Furnace pressure switch playing up

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Old 12-02-14, 10:11 AM
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Furnace pressure switch playing up

Hiya,

New to the forum and first time poster here, looks like a handy place to have saved in the bookmarks!

First time working on a furnace too, before yesterday I'd never looked at one as they're not common in NZ, so forgive me if I get the terminology or whatever wrong. I woke up to a room of 10 degrees so, popped the covers off the furnace and started digging through manuals online.

It's a Lennox ML193UH, and I'm 99% convinced it's the pressure switch playing up. I checked vents and condensation trap which were all cleared and still couldn't get it to light, then tried jumping the pressure switch terminals and it fired and ran normally, until I removed the meter where it flamed out. I pulled the pressure lines and blew on positive, sucked on the negative, both trip the switch soundly and confirmed this with continuity check. I noticed air leaking from the switch end of the positive hose, not much but enough to be obvious, so I reseated the pressure line and assumed this was the fault, connected it all back up and it fired up and ran normal. I ran the furnace with the meter connected and the air inducer does indeed now trip the switch.
However.. Once it got the house warm it stopped then I noticed the temp drop again, it seems it didn't fire up.

I don't know what I did exactly, a bit more poking around with the meter, maybe just bumping it did it, but I cycled the power anyway and it lit up and has been functioning normally since. Multiple starts and stops over 24 hours now. I'm just not sure why it stopped that one more time, and wondered if there's anything else I should do or check?

I had another question just out of interest, would the furnace stop if the exhaust vent became blocked? As far as I understand that pressure switch it is only detecting the function of the air inducing fan. There's no carbon monoxide detectors in the house, something I'll mention to the land lord probably.

I did try calling the heating company first, they said they couldn't get out here until next week earliest. It's been as low as -20C this week so that wasn't going to cut it
 
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Old 12-02-14, 10:42 AM
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Welcome to the forum!! One thing you can try is to remove the pressure switch hose where it connects to the inducer motor port. The small hole in that port gets clogged up with gunk and can prevent the pressure switch from closing properly. You can use an opened paper clip to ream it out making sure it is clear all the way though. Also, make sure there are no cracks or splits in the pressure switch hose.

I had another question just out of interest, would the furnace stop if the exhaust vent became blocked? As far as I understand that pressure switch it is only detecting the function of the air inducing fan.
Yes, a blocked vent or air intake pipe will definitely cause the pressure switch to open and shut down the gas valve. Do you see any red and green lights flashing on the control board that will indicate a possible fault?

Here are links to some info that may be of assistance to you:

https://www.lennox.com/pdfs/installa...L193UH_IOM.pdf
http://www.heatingandairparts.com/do...%20SERVICE.pdf
 
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Old 12-02-14, 12:42 PM
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I blocked the end of both pressure hoses and blew on them they are sealed, just had that small leak where it fitted over the nozzle which is gone now. I'll poke a paper clip through the nozzles, there's nothing behind them I can damage?

Yes, a blocked vent or air intake pipe will definitely cause the pressure switch to open and shut down the gas valve. Do you see any red and green lights flashing on the control board that will indicate a possible fault?
The two nozzles are right next to each other it just didn't seem like the exhaust could have any influence on the pressures when the ports are on the intake side? I guess if it blocks the pressure backs all the way up to there though?

I've looked through both of those documents already - thanks though. There was an LED error code, the slow green flash with no red, listed 4 potential problems pressure switch being one of them which led me to the problem.
 
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