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Intermittent Furnace Operation - Pressure Switch

reins8reins8 Posts: 11Member
edited March 12 in Gas Heating
Hello All,

I have noticed an intermittent issue twice in the past week where my furnace has not been kicking on with a command for heating.

I typically hold my house at 70degF and the past two times this issue has arrised, the thermostat reading has dropped to 63degF before the furnace comes back to life. I have a one stage Coleman 80% efficiency furnace.

Upon first inspecting the furnace, the inducer motor is running and I'm getting three blinks out of the troubleshooting light. This means that the pressure switch did not close or has reopened.

During the past two instances of this issue, the furnace has come back to life after the inducer motor running for 10-15 minutes.

Things I've checked so far...

-I've checked the connections on the pressure switch. All connectors are firmly attached.
-I've checked the pressure switch hose for cracks and don't see any.
-I've removed the end of the pressure switch hose that connects to the inducer fan housing and used a wire to clear any debris that might be blocking the port, but doing this had no impact after I re-attached the hose.
-I've done a rough check of the pressure switch function by lightly sucking on the pressure switch hose, and when I do so, I can hear the pressure switch click closed.
-My water heater shares an exhaust pipe with the furnace, and when the furnace does come back to life, I don't feel much hot air coming out of the natural draft on the water heater. So, for this reason, I don't think I have any major back draft or blockage on the exhaust pipe.
-I've verified that the inducer motor is getting 120VAC

The furnace is running now and the house is back up to temp, but this is something that will likely happen again and I'd like to get ahead of it before it does.

What are your suggestions? Thanks in advance.


  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,634Member
    If you are getting ANY hot air coming out of the water heater connection you need to shut the furnace down and have things checked immediately.

    I would strongly suggest you take down all the flue piping from the furnace and water heater and make sure it is not plugged. You could have a squirrel of bird or something in there.

    You chimney needs to be checked as well for blockage and broken flue tile.

    Your best bet is a chimney sweep to check the flue pipe and the chimney.

    Then an HVAC tech to check the furnace and the air switch

    Any "HOT AIR" backing out of your water heater flue contains CO---carbon monoxide.

  • JellisJellis Posts: 110Member
    Did you check if the "pressure switch hose" was plugged?
    what you are describing points me to this being your problem.
  • icy78icy78 Posts: 251Member
    What @EBEBRATT-Ed said.

    80% furnaces use flue draft . The flue pipe and chimney need to be clear and installed correctly.
    You could also be experiencing too much draft. Or too little combustion air.
    You may also have a heat exchanger issue.
    Or draft inducer issue. Or a pressure switch itself issue. Or a water heater issue.
    Best to call somebody in that has a combustion analyzer and knows how to use it and what the readings mean. And can diagnose this for you.
  • reins8reins8 Posts: 11Member
    Thanks, all.

    To clarify things, I'm finding no hot air coming out of the natural draft on the water heater. In addition to this, I've been able to inspect my chimney by removing a sheetmetal plug at the very bottom of the chimney. After a short incline run of duct work from my furnace, my chimney is a completely vertical pipe to the roof. So, a visual inspection was straight forward and I didn't see any blockages.

    I've checked the pressure switch hose for blockages and found none.

    After finding another heating issue this evening, the only thing I've found to help coax the furnace back to life is to remove the pressure switch hose from the inducer fan box several times. Each time I disconnect and reconnect the hose I hear a click. But after doing this several times, I can get the furnace to progress through the rest of its sequence of events (igniter lighting, gas valve opening, etc).

    So, I don't think I have an exhaust blockage, and it seems that the inducer motor is creating enough vacuum to close the pressure switch (thus the click). Right now I'm thinking my pressure switch needs replacing.

    Any other thoughts?
  • ratioratio Posts: 1,936Member
    Remove the wires from the pressure switch & put your meter across the same terminals, then repeat your test of removing & replacing the hose. The resistance should show open when the switch is off & repeatably very low resistance when closed. I think you'll find that the contacts aren't properly closing electrically, even though it's mechanically closing (clicking).
  • DZoroDZoro Posts: 752Member
    The only way to be sure on the pressure switch failure is to put a magnahelic on the hose. Check the reading on it and compare to the pressure switch requirement.
    Sounds like a possible sticky switch, but you must confirm the readings also.
  • reins8reins8 Posts: 11Member
    I took ratios advice and made resistance measurements while the vacuum tube was connected/disconnected. With the inducer motor running, I’d see both open and low resistance measurements after connecting/disconnecting the tube several times.

    I replaced the pressure switch yesterday afternoon and haven’t had any issues since.

    Thanks for the input, everyone!
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