# What causes a pressure switch to stay open?



## biophile (Dec 29, 2011)

My furnace stopped working and gave me an error code that the pressure switch was stuck open. After doing a little bit of research I fixed the problem by blowing in the small hose that connects to the pressure switch, the furnace has been on all day and has run with no problems. What could be the cause of this happening? I would hate to just let this go and something more serious happen in the future. 

Thanks for your help, 
Amanda


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## Doc Holliday (Mar 12, 2011)

Sediment, most likely. If you were to remove your exhaust pipe from your furnace and tap it with your hand a few times, slightly hard, you'd see some sediment fall out. That stuff builds up in the draft inducer motor and the pressure switch. 

Sticking a straightened paper clip into the port where the tube connects to on the pressure AND the port on the exhaust motor, scraping the sides of the port and back and forth, in and out, you can usually clear that debris away.


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## biophile (Dec 29, 2011)

Thank you so much!


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## Houston204 (Oct 18, 2009)

Moisture, rust left behind by moisture, venting restriction, poor vacuum hose connection, (backed up condensation if this is a condensing furnace).

I'd use a paper clip to clean that port between the furnace and hose to the presssure switch, and measure the vacuum to the pressure switch with a manometer.


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## Houston204 (Oct 18, 2009)

Doc, your name fits. You type too fast for me.:thumbsup:


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## Doc Holliday (Mar 12, 2011)

Houston204 said:


> Doc, your name fits. You type too fast for me.:thumbsup:


Pew, pew!! :laughing:


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## biophile (Dec 29, 2011)

It ran for about 9-10 hours just fine and then it shut off again. I went ahead and tried to clean it like you said but to be honest with you, I really don't think I cleaned anything out. I tapped with a screw driver also to loosen up any debris that may have been stuck. Do you think that using something that evaporates quickly (like alcohol) on a qtip and letting it dry for about 15 minutes before turning it back on would be more efficient? It's running again for now, will let you know if it shuts off again tonight. 

Thanks for your help, 
Amanda


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## Doc Holliday (Mar 12, 2011)

Never tried the alcohol thing, never even heard or thought of it before. I'm not sure if using it will do any harm or not. It could be worth a try, sounds feasible, but I'd hate to be the one to say try it and then the pressure switch fails to work ever again. 

I can't see a reason why it would not work though, alcohol does evaporate. I wouldn't use a qtip as I wouldn't want any of the fibers getting left in there. I'd just drop a few drops into the switch, after I removed it from the furnace (very easy to do, just remember-write down or take a picture- where each wire goes to, each terminal is marked NO and C), and slowly roll it around.

There are usually two screws for the mounting of the pressure switch, either 1/4" or 5/16". Nutdivers are best. 

It's your call if you want to use alcohol, I have to say that.


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## Doc Holliday (Mar 12, 2011)

ps, the replacement pressure switch I purchased today for a customer cost $47, just so you have an idea. If and when you decide to replace it you will need the model and serial number of your furnace along with any data on the pressure switch such as the .4" or .35" or whatever is on it. That is the amount of pressure required to close that switch, in water columns. Your draft inducer motor, exhaust motor, produces that negative pressure or is supposed to. 

Again, the sediment can also get stuck and usually always does in the port (where the tube connects) of the draft inducer which then does not allow the switch to close as it's blocked.

And not closing means "stuck open."


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## Doc Holliday (Mar 12, 2011)

Amanda, try this. Work on the draft inducer versus the pressure switch. Pull the tube off of the pressure switch, leaving the tube connected to the draft inducer. Suck and blow on the tube into the draft inducer. If there is any difficulty in either direction than it's the draft inducer and not the pressure switch. You should feel little to zero resistance, like blowing through a drinking straw.

Now the build up in the draft inducer port is usually tough. You can easily tell how far you need to push a small nail through to clear it without hurting anything and believe me, the sediment build up in the draft inducer can be like a brick. It is tough but it will break loose with a good shove. You'll feel it clear.

You may need something a bit sturdier than a paper clip to do this if the build up is bad.

Let us know how it turns out.


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## harleyrider (Feb 20, 2007)

biophile said:


> It ran for about 9-10 hours just fine and then it shut off again. I went ahead and tried to clean it like you said but to be honest with you, I really don't think I cleaned anything out. I tapped with a screw driver also to loosen up any debris that may have been stuck. Do you think that using something that evaporates quickly (like alcohol) on a qtip and letting it dry for about 15 minutes before turning it back on would be more efficient? It's running again for now, will let you know if it shuts off again tonight.
> 
> Thanks for your help,
> Amanda


 
How about a make and model number?


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## biophile (Dec 29, 2011)

Unfortunately, it shut off again... It's definitely not clogged with anything, it was very easy to blow through. It's gradually getting worse though, yesterday it ran for about 9 hours, the second time only about 3, and now it only will run for about 15 min before it shuts back off. Goodman Mod.# CAPF3636A6AA I guess I need to replace the pressure switch? :/


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## Doc Holliday (Mar 12, 2011)

Yes, buy a new switch.


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