I've installed a Honeywell whole house humidifier. For the time being, I've left off the air pressure switch that goes between the humidistat and the humidifier. It appears to be working fine without it and seems like just another fail point. Haven't been able to find much on the use/necessity.

Can anyone provide guidance?

  • My understanding is that the purpose of the air pressure switch is to turn off the humidifier if the furnace is not running, many times the furnace itself provides that function meaning that an air pressure switch is not necessary. What you don’t want is water running (i.e. waste) when the furnace blower has shut off.
    – Tyson
    Commented Jun 7, 2018 at 22:32

1 Answer 1


The switch is not required for proper operation. I believe it's intended as a fail safe in the event the humidifier gets turned on without the HVAC fan enabled. In this scenario, the humidifier could theoretically flood the duct. If wired properly, I can't think how this scenario would occur, but I suppose a specific type of system failure could.

  • 1
    Some times there is just a vane switch that toggles with airflow loose the air flow while the system is running and over time there will be water buildup even with the ultrasonic models.
    – Ed Beal
    Commented Dec 27, 2017 at 23:13
  • Sometimes it just tied to the power for the air handler blower.
    – fixer1234
    Commented Aug 12, 2018 at 8:28

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