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I'm looking control a 120 V device with an HVAC thermostat.

The device is a scrubber that cleans air and should\must only run when the HVAC is running (running in the cool state, heat state, or just fan state). The air scrubber is a 120V device. It is currently connected to mains via a SPST relay (image included below).

To activate the device, a home automation system determines if the smart thermostat is in an ON state (fan, cool, heat) and closes the relay (its a NO relay), activating the device.

I have a need to change the thermostat which will remove the ability for the home automation system to know if the HVAC is running and close the relay.

To rephrase my question: Is it possible to accomplish this by wiring the NO SPST relay to the thermostat wiring directly? Since the thermostat will only shunt one contact per mode (example R to W, or R to O) do I require a different type of relay?

Very rough sketch of how it is currently connected. Current Wiring from fan/scrubber to HA system

Original Question

The thermostat has a red, white, yellow, green, blue wire. If I understand the red is 24 V, white is heat, yellow cool, green fan, blue common.

The 120 V device is connected to a relay such that i now have a black and red wire. When i close the circuit between the two wires it activates the relay turning on the 120 V scrubber.

Where I am having trouble is how i can correctly wire this into the thermostat?

  • Connect the black (relay) to the common.
  • Connect the red (relay) to the green (fan).
  • Will the thermostat energize green if the fan is in "auto" and the unit calls for cool, or does it only energize the white leg?
  • How do I connect the red(relay) such that the relay enables when the thermostat calls for heat, cool OR fan?
  • Do I need a different type of relay? A multi-input - single output relay? (Is this a thing, what is it called?)

Appreciate input. Trying to complete a task (and learn).

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  • Can you provide a copy of the wiring diagram for your air handler?
    – JACK
    Jun 23 at 16:46
  • Can you provide a wiring diagram for your scrubber and relay?
    – JACK
    Jun 23 at 17:03
  • How much power does the scrubber use? What are the specs on the current relay? Jun 23 at 17:08
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    I would suggest NOT experimenting, unless you like to buy new thermostats and furnace / A/C control boards LOL. It sounds like your thingamajig supplies its own power to the red+black wires, and that is NOT compatible with standard thermostats, so I don't know what the designers are thinking, LOL. Is this thing homebrew or Chinese? Jun 23 at 22:11

2 Answers 2

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The problem is that your gadget is not designed to work with normal North American thermostats and furnace controls. It is actually supplying its own voltage on red and black, at ??? who knows voltage, might be 120V or even 240V. We can't just tie it into thermostat wiring, or it would fry everything, or this rogue power source could actuate devices. Your fan won't shut off or your A/C won't stop running. Bad design, not indended for North American HVAC systems. Is this thing even UL Listed? Shouldn't be wired to our power if it's not.

Get 2 UL listed SPST relays such as RiB designed to junction box mount, that have 24V coils. Enclose all of this in proper enclosures per NEC. No science projects. No "clean it up someday".

On one relay, connect coil to W and C.

On the other relay, connect coil to G and C.

On the contacts of the relays, parallel them to the black and red wires to your device.

One relay will pick up during furnace Call for Heat. The other relay will pick up during thermostat Call for Fan. This also means the cleaner will work if someone flips the thermostat "Fan Only" switch to the on position. If you don't think that's a cool feature, connect the relay to Y instead of G.


What's the deal with that?

Thermostats have a simple logic.

  • When heat is desired, they shunt R to W. (W=call for heat.)
  • When the switch is placed "Fan Only", the thermostat shunts R to G. The thermostat does not do this when heat is called for, because the furnace has internal logic of when to turn its own fan on, i.e. after the heat exchanger has warmed up so you don't get an initial blast of cool air, and so the after-shutdown remaining heat is fully distributed and not wasted.
  • When chill is desired, the thermostat shunts R to Y and also R to G. Why does it do this? Because the furnace has zero awareness that the air conditioner even exists. The A/C system relies on the furnace's air handler, since the condenser sits above the furnace in the air handling stack. Thus, if nobody told the furnace "run the fan", it wouldn't!
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  • For the OP -- a RIBTU2C is the part they need, as they need high voltage/low voltage separation inside their relay box Jun 24 at 2:15
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The fan (air recycler) is always on in any HVAC mode (heating or cooling or Fan only).

You can tap off your 120V from the air recycler fan, this might be a problem with power consumption.

If the power consumption from your Air cleaner is too much ?, use a 24 V Relay connected to the fan controller relay to switch separate 120 V for your Air Cleaner.

All of this does not require wiring change from the Thermostat, it is already there.

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