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Connecting Dehumidifier and A/C to single 24V thermostat

Hi-

I need to hire an electrician but first I want to see if what I'm proposing is theoretically possible, and also what parts I need to buy (the install is in a remote area so I'd like to have the parts on-hand).

Here's the situation. I have an air conditioning unit (110V) which is in the ceiling of my condo. It is in a very humid area, so management also installed a 110v dehumidifier in the same ceiling area. The dehumidifier doesn't have enough fan power on its own to really move the dry air into the room, thus it really is effective if it only runs when the air conditioner is running. What I'd like to do is have the dehumidifier only come on when the air conditioner is running (assuming the hygrometer humidity measurement indicates it needs to). I'd end up with these scenarios

1. Too warm & too humid - A/C turns on, dehumidifier gets power and also turns on
2. Too warm but not too humid - A/C turns on, dehumidifier gets power but doesn't turn on since it is not necessary
3. Cool enough but too humid - Nothing would turn on (very unlikely)

I'm thinking the 24vac from the thermostat would need to be split somehow to power both, but then I must need some relays and transformers to make the magic happen and not blow something up.

Thanks,
Bill

Comments

  • Jamie HallJamie Hall Member Posts: 12,340
    The thermostat needs to operate a couple of relays -- one to the A/C control and one to the dehumidifier. The thermostat alone, and it's power supply, wouldn't really be enough to do both.

    The thermostat would operate one relay which would turn on the A/C. The other relay would allow the dehumidifier to run, but the dehumidifier would also be controlled by its humidistat.

    Without the wiring diagrams for the two units, I can't really go farther.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • mexicalibillmexicalibill Member Posts: 3
    The dehumidifier is a Santa Fe Compact 2. The full manual is here: https://www.sylvane.com/media/documents/products/santa-fe-compact70-owners-manual.pdf

    and the wiring diagram from the manual looks like this:



    I don't know what model the air conditioner is. I do know both units are on the same 15A circuit, so it must not be anything too powerful.

    If I split the 24V from the thermostat, won't each line only have 12V? So then I'd need a 12V relay + 12 -> 24V transformer on the A/C line, and a relay + 12V -> 110V transformer for the dehumidifier? I was having a little finding those things but I might be looking in the wrong places or have the names wrong.

    Thanks,
    Bill
  • ratioratio Member Posts: 2,279
    edited December 2019
    No, it doesn't work that way.

    Also, going from the schematic you posted, that dehumidifier has line voltage controls—the humidistat switches the 120 volt power. While it's certainly possible to make them work together, you're going to need someone with controls experience. An electrician may not be familiar with what you want to happen, or know all the implications thereof.

  • HVACNUTHVACNUT Member Posts: 3,063
    This is from Aprilaire. Dont know if Santa Fe is comparable.
    Adding dehumidification to an existing distribution system.
  • Jamie HallJamie Hall Member Posts: 12,340
    You definitely need an electrician, unless you are very happy and competent working with 120 volt power circuits. What is perhaps a little more serious is that the dehumidifier is wired in such a way that you can't safely just disconnect power to it and expect it to be happy. You will need to interrupt the circuit to the dehumidistat internally with your relay. This will void whatever warranty there may be on the system... since you will be creating a new control system.

    Not that it can't be done. It can. Your thermostat would operate -- on 24 volts which it will still have -- two relays: one will simply take the place of the present thermostat circuit from the air conditioner. The other will be a 24 volt coil like the first (and the two relay coils wired in parallel) but will interrupt the dehumidistat circuit in the control board for the dehumidifier.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • mexicalibillmexicalibill Member Posts: 3
    Couldn't I use something like this to control power to the dehumidifier from the "fan" line coming from the thermostat?

    https://www.supplyhouse.com/Honeywell-Aube-RC840T-120-120v-Relay-w-Built-In-24V-Transformer
  • pecmsgpecmsg Member Posts: 1,233
    Assuming you have a small Whole House dehumidifier how is it connected? It should have a separate return and discharge into the supply duct. Its only to dehumidify and need very little air flow.
  • Jamie HallJamie Hall Member Posts: 12,340

    Couldn't I use something like this to control power to the dehumidifier from the "fan" line coming from the thermostat?

    https://www.supplyhouse.com/Honeywell-Aube-RC840T-120-120v-Relay-w-Built-In-24V-Transformer

    Kindly note my comment above. Your dehumidifier will not take kindly to having its power interrupted routinely (now and then, such as a power failure, probably OK -- but on a routine basis, probably not). The relay will work -- but must be in the circuit containing the dehumidifier switch, not the overall power circuit to the unit.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
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