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dmichel 02-11-2013 04:35 PM

Bryant Furnace Pressure Switch Problem
Hi All,

I have a problem with my home furnace I'm hoping someone can help me with. It's a Bryant 340MAV 036080, about 12 years old. It's not given any problems since I've been here until now.

We had a large snow storm a couple days ago, and when blowing the slow I inadvertently covered the inlet pipe. No big deal I figured, and cleared out the area around it, and made sure it wasn't plugged. Later that day I realized there was no warm air coming.

I check the furnace and had a code 31, indicating a pressure switch malfunction. I figured some snow got into the pipe, so I put a space heater in the area over night. No luck the next morning. The pressure switch is a differential switch, and in playing I realized if I disconnected the end going to the combustion chamber plenum, it would switch. So my furnace ran lots and I was hoping if it was snow related it would clear itself out. But no luck. I hook up the tube to the plenum the differential pressure drops just enough to open the switch. I check the pressure with a manometer and I have about 1"WC, and I think I should be closer to 1.8 or 2" WC.

Anyone have any suggestions on what might be wrong? Any good ideas on how to check the entire length of the exhaust piping? The only thing I'm left thinking is that something is partially blocking that.


beenthere 02-11-2013 04:40 PM

Shop vac. Turn off furnace. Then connect/place exhaust of shop vac to blow into intake pipe. let it run for 10 minutes or so. Then switch it to vacuum, and see if it draws out any water. Intakes aren't always slanted enough for water to drain out of them.

Might want to also vacuum the exhaust pipe.

dmichel 02-11-2013 04:49 PM

Thanks for the advice, the more I'm thinking about it the more I'm thinking something is blocking the exhaust. And I have a year old who likes put stones down holes......damn, now to figure out how to get them out.

yuri 02-11-2013 04:59 PM

1 Attachment(s)
Carefully use a hair dryer or paint stripper heat gun and blow heat in while it is trying to run works also. Not likely the kid put stones in if it hapened during the snow storm but you never know. Does it have a concentric vent kit, pipe inside pipe like this pic. If so the air goes in the back and you may need to pull the hood off and get in there as the ice can be in a foot or two. I have seen them recirculate the exhaust into the intake and get 3 feet of ice in there. The inlet can also get leaves/spiders webs in there. Most of those furnaces have a screen where the intake pipe attaches to the burner box that may have debris in it. Trick is to get at that screen.

dmichel 02-11-2013 06:49 PM

Thanks for the reply. Sure it enough it was rocks in the exhaust pipe. My son must have shoved them in there in the summer, and it was just borderline creating enough backpressure. The storm was a coincidence I believe. I had the cut off the pipe outside, but at least its working. Now to repair that!

jagans 02-11-2013 07:51 PM

I used a leaf blower. Blew the heck out of those pipes. Hooked back up after cutting with 3 inch fernco couplings. :laughing:

bobinphx 02-12-2013 03:43 PM

as a kid, we had an in the wall vacuum system. I found that a flashlight and a matchbox car,was way to much fun to watch as the cars disapeared into the wall.

dad didnt think it so fun when the vacuum repair guy found about 2 dozen hot wheels in the pipe systems... got my pants dusted off good for that one!!!!!!

bob in phx.

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