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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.
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
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 .