# Furnace Exhaust Freezing Up



## PerchPatrol (Mar 31, 2006)

I have an issue an a cabin with the 4" PVC exhaust form a hi-effiency furnace making icicles & gradually freezing up.

It travels quite a ways through an unheated attic before exiting the side of the house. It does slope back towards the furnace slightly, but the steam condenses and slowly and steadily builds ice at the very end of the pipe, choking off the flow. 18" sticks through the side of the house at the gable end.

The ppl I have watching over the place are taking off to florida for a month & I am now scrambling to get a quick fix in.

Last siummer I insulated the pipe in the attic in the hope that it would keep the heat in the pipe better, but it didnt help. 

I also kept the heat up a litttle higher, but I think that actually is sppeding up the ice formation.

I was thinking of wrapping the 18" of pipe that is outside the house in heat trace, then covering it with insulation to get me through March.

Does anyone have any thoughts?


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## Luv2hunteup (Mar 22, 2003)

Insulation with heat trace should solve the issue. When you get time pitch the drain away from the furnace.


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## GRUNDY (Jun 18, 2005)

Check the installation manual. There is probably a limit of unconditioned space that the vent pipe can run through, or it may not be recommended to run horizontally through unconditioned space. its been a while since I've been out of the trade.

The guide will help you make the best decision on how to remedy the problem.

B


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## miko450fish (Dec 21, 2010)

Maybe heat tape on the pipe?


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## SCOOTERBUM (Dec 10, 2014)

Why not go vertical?


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## GRUNDY (Jun 18, 2005)

Here's an exceerpt form some random 90% install guide.

"When the vent pipe is exposed to temperatures below
freezing, i.e., when it passes through unheated spaces,
etc., the pipe must be insulated with 1/2 inch (22.7 mm)
thick Armaflex-type insulation or equal. If the space is
heated sufficiently to prevent freezing, then the insulation
would not be required. If domestic water pipes are
not protected from freezing then it is assumed the
space meets the condition of a heated space."


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## grapestomper (Jan 9, 2012)

I am pretty sure the pipe is suppose to slope to the furnace. 
Maybe the last little bit doesn't matter but I was told by a furnace guy 
the pipe slopes back to the furnace.


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## PerchPatrol (Mar 31, 2006)

Thanks everyone,

The pipe does slope toward the furnace as it supposed to. 

The insulation I wrapped th epipe with was the pink stuff I cut into strips and tie - wrapped around the pipe.

We shall see.


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## Tracker83 (Jun 21, 2005)

I would think your 18" could be reduced. That would help some. I know on my house they only stick out about 8" past the wall. I would shorten it to the minimum allowed by code.


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## SCOOTERBUM (Dec 10, 2014)

SCOOTERBUM said:


> Why not go vertical?


.....?


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## grapestomper (Jan 9, 2012)

Checked mine at lunch time. Sticks out 7-8 inches from the side of the house.


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## kracker (Jan 25, 2013)

The exhaust pipe has to have a 1/4" per foot pitch down towards the furnace. The pipe should not stick out of the building more than a foot. I would also check into the size specs for the exhaust pipe.....4" seems big. 3" pipe is usually the biggest ever needed but different manufacturers have different specs.....in 20 years in hvac, I have never seen 4" pvc used. All furnace vent pipes in unconditioned spaces need to have armaflex pipe insulation on it.


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## Petronius (Oct 13, 2010)

grapestomper said:


> I am pretty sure the pipe is suppose to slope to the furnace.
> Maybe the last little bit doesn't matter but I was told by a furnace guy
> the pipe slopes back to the furnace.


I have a Carrier Weathermaker 9200. My exhaust pipe slopes slightly back to the furnace also and only sticks out of the side of the house about 8" with the cap on it. Condensation runs back into the furnace and drains out through the drain hose. I had an issue with this a few days ago. The water is supposed to collect in the bottom of the housing surrounding the induction fan, then drains into a plastic trap before draining out. There is a pressure switch that controls the operation to allow the furnace to fire up. When water builds up or the exhaust pipe gets plugged (snow, ice or debris), the furnace light near the bottom of the furnace starts blinking a 31 code, three short and one long blink. The furnace then does not operate properly. In my case, the plastic trap (which helps keep pressure in the system) got plugged with junk (things grow in the water over time and you get a slimy buildup). Once I figured this out, I cleaned the trap and made sure the hoses were clean. There is a hose in my furnace to allow you to pour a little bleach in the system to keep it clean. The furnace runs great now.

The further away the exhaust pipe is from the house, the easier it is for the water vapor to freeze. If it clogs up the pipe, intake air and exhaust can not flow through and it will cause the furnace to stop.


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## Hauptmann6 (Oct 19, 2012)

is there a place that would be a shorter run for it to get outside? 

I am assuming your furnace is in the attic. Correct?


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## ESOX (Nov 20, 2000)

The exhaust pipe must be positioned or terminated in such a manner that the heated, vapor laden exhaust air cannot simply rise and reenter the house through a soffit intake vent. That could cause potential problems with both CO, moisture issues and mold.


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## Fabner1 (Jan 24, 2009)

If the inside of the pipe is black with soot check the heat exchanger for cracks.

Old


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## PerchPatrol (Mar 31, 2006)

Hauptmann6 said:


> is there a place that would be a shorter run for it to get outside?
> 
> I am assuming your furnace is in the attic. Correct?


Furnace is on main floor of house - on inside wall. They had to go up through ceiling, then across attic to exit at gable end. A shorter run would have been to the "front" of the house, but that is the lake side, and more windy. Maybe they went the other direction because of that reason.


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## PerchPatrol (Mar 31, 2006)

I went up there Saturday and wrapped the end of the pipe that sticks out with heat tape, then insulation. Maybe it would have worked by only insulating it, but I didn't want to risk it. 

Someone from the area went over there this AM and told me that there was no ice, so the heat tape must have worked. 

The pipe sticks out 18" - maybe because the eaves stick out that much as well, though they are a bit above the vent pipe. 

I was hesitant to cut the pipe back a few inches, but I think that would have worked to prevent freezing.

I am so glad to finally not have to worry from afar about this thing.

Thanks for all the input everyone.


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## JUSTCATCHINUM (Feb 19, 2004)

PM sent.


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## riverman (Jan 9, 2002)

Friend in the business for over twenty years says cutting the exhaust pipe at a 30 or so degree angle will eliminate that problem most of the time.


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## PerchPatrol (Mar 31, 2006)

riverman said:


> Friend in the business for over twenty years says cutting the exhaust pipe at a 30 or so degree angle will eliminate that problem most of the time.


Good Idea!

The "longer" side on top? 

ie. cut at an angle from the top of the tube toward the house?


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## riverman (Jan 9, 2002)

PerchPatrol said:


> Good Idea!
> 
> The "longer" side on top?
> 
> ie. cut at an angle from the top of the tube toward the house?


yes


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## Petronius (Oct 13, 2010)

riverman said:


> Friend in the business for over twenty years says cutting the exhaust pipe at a 30 or so degree angle will eliminate that problem most of the time.


My exhaust pipe runs inside the intake pipe starting from inside the house until it ends outside. Also, it has a cap on the outside that holds the two pipes together.


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## PerchPatrol (Mar 31, 2006)

petronius said:


> My exhaust pipe runs inside the intake pipe starting from inside the house until it ends outside. Also, it has a cap on the outside that holds the two pipes together.


 
Mine are side by side about 1' apart or so. 4" pvc. They exit the house about 12-15' off the ground, after going across the unheated attic.


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## Forest Meister (Mar 7, 2010)

I had the same problem a year or two after my high efficiency furnace was installed. We had an unusually cold stretch of weather and the pipe would ice up a couple times a day. My furnace man came back multiple times and checked everything. Two or three times he increased the angle of the pipe back to the furnace. When all else failed he went outside with a hacksaw and cut about 6" off the 18" that was outside the house. That was almost 20 years ago and I have not had any trouble since. FM


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## PerchPatrol (Mar 31, 2006)

Just an update -

Looks like the heat tape worked - no more icicles!

I think this summer I will try cutting the pipe at a 30 degree angle as was suggested.

Thanks for the ideas everyone!


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