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Old 12-22-2008, 09:40 AM   #1
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Furnace shuts off due to clogged exhaust


When it is very cold, the exhaust PVC pipe (the one that comes out from the burner compartment) on my Armstrong high efficiency furnace gets clogged with snow/ice. I guess the condensation freezes. This causes the furnace to shut off until I go outside and clear it and reset the furnace.

This happens when it is usually below 10 degrees outside. There is a grate shoved in there to keep birds/rodents out. It seems like that makes (or even causes) the problem worse as the ice starts to form on the grid of the grate. Any ideas?

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Old 12-22-2008, 09:54 AM   #2
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Furnace shuts off due to clogged exhaust


Id take off the grate then. Just put it back on at the end of the season. No birds or anything are going to be moving in this time of year..
Just my .02

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Old 12-22-2008, 10:00 AM   #3
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Furnace shuts off due to clogged exhaust


Does the exhaust elbow exit the home at the outside and travel up the outer wall more than a foot? If the outside termination riser is too high it must be insulataed.

Another culprit could be the point it exits the home. If there is no slope towards furnace water could be collecting there and freeze only when it done close to single digits. IF its just freezing at the PVC termination opening I would take a close look at whether or not the screen is the root cause.

My line of furnaces call for only a screen in the intake.

Another thing you can due (assuming you have two inch PVC right now) is to increase the PVC to three inch at the termination.

But answer this: How high is the termination above the snow line.

Is the termination close to a corner? (air currents could cause he condensate to bounce back on to termination opening and freeze).

Is the termination pointing away from the home or towards the the ground?

Describe how many elbows and how long the PVC pipe runs are.

I'll wait for you r response.
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Old 12-22-2008, 10:01 AM   #4
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Furnace shuts off due to clogged exhaust


I've been hunting around the web and several have said that winter is when rodents/birds will try to get into a warm place. That is my concern.

To clarify, the exhaust is an upside down J PVC pipe. I would guess it is a 4 inch pipe. The grate is wire grid with the squares maybe the size of a pinky finger.
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Old 12-22-2008, 10:05 AM   #5
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Furnace shuts off due to clogged exhaust


Four inch is not common. But how long is the PVC from where it exists the home to the elbow where the fumes exit?
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Old 12-22-2008, 10:07 AM   #6
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Furnace shuts off due to clogged exhaust


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Does the exhaust elbow exit the home at the outside and travel up the outer wall more than a foot? If the outside termination riser is too high it must be insulataed.

Another culprit could be the point it exits the home. If there is no slope towards furnace water could be collecting there and freeze only when it done close to single digits. IF its just freezing at the PVC termination opening I would take a close look at whether or not the screen is the root cause.

My line of furnaces call for only a screen in the intake.

Another thing you can due (assuming you have two inch PVC right now) is to increase the PVC to three inch at the termination.

But answer this: How high is the termination above the snow line.

Is the termination close to a corner? (air currents could cause he condensate to bounce back on to termination opening and freeze).

Is the termination pointing away from the home or towards the the ground?

Describe how many elbows and how long the PVC pipe runs are.

I'll wait for you r response.

The pipe exits the house about 3 feet above the ground. I believe it is a 3 or 4 inch PVC pipe. It is basically an upside down J so it points downward. It is on the side of the house not near a corner. I would say the PVC goes up about 7 feet from the furnance before exiting the house. It is a pretty straight run of PVC (I am at work and at least that's how I remember it).

Thanks
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Old 12-22-2008, 10:13 AM   #7
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Furnace shuts off due to clogged exhaust


A couple of observations based on my son's probelms a couple of years ago. His exhaust was "plugged" in cold weather. He found a couple of things.

1. His "horizontal" exhaust run in the house was not installed correctly and had a sag. The low area filled with condensation and cut down the ability to exhaust. It was a long run inside and he had to remove the ceiling drywall to correct the sag. It was not frozen, but in very cold weather, the length and lower temperature of the rooms it ran through cause some condensation that could not drain and was trapped. The more trapped, the more weight and the greater the sag.

In doing the repairs, he discoverd a dead "critter" that may have entered before the heating season.

2. He put a "critter" cage that was not jammed inside, but projected out beyond (3" to 6") the end of the face that did not collect much frost. When the days get shorter the critters look for a home before the heating systems are on and they can get anywhere if there is no protection.

Dick
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Old 12-22-2008, 10:24 AM   #8
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Furnace shuts off due to clogged exhaust


A "J" would indicate two 90* elbows pointing the ground if I am understanding you. If that the case the exhaust could be "bouncing " back up and refreezing on screen.

Do you have one or two pipes going our? Are you sure you are not mistaking the the intake air for the exhaust?
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Old 12-22-2008, 11:16 AM   #9
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Furnace shuts off due to clogged exhaust


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A "J" would indicate two 90* elbows pointing the ground if I am understanding you. If that the case the exhaust could be "bouncing " back up and refreezing on screen.

Do you have one or two pipes going our? Are you sure you are not mistaking the the intake air for the exhaust?
You are correct that there are 2 elbows in order to get the pipe pointing downward on the outside of the house.

There are 2 pipes. There is one that goes to the inducer. the other goes into the area where the igniters are. The one that is clogging is the one coming/going to the igniter area. I believe it is the exhaust, but am not positive.
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Old 12-22-2008, 11:35 AM   #10
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Furnace shuts off due to clogged exhaust


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In doing the repairs, he discoverd a dead "critter" that may have entered before the heating season.

2. He put a "critter" cage that was not jammed inside, but projected out beyond (3" to 6") the end of the face that did not collect much frost. When the days get shorter the critters look for a home before the heating systems are on and they can get anywhere if there is no protection.

Dick
I was trying to think of what will keep critters out but not allow ice to form. Maybe something like fishing line strung in a star-like pattern on the end of the pipe?
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Old 12-22-2008, 03:24 PM   #11
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Furnace shuts off due to clogged exhaust


The pipe you are referring to is the intake pipe. I suspect it is recirculating some steam that is being exhausted. The intake pipe should have an elbow pointed to the ground. The exhaust should extend straight out past the intake pipe so it won't recirculate.
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Old 12-22-2008, 06:44 PM   #12
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Furnace shuts off due to clogged exhaust


What is the make and model # of the unit. Different manufacturers vent their units differently. Post some pics of it and we can help better.
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Old 12-22-2008, 11:32 PM   #13
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Furnace shuts off due to clogged exhaust


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You are correct that there are 2 elbows in order to get the pipe pointing downward on the outside of the house.

There are 2 pipes. There is one that goes to the inducer. the other goes into the area where the igniters are. The one that is clogging is the one coming/going to the igniter area. I believe it is the exhaust, but am not positive.

I am thinking that also You are recirculating the exhaust into the intake.

The PVC pipes are probably too close together and need to have some distance put between them.
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Old 12-23-2008, 01:30 PM   #14
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Furnace shuts off due to clogged exhaust


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I am thinking that also You are recirculating the exhaust into the intake.

The PVC pipes are probably too close together and need to have some distance put between them.

So, the pipe connected to the inducer is the exhaust? I would have thought it was the other way around.

The exhaust and intake are not close to each other. They are maybe 3-4 feet apart.

The only other thing close is the hot water tank exhaust. That points in the other direction though. One is down to the left, the other down to the right.

The furnace is an ultra V advantage 93 by Armstrong high efficiency.
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Old 01-09-2010, 09:03 AM   #15
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Furnace shuts off due to clogged exhaust


I was reading the threads posted and have a similar problem. We have a house that we have had a few years now. We have two 2" PVC pipes on the South side of our house. Intake and exhaust. They are about 16" apart and about 3' above the ground. When we got the house some one had broken the longer pipe and it had sharp edges. We had concerns with birds or wasps nesting, so we cut off the broken pipe about a foot from the exterior wall and then put sweep elbows on both pipes. We never had any problems until this winter when it has been exceptionally cold and we have had more snow. Twice in the past 2 months the intake has frozen and choked the furnace.

I read that the exhaust should be straight not elbowed, which does make sense now. Any ideas about the safety issue of having a pipe sticking out over the sidewalk? Nesting concerns?

Also from reading your threads, i am wondering if we have some water build up in the pipe down by the furnace. I should ad this is a basement furnace normal air flow. There is not a floor drain, but we have added a condensation pump, which seem to be working fine. How can we tell if there is water buildup in the exhaust vent...from the furnace up approx 6', elbow, then approx 6' outside, and the sweep elbow on the bottom.

Dorothy and Toto are getting cold...any help is much appreciated.

OzMan

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