Furnace pressure switch question


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Old 05-18-16, 04:24 PM
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Furnace pressure switch question

I'm having problems with my gas furnace starting. To check the pressure switch I've tried sucking on the inlet port. I can hear it click and it does work but shouldn't the switch keep its suction? Once I create a vacuum (by sucking) I'll then place my tongue over the inlet. Shouldn't the vacuum remain? It does not hold vacuum very long. Is this a sign of a faulty diaphragm in the pressure switch?

Thanks for any help that you can give..... STEPFORWARD
 
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Old 05-18-16, 07:25 PM
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Welcome to the forums.

I think most pressure switches will bleed the vacuum so in the case of a plugged line or inducer fitting..... the switch is not kept closed.

What is the problem you are having ?
Pressure switch fault code ?
 
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Old 05-19-16, 05:20 AM
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Yes, most pressure switches I've tested bleed the vacuum. It's a safety feature. They must see a constant suction in order to keep the switch open/closed.

Quite often I don't find a problem with the switch itself. Often it's an obstruction in the tubing leading to it. Sometimes it's a hard blockage and other times water in the line which can cause intermittent operation.
 
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Old 05-19-16, 08:10 AM
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Thanks guys for your responses. I have a 90% efficient furnace.

The error code is blinking 3 times. The problem is the furnace induction motor starts but there is no ignition. When I take off the exhaust line from the induction motor then the furnace will ignite.

I was hoping its not in the exhaust pipe. The pipe is not a straight shot to the exterior wall. It is long and has many bends in it. I've checked the end of the pipe and it looks good.

I also checked the rubber hose from the induction motor to the pressure switch. I did cut the two ends to make sure there is a good seal. Hose is good. I took a plastic straw (one that comes with a spray can like wd40) and I then ran it through the inlet hole where the pressure tube connects on the induction motor. It went all the way in with no problem hitting the blades of the motor. There seems to be no blockage.

Any suggestion? Other than the exhaust pipe? If not ... what's the best way to clean or check out the exhaust?

Thanks again for your responses, any other suggestions will be appreciated.
 
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Old 05-19-16, 08:24 AM
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Regarding the exhaust line, you could try disconnecting from the inducer and use a shop vac to apply suction. Usually best to do it from the outlet side because if a bird or critter built a nest it will usually be closer to that end. If there is an obstruction you can usually tell from the sound of the shop vac. Just applying suction may clear a minor obstruction. If it seems there is an obstruction and the vac didn't clear it, you can try compressed air from the other end (if you have a compressor). After that your options are a snake of some sort. Having a lot of bends in the exhaust is not really ideal, but there may be no way around it. You may have to cut the pipe to get at the section that's plugged.

If it seems the exhaust line is clear, you can suck a small piece of rag tied to a light string through with the vac to be sure, but be I'd be a little leery of trying this with a lot of turns in the pipe.
 
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Old 05-20-16, 11:02 AM
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Thanks Carbide for your input. I ordered a new pressure switch even though I have my doubts on whether the old one is good or not. Being that its now summer and I don't need the heat right now I'm going to try the pressure switch even tho its a long shot.

I'm afraid cleaning the exhaust pipe will become quite a chore and I'd like to avoid that if I can. The pressure switch is under $20 so I'm thinking what the heck......lets give it a shot first. But if needed I'll definitely go with your suggestion on using a shop vac.

I'll post again here with an update after I get the part in (coming by mail), and installed.

Thanks all for you help ..... much appreciated. Glad I found this site.
 
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Old 05-25-16, 01:32 PM
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It is very unlikely that the pressure switch is the problem.


The pressure switch checks to verify that the combustion air and exhaust systems of the furnace are working properly. That can be caused by a variety of different problems, and can require significant skill to eliminate possible problems until the actual problem is revealed, and then correct that.

Frankly, I'd recommend AGAINST replacing the pressure switch. You can easily get the wrong one, which will cause more problems, and in any case it's a waste of time and money.
 
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Old 05-28-16, 01:57 PM
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Before you do anything, try by passing the Pressure Switch, connect the wire from the pressure switch to the wire going to the Control. If it works and ignites, then its the Pressure switch.
 
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Old 05-28-16, 03:15 PM
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As confirmed back in post 4..... it's a pressure switch problem.
 
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Old 05-29-16, 03:35 AM
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Where did you find a Pressure swictch for $20.00, I may need one .
 
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Old 05-29-16, 02:16 PM
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Just an update here. First I'd like to thank everyone who responded and gave me good advice. I'm sure I would have never figured out my problem w/o your help and this site.

I was able to locate a video camera from a auto mechanic. The type that has the monitor with a 3 foot cable attached to the camera. I was able to go into the exhaust pipe and around the 90 degree elbow ..... sure enough there is a nest there. In the 17 plus yrs living in this house never seen that before. I had my doubts but sure enough that seems to be the problem.

I guess I now have an extra pressure switch on hand. But that's okay, its allot cheaper than calling in a furnace man to do the job. Thanks again all for the great help and communication.
 
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Old 05-29-16, 02:21 PM
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Bilhar .... the switch i found was on ebay through a Co. called ... "Sellersburg Heating & A/C Inc."
 
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Old 05-29-16, 04:49 PM
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Bilhar..... to date.... I have never needed to change a pressure switch. They are extremely reliable.
I have spent a few dollars and bought a manometer. This is a gauge for measuring vacuum and enabling servicing of the system without parts swapping.

Stepforward... thanks for the update on your furnace problem.
 
 

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