# Furnace shuts off due to clogged exhaust



## michaeljc70

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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## Tommy2

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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## hvaclover

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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## michaeljc70

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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## hvaclover

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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## michaeljc70

hvaclover said:


> 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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## concretemasonry

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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## hvaclover

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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## michaeljc70

hvaclover said:


> 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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## michaeljc70

concretemasonry said:


> 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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## SKIP4661

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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## yuri

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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## hvaclover

michaeljc70 said:


> 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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## michaeljc70

hvaclover said:


> 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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## OzMan

*Frozen PVC Exhaust Pipes*

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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## Marty S.

Ozman, you need to get a copy of the instalation manual. Some furnaces have a 6" max between vents, some reduce the exhaust to the next size down on the end for increased throw to avoid recirculation. Most that I have installed want the exhaust vent extended 8-10" past the intake. Each manufacturer has their own way of venting and those specific directions must be followed. As a general rule the vents can exit over a walkway of yours but not a common or public sidewalk, too much liability of ice forming on the walkway.


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## SPS-1

I put screening on my furnace and tankless water heater exhaust ( and intake ) for the same reason. I don't have a problem with condesnsation though. My mesh is fiberglass. Is your mesh perhaps steel, and possibly it freezes over more readily because it transfers heat quicker ? 

My furnace is handling it fine, but the tankless shot some dust/debis out and shut-down the unit. I took the screeing off. Only a very small obstruction would shut it down. 

I think the true answer is like Tommy2 said -- remove the grating. Then in the spring come up with a better critter trap.
I need to do that too.


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## beenthere

michaeljc70 said:


> 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.


We pull the screen out after inspection. Because customers hate to be without heat because that stupid screen causes the freeze up.


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## yuri

Park one of those fake Owls on the pipe and that will keep the critters away. LOL:thumbup:


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## hvaclover

wat going on?


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## BillBrakes

*Furnace Intake Plugged*

I live in Winnipeg, Manitoba, plenty cold here. I've had similar issues with my HE furnace and here's what I found: My pipes are 2" ABS, the exhaust not far from the intake, but both are installed correctly. I found as others have that the moist exhaust was getting sucked into the intake causing a restriction from the formed ice/frost. My intake fan could not draw enough air to begin combustion so the furnace would shut down before the burners would light. I thought I solved this by placing (upright) a piece of 2'x4' plywood between the pipes, and for the most part it's worked. Instead of 3-4 times a winter, now I'm down to being plugged once or so. I believe the culprit now is high humidy when cold. That can form a "Hoar Frost"( very white trees, etc,) and also seems to be able to plug my intake (again). Scrapping out the elbow always works, but now I'm scared to go away in winter with my house unattended. Thinking of possible trying a short run of heat tape to see if it helps. I'm a bit scared of too much heat on the ABS from the tape, anyone else try this yet??


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## Ralph_handy

*Installation instructions for the air intake vent*

When the temperature gets near zero F, the intake pipe of my high efficiency furnace gets plugged with snow-like flakes and stops. Combustion produces some water vapor. The cold air condenses the water which would gradually accumulate in the intake pipe as snow-like flakes. This has been a repeating problem - fortunately it does not get that cold every winter here. It happened a couple of times during recent cold snaps (where is global warming when you need it ). After some prodding from my wife, I got out the furnace installation instructions. My vent and exhaust pipes were not properly installed and much too close together. Today, I installed 3" PVC pipe according to the attached instructions. Tonight is supposed to be cold. Will let you know if the problem is solved.

Note: The PVC is press-fit together and not glued. May have to disassemble to clean sometime in the future.

Update: It was 3F when I got up the next morning, and the furnace was working. Looks like extending the intake vent worked!
I examined the PVC fittings later in the day. This design is more sophisticated then it initially appeared. Some snow flakes accumulated on the inside walls of the section of the T that is vertical - where the weight would cause them to fall to the ground before causing a blockage. But, none in the intake.

Science behind why the tee is critical: 
Upon further consideration, it occurred to me that the condensation from the exhaust is not the cause of this problem – at best a minor contribution. In an earlier post, the person who put a large piece of plywood between the exhaust and intake continued to have problems. After digging deep into my engineering education, I think I have the explanation including why the tee works.
The outside air during the cold winter has high humidity. This is counter intuitive because the heated air inside the house is dry. When cold outside air enters the house, it is heated. It expands and can hold more moisture – thus lower humidity.
During the daytime in the winter, the outside air absorbs water from the snow and other sources, and reaches a high humidity – often near 100%. As night falls, the outside temperature drops along with the air’s ability to hold water. The air can reach a state where it is “super saturated” with a humidity above 100%. This excess humidity is unstable, and a disturbance will cause the water to condense on any object (you may have noticed frozen dew on the grass or tree branches early in the morning before the outside air warms). 
Air sucked into an intake vent has turbulence that will cause the super saturated air to condensate on the inside of the vent. Grating installed to block critters would be particularly problematic. The ingenuous tee attracts this condensation in a place where it does no harm i.e., the condensate falls to the ground before it builds-up enough to block airflow. After the tee, the air is no longer super saturated and enters the intake without a build-up of snow-like flakes.


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## biggles

turn the intake 180 degrees and put a 3-4' length on it and a rain cap


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