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quantumspores 10-09-2012 09:33 AM

Goodman "pressure switch stuck open" indicator
Hi Folks,

I've got an irritating "pressure switch" problem. The furnace (Goodman GMS80904BN) will not fire up. The indicator light is complaining that the pressure switch is stuck open. I've tried blowing in and out of the tube from the PS to the fan and it is clear. You can hear it click when you blow/suck on the tube. As a test I swapped this switch with the one in my downstairs furnace, and still the complaint is the same - so it does not seem to be the switch. I can see the induction fan running. I pulled the fan out and checked it, there is nothing occluding it. The intake pipe looks clear too.

What am I missing here?


techpappy 10-09-2012 10:15 AM

clean out sensing orifices where pressure switch tube connects. Use extended paper clip or similar wire.

quantumspores 10-09-2012 10:57 AM

Tried cleaning the hole with a paperclip, both no blockages found and no change.

quantumspores 10-09-2012 11:29 AM

Now i'm slightly more confused. I decide to bypass the pressure switch. I turned to system on, ran a connector between the two wires on the pressure switch. This ignited the burners. I then removed the connection and reconnect the wires to the pressure switch. The igniters went off and the system ran normally for a few minutes, then shut off.

Makes sense to anyone?

how 10-09-2012 12:22 PM

Your pressure switch is not operating properly because either
the switch is failing in such a way that it now requires more vaccum than the ID can provide
The ID is providing less suction that it originally did.
In the field a T'ed manometer test compared against the ratings on the PS and the ID tells us which is which.
ID problems can include debris inside ID PS outlet/ restricted vent or intake/ missing ID blades (usually creates noice)/ fan loose on ID shaft/ failing ID gasket to furnace/ failing outlet restrictor plate/ exchanger crack/ failing ID motor causing reduced rpm/ run cap if it has one/ PS hose being cracked, split or too severley bent/

quantumspores 10-09-2012 12:33 PM

Considering I swapped the pressure switch for a known working switch and there was no change that would lead towards some problem with the induction blower fan. Would you agree? If so, since I can see the fan working and it appears to be fairly clean is there anything I can do to help? Perhaps grease or lubricate it?

techpappy 10-09-2012 12:53 PM

as stated the fan mounting gasket installed and leak free

switch could be failing intermittently...try continuity across switch several times by manually activating it on and off..10 times or more....did you check the hoses for cracks? just probably did..pull and clean fan and check discharge piping beyond the fan

how 10-09-2012 01:02 PM

Yes I'd agree that it's likely related to something other than the PS.

If the ID motor has oil ports, (most don't) they will be obvious.
Some people here have had success WD spraying the ID shaft when they've come across a struggeling ID motor. Usually it's just a temp fix though.

quantumspores 10-09-2012 04:00 PM

1). I checked the hose for cracks - there are none.
2). I pulled the ID off and there are no visible problems
3). when the ID is running you can hear/feel suction at the end of the hose that connects to the PS.
4). The fiberglass gasket around the ID was worn out, so I made a new one out of rubber as a test - no change
5). if I manually close the PS valve (using lip suction) the system works properly - so there does seem to be something reducing the amount of suction required.

techpappy 10-09-2012 04:39 PM

Not sure on this but, the rubber gasket may be too thick thereby increasing the clearance of the fan blades and decreasing air flow...

suggest you try existing PS on other furnace to see if it works consistently..if it does then you are back to the ID fan..if it doesn't then it's the PS

btw did you look at the discharge end of the vent pipe to see if any signs of blockage....favorite place for kids to put balls or other items...also bird or hornets nests

beenthere 10-09-2012 04:40 PM

Could be a cracked heat exchanger. or a restriction in the flue/chimney.

quantumspores 10-09-2012 06:33 PM

Just to confirm I put the PS in the second furnace and it works properly there. And still, the other PS has no effect in the failing furnace. I think I have a vacuum gauge, I'll try and determine the pressure from each furnace at various points.

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