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The Home Smithy
The Home Smithy, Handyman
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I have a high efficiency furnace that uses PVC as a flue exhaust.

Customer Question

I have a high efficiency furnace that uses PVC as a flue exhaust. The exhaust runs vertically through the roof, and at about 20 inches above the roof it terminates in a 135 degree PVC gooseneck. All of which is fine 9 months out of the year.   I live in Minnesota, and it gets quite cold in the winter. When it does so, the moist exhaust freezes on my roof, which subsequently creates an ice-dam on the roof, which creates water leakage into the house in the spring. What can I do besides re-routing the exhaust line?
Submitted: 8 years ago.
Category: HVAC
Expert:  tomgoe replied 8 years ago.

Hello paul arthur, ,

I have the same set up at my house but mine goes through a masonry chimney but is still coming out of my roof . On mine it is simply sticking straight up - no bends, 90's or anything . Does water get in it ? - yes but it goes down the pipe and ends up in my floor drain . I would eliminate the 135 degree and if you prefer use one 45 degree to point this exhaust slightly up and out so it disperses over a large area instead of one spot on the roof.

Customer: replied 8 years ago.
Since I was unsure of what exists at the high effiiciency furnace end of my flue exhaust pipe, it was not clear to me that I could eliminate (or replace with a 45 degree bend) the existing bend. The only other issue I can think of (besides rain getting into an open vent) is debris from birds, if for example a bird were to use the open exhaust vent as a perch.

I know that this is not a problem with the open roof vents from my sewage line, but was not sure that the same logic could apply to my high efficiency furnace.

Your thoughts?
Expert:  The Home Smithy replied 8 years ago.


Tomgoe is correct for a pipe that goes up through a chimney. The warmth from the chimney will also mitigate any iceing near the stack.

But in your case the downward bend is to keep the pipe from being clogged by its own moisture. It acts like a heat trap and keeps the ice from forming on the pipe opening. Without it the warm air will condense right at the top of the pipe and eventually plug up the pipe like sticking a cork in it.

The better solution in your case is to raise the pipe up to about 4' from the roof. Thos will allow the moisture to freeze before it hits the roof and then melt off normally like snow would.

If it continues to form an ice dam then the problem is a lack of insulation allowing the roof to be warmed. This in turn melts any snow or ice and subsiquently refreezing it that night.

Another solution would be to place a piece of 5 mil plastic over the stack with plenty of area around it to keep the moisture off of the roofing tiles. Making a frame around the plastic with some 1 1/2" x 1/4" slats will hold it in place.

Best regards, THS

Customer: replied 8 years ago.
OK, I think we're getting closer to an answer that works . . .

Here is a little more detail regarding my situation.

The vertical pipe is located within an exterior wall. That means that, where it comes through the roof, it is right at the junction of roof whose underside is in the interior of the attic, vs. roof that is the overhang and is therefore not insultated. That is the reason for the freezing when the moist air hits the cold, Minnesota roof in January.

Right now the vertical portion of the pipe rises about 20 inches above the roof. I understand why going to four feet would work . . . the condensation would freeze before it got to the roof and effectively become snow (if it works really well perhaps I can license this approach and get extra snow-making revenue for downhill skiing!).

Alterntively, could I replace the 135 degree bend at the top of my 20 inch rise above the roof, with a 90 degree bend, or a 45 degree bend. This would help a lot in terms of directing the air away from the roof rather than down toward it . . . but does have too much of a negative effect on the heat trap capability that you described? Or is this not knowable without trial-and-error . . . one conseqence of which is I might find the exhaust vent frozen over?
Expert:  The Home Smithy replied 8 years ago.

one conseqence of which is I might find the exhaust vent frozen over?


Here where I live we dont get anywhere near the cold extreams you do but we still use the bend in the pipe on our stacks. Mostly because it precludes a lot of moisture condensing back into the pipe and this has the potential for causing other problems in the future. The best thing to do with any moisture laden air is vent the moisture so that it stays out of the home.

The best soliution is as I mantioned before. Raise the pipe. If you like you can add a piece inbetween and not cement it except for one connector on the top of the bottom section coming from thr house, and another connector at the top of the extention. Then just install it during the colder months of the year. Of course wind may play a factor in that idea. It would never happen here. But then we are famous for our constantly windy conditions, our train track loop, and somewhat for our wind turbines. That and Enron but we dont mention that .


The Home Smithy, Handyman
Category: HVAC
Satisfied Customers: 9622
Experience: Handyman
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