energy.

    Mine is the White Rodgers. It calls for cooling up to 3F below the setpoint when the humidity is at least 2%RH above setpoint and runs until it's either 3F below temp setpoint or its' lowered the humidity 2%RH belwo humidity setpoint. The humidity can be set in 1% increments. That's one thing I found annoying on the Honeywells. While I liked the way they staged equipment and controlled temeprature, with humidity, you were stuck with 5% increments.

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