Furnace pressure switch, how to tell what connections go where?

Discussion in 'HVAC Heating & Cooling' started by plumb krazy, Jan 11, 2021.

  1. plumb krazy

    plumb krazy New Member

    Mar 18, 2020
    Rural Utah
    I have been trying to learn more about the furnace in our house.

    I have had a few techs come out and they have opined that the pressure switch connected to the combustion blower may or may not be giving intermittent problems. The furnace does occasionally display a code indicating that the pressure switch is stuck, but after the testing is done the switch appears to operate and the furnace will work again for a couple weeks.
    I have asked several times, if they would just please put a new switch in, and they say "well we would have to order that" and "call us if it stops working again".
    We live in a very remote area, and I am not trying to complain etc. This is just the way things happen out here.

    So, to make a long story short I ordered a pressure switch and would like to give it a try. The switch was one of the many "replaced with" equivalents that was listed by the furnace manufacturer, and it has a slightly different form factor regarding the electrical tabs and no documentation.

    My question; is there a formal way to determine which connections go where?

    I have a wiring layout for the furnace, and the switch itself has a Normally Open and Common tab.

    It seems as if many of the techs regard the connections on the SPST with the casual assumption that either electrical configuration will work in the Open/Closed operation but it seems to me that the manufacturer would have had a more specific intention.

    My intuition is to connect the +24vAC lead to the Normally Open connection and the return lead to the Common connection on the Pressure switch.

    Do you have any thoughts?

    Thank you.
  2. Reach4

    Reach4 Well-Known Member

    Sep 25, 2013
    There are only two terminals on the old and new pressure switch? In a normal switch, it does not matter which terminal is which. It is possible for a sensor to have some polarity difference, but not a simple switch.

    Try checking the ohms between the terminals. Is it near zero as it sits? In a simple switch, the resistance between terminals will be the same when you reverse the meter leads.

    If you blow or suck the air port with your mouth, does that resistance change? I am not a pro.
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  4. plumb krazy

    plumb krazy New Member

    Mar 18, 2020
    Rural Utah
    Thank you for the reply.
    Yes, it is a simple open closed switch and it will work with the connections made in either configuration. I am asking if there is a best practices hookup configuration.

    The diaphragm mechanism holds the switch open until a pressure differential is sensed and the switch closes to complete the circuit.

    Thank you!
  5. fitter30

    fitter30 Well-Known Member

    Feb 2, 2020
    Retired service tech
    Peace valley missouri
    Blow gently in each port with a short piece of hose can hear the switch make. Take a pic of the wiring diagram.
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