I recently noticed the whole house humidifier (installed by the previous owner) stopped working (Honeywell 280A). Filter dry, no flow, no click from the solonoid valve.

I did some investigation and figured I had a bad solonoid valve.

After installation, I noticed a bad smell (burning plastic / electric) that seemed to be coming from the board in the housing next to the air pressure switch. After looking closely, one of the components looked toasty.

I did tons of searching to try and find this part but no joy.

After getting really frustrated, I finally wondered.. what the heck does this board do anyway? And do I need it? after all, the humidostat and air pressure switch are just a series of switches that need to be closed to start water flow.... (right?).

So I omitted the board, and simply put the humidostat and air pressure switch in series and... everything seems to work great.

My question is: - What function does this board fulfill? - Why can I not find it anywhere for sale? (Every offering for an air pressure switch only includes the mechanical air pressure switch, and not this board). - Am I a fool to run the system in the (simplistic) way I have it set up currently?

(Pic of PCB attached). Thanks in advance! --Dave HoneywellAirPressureSwitchMysteryPcb

  • 1
    That is an awful lot going on -- in order to answer the question, we'll probably need much closer-up views of both sides of the board Commented Mar 12, 2018 at 2:48
  • Yeah, I realize I didn't give a lot of detail about components on the board or layout.. I was more wondering if some kind of control circuit was common in a setup like this. From what I can tell, the air pressure switch is usually sold as a "dumb switch", but mine seemed to have this control circuit. Notably, the model I have brags about low water consumption. I wonder if this circuit has some timing/interval kind of functionality that reduces the amount of time the valve is open?
    – pettheory
    Commented Mar 13, 2018 at 18:35

1 Answer 1


The board is the device that inturpets the humidity levels from a sensor and turns on the water solenoid, most units also have a flowswitch to turn the system off if there is no air flow. I would want at least an air flow switch controlling the water valve so the system is not flooding the duct work when no air is flowing , I know it is very small amounts of water but over time it candbuild up. Other than flooding the duct I do not think it will cause a problem to bypass the board.

  • Thanks for the response. Does the air pressure switch not perform essentially the same function as a flow switch? I'm wondering if this board adds some timing function or perhaps switches the solonoid on/off a few times instead of just leaving the solonoid valve open the whole time that air is flowing?
    – pettheory
    Commented Mar 13, 2018 at 15:31
  • It should be a combination of pressure/flow and a humidity level sensor you don't want the inside of your home to be a rain forest.
    – Ed Beal
    Commented Mar 13, 2018 at 18:44

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