Carrier furnace code 31, plugged negative pressure port

Old 01-06-14, 09:34 PM
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Carrier furnace code 31, plugged negative pressure port

Hi guys,

I have a Carrier furnace that's about 6 years old. I've never had any problems with it until October of this year. Tonight is the third time I've had this problem.

The plastic port/nipple that supplies negative pressure to the pressure switch gets plugged with what looks to be some kind of white scale. Not right at the switch, but where the tubing plugs into the furnace. I unhook the tube and run a piece of wire coat hanger into the nipple to bust the blockage up and the furnace starts operating like normal. But, like I said, now that it's had this problem once, it's doing it very often.

On my furnace this port is a little hard to get to, the way its situated in my furnace closet I can't even actually get my eyes on it. And to get to the back side of it I would have to tear the whole furnace apart I think. Also, I don't find any moisture in the pressure tubes when I take them apart like I would expect because of the apparent scale build up.Anybody have any suggestions on what I could do to keep this from happening again in the near future?

Old 01-07-14, 10:35 AM
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I'm not a professional, but since no one else has responded, let me take a shot.

It seems like you may have some condensate from your vent running back down to the area where the tube to the switch is. The hot vent gases re-evaporate that moisture, leaving the white scale which eventually blocks the tube to the switch. This may only run back when it is extremely cold, causing more condensation in the vent stack before the gas exits the stack.

The solution would be to either divert the condensation so that it does not run back to where that tubing connects. Or provide a drain so the condensation does not sit there and re-evaporate (leaving the scale). Or....find some way to insulate the vent pipe to keep the vent pipe warmer so the condensation does not occur in the first place. Or all of the above?

Hard to be sure what is most practical without knowing the exact configuration of your vent pipe.
Old 01-07-14, 11:06 AM
Join Date: Oct 2002
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If your furnace is a high efficiency type, there should be trap which is attached to your inducer motor and to the exhaust vent. It then runs to a drain. Check the trap to see if it is clogged and preventing the condensate from draining properly.

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