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Thread: Control 2 wire humidifier with P374-1800

  1. #1
    Junior Member

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    Control 2 wire humidifier with P374-1800

    I have a P374-1800 thermostat and a Honeywell humidifier with its own 2 wire control. I am trying to find out if I can control the 2 wire humidifier with this thermostat if I buy the humidifier add-on board.

  2. #2
    HVAC Thermostat Specialist

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    Yes you can. Does your humidifier have a dedicated 24vac transformer?

    I found this on page 18.3 of the P374-1800 owners manual.
    If your HVAC unit is equipped with a humidification system and the Humidity Module (sold separately) has been installed, the thermostat will provide power to the MISC1, MISC2, or MISC3 terminal of the thermostat when the humidity in the home falls below the humidity setpoint you have chosen. The value for this setpoint ranges from 0% to 60%. If no humidity is desired or if a humidification system has not been installed, set the value to OFF.

  3. #3
    Junior Member

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    I am not sure if it has a dedicated 24 volt transformer or not. It is a Honeywell TrueSteam humidifier and has a H8908A/B humidistat, it is wired up with two wires, a black and orange, and turns on the furnace fan, low speed, when calling for humidity, even if the furnace is not running. I know the totaline thermostat will control a humidifier, but it seems to be talking about only connecting one wire. Since the control now has two wires I am assuming that it has its own transformer. Where would I connect the second wire?

  4. #4
    HVAC Thermostat Specialist

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    One way would be to install a relay. There is a thread here on this forum that will give you a idea on how to do it. If you need a diagram for your unit let us know. Here is the thread:

    This relay will work: Industrial Grade 6AZU2 Enclosed Fan Relay, SPNO, 24V

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