Pressure Switch Open

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  #1  
Old 01-17-16, 08:46 AM
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Pressure Switch Open

I have a Lennox gas furnace system that's about 23 years old. The lights indicate "Pressure Switch Open". When the system is running there are no issues. Some times it will startup on its own but when it doesn't nothing is happening, no exhaust fan turning, blower motor turning or anything. If I just remove the vacuum tube that connects the exhaust fan to the pressure switch the exhaust fan kicks on and as long as I reconnect the vacuum tube the system starts up and runs just fine. The lights also indicate everything is fine. I've checked the vacuum tube and it's clear. Should I replace the pressure switch?

Thanks!
 
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  #2  
Old 01-17-16, 09:56 AM
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It's possible the switch is bad but not likely. What's the make and model number on the furnace?
 
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Old 01-17-16, 10:17 AM
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Welcome to the forums.

Remove the hose from the draft inducer and use a strong paper clip or small drill bit to clean the tiny hole in the nipple. From years of use that small orifice becomes clogged with rust.
 
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Old 01-17-16, 01:52 PM
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OK, I did that and I'll let you know if it works. It'll take at least a week of working to make me feel it's made a difference. I didn't think that would be an issue because the exhaust fan isn't running and that fan starts up once I disconnect the vacuum tube.
 
  #5  
Old 01-17-16, 01:59 PM
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I've read that it's not likely the switch but I'm at a lose for what else it could be seeing that the system isn't doing anything except giving me the error code. It's a model 80MGFF4-75A-1 and it's a series 80MGF. I'm wondering if the pressure switch may have been switched in the past. Both bolt holes don't line up to where it's mounted.

Thanks -
 
  #6  
Old 01-19-16, 01:11 PM
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<<I'm wondering if the pressure switch may have been switched in the past. Both bolt holes don't line up to where it's mounted.
>>


Interesting observation.


It's all too common for people to change out pressure switches, and it's usually a mistake wasting time and money.

It also risks getting and installing the wrong pressure switch as you suggest, which could make the furnace operate erratically as you describe, although I'm not suggesting that is the problem!
 
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Old 01-19-16, 01:16 PM
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<<Some times it will startup on its own but when it doesn't nothing is happening, no exhaust fan turning, blower motor turning or anything. >>


When the thermostat is calling for heat, the inducer motor should turn on and stay on as long as the thermostat is calling for heat.

The thermostat should switch 24 VAC to the W terminal, and when that voltage is present the inducer motor should turn on and stay on. That might be one problem.
 
  #8  
Old 01-19-16, 04:51 PM
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Maybe a clue

The system was not working again but this time I turned the thermostat off and then checked the status lights and both lights were flashing green which means all systems are good but when I turn it back on it goes back to pressure switch open error. It's not the thermostat because we have 2 and the other one acts the same way on the problem furnace but works fine on the other furnace.
Thanks for all comments
 
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Old 01-19-16, 09:09 PM
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I didn't think that would be an issue because the exhaust fan isn't running and that fan starts up once I disconnect the vacuum tube.
I don't understand what this means.

Disconnecting the hose should not cause the draft inducer to run. The cleaning should have been done with the furnace power off.

I mentioned cleaning the fitting where the rubber lines connects to the inducer since that is a very common problem especially on an older furnace.
 
  #10  
Old 01-20-16, 07:02 AM
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Hi Pete,
I turned off the system before cleaning the nipple on the draft inducer but it still has the same problem.
Thanks for the advice -
 
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