Wiring question

Discussion in 'General Discussions' started by geome, Sep 9, 2009.

  1. geome

    geome Member Forum Leader

    The dehumidification feature on our Prestige thermostat (using the AC with a low* speed fan) requires that a connection be made between the thermostat and a dehumidification terminal on the equipment for low speed fan operation. This connection is currently not wired and consequently this feature is not working.

    * In our case, I assume we want medium speed as opposed to low or high speed. My understanding is that low is mainly used during startup and shutdown with an ECM blower.

    Does this terminal exist on a WF All-In-One Envision dual stage unit with ECM? I see a wiring schematic in the installation manual, but not sure how to read it. I can do this type of wiring if I know exactly what to wire. [​IMG] If there is no such terminal, is there other way to connect this? Could the 3 accessory terminals help somehow? Below is a link to the Envision manual. I’m looking at page 27:


    I have the same question for a split unit. How could this be wired as well? Would this terminal be located in the compressor section or in the air handler section?

    In case anyone needs to see it, here is a link to the Prestige manual. Please see the middle of page 7, on the right hand side of the page for the wiring instructions that I am referring to.
    http://www.forwardthinking.honeywell.co ... 057efs.pdf

    Any help would be greatly appreciated. Thanks

    P.S. I am aware of the WF dipswitch that reduces the fan speed by 15% in AC mode. Not interested in going that route as I would rather use the thermostat if possible.
  2. teetech

    teetech Member Forum Leader

    There is no terminal om the Envision logic board for dehumidify. Your options are dehumid switch (which only controls fan speed) or connect to a separate dehumidifier. A dehumidifier would cost less to operate that the envision compressor.

    True dehumidification on a geo unit involves a reheat coil, that's a whole new ballgame. If you have a humidity problem a separate dehumidifier would be your best bet.

    Same situation withe the split unit (fan speed only)
    Remember if you slow your fan too much you will freeze your coil.
  3. geome

    geome Member Forum Leader

    If no dehumidification terminal, how could I still get this to work with the equipment I have?

    I would be happy with the unit running in first stage (that would be medium fan speed anyway). Could I jump DHM2 to Y? Would there be a voltage problem if the thermostat called for normal first stage AND this dehumidification at the same time? If so, could this problem be overcome?

    What do the accessory terminals do? Would they turn on the system to first stage if I wired DHM2 to one of the terminals?

    I can ask WF, but thought someone here may have already come across this, or can come up with a "thinking outside the box" solution. Maybe this can't be done.
  4. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Not a helpful reply here, just a comment that I find it odd this thermostat would require hard wiring as a work around to what should be a programming function. t

    One could, I guess, setup the "on" blower selection on the thermostat to trigger the compressor to run as well, but with all the caveats that I've not fully grasped yet ;) . This blower on function on most thermostats is designed to just run the blower at low speeds - ie when painting the house or such.

    I'm sure there is a reason this is not done though...
  5. geome

    geome Member Forum Leader

    Me too. It should just trigger "Y", in my opinion. The same thermostat terminals are used for a dedicated dehumidifier, so maybe cost was the reason it was done this way instead of doing the additional programming.

    The functions to select the humidity percent and overcool range (1 to 3 degrees) are neat and it should work automatically if I could just get it wired properly. It's frustrating. Would jumping DHM2 to Y be a bad idea per my previous reply?

    I will call WF to see if they can come up with a solution if no one here can think of one. I just need to know how to get DHM2 to trigger first stage (without causing a problem with the thermostat or the Envision).

    Also, does anyone know what the 3 accessory terminals do on the Envision?
  6. moondawg

    moondawg Member

    NC, NO, and COM *likely* stand for Normally Closed, Normally Open, and Common.

    You can verify with a multimeter, but I bet that those terminals change state when the fan and/or compressor is running.... used to control power to something like an external humidifier.
  7. geome

    geome Member Forum Leader

    I think you're right moondawg - thanks. No help from the accessory terminals, unfortunately.

    This is how I see the system operating if I jump DHM2 to Y. Basically, instead of DHM2 energizing a whole house dehumidifier, it would energize Y, and start first stage. Can someone tell me if this seems correct or not? I really don't want to damage anything.

    Call for cooling:
    A connection is made in the thermostat between R&Rc and Y = 1st stage runs.

    Call for dehumidification:
    A connection is made in the thermostat between R&Rc (jumped to DHM1) and DHM2 (jumped to Y) = 1st stage runs.

    A call for cooling and dehumidification at the same time should have no adverse effects on the thermostat or the Envision unit since power comes from the same source R&Rc. 1st stage will still run.

    I would need to have the system in cooling mode (not automatic or heat) so that O is energized as well.
  8. geome

    geome Member Forum Leader

    HOLD EVERYTHING. Sometimes the dehumidification feature worked, but most of the time it didn't. I just switched the system from automatic to cool, and the system immediately turned on due to a dehumidification call, not a temperature call. I will update everyone on what happens, but the solution may be as simple as keeping the system in cooling mode instead of automatic mode. The Honeywell manual says nothing about this! :evil: Thanks to everyone that has contributed to this thread thus far.
  9. teetech

    teetech Member Forum Leader

    You have the right Idea (O is needed for cooling). You can wire it that way, but you may over cool the space.

    Also correct, this is a dry contact and can trac compressor or fan depending on dip switch setting SW2 #3. Not of any use for dehumidify.

    Basically what you are doing here is giving a Y1 cooling call whenever the stat calls for dehumidification. I don't know how well that will work, but you will soon find out. It's about your only option to run the unit with the dehumidification terminal. Let us know how it works.
  10. geome

    geome Member Forum Leader

    Just a follow up in case this helps someone at some point. The dehumidification feature seems to be functioning properly in cool mode. In automatic mode it still seems hit or miss - the thermostat may need a cooling call before the dehumidification feature will engage, but I'm not sure what's going on. I've given up on additional testing since the cool mode works well with this feature.

    Additionally, when the overcool temperature is set to 1 degree (1, 2, & 3 degrees are the available options), the thermostat does not always register a temperature change on the thermostat. This may be due to "less aggressive" cooling being set and the feature turning off before one full degree is reached. It may also be because the temperature reading when the overcool starts may be on the high side of a given temperature (i.e. 74.9 vs 74.0), so even though the temperature may be 74 when the overcool starts, it may still be 74 when it has finished. It could be a combination of these things or possibly something else.

    When I set the overcool to 2 degrees, the thermostat always registers a temperature drop of 1 or 2 degrees. This is where I have it set now.

    The one observation I made so far was that an approximate 2 degree temperature overcool reduced the humidity level by 3%. Since the temperature was reduced by 2 degrees as well, it made the house comfortable. I like the feature and wished it worked reliably in automatic mode too.

Share This Page