# gas furnace roof vent



## jimbo

my funace vent is only 12" above my roof. the pamplet at lowes said 12" was enough for a gas furnace but it's causing my ice dams in the winter by melting all the snow around it. so i want to go higher. at 12",a 4 ft rad touches the roof. a 12" extension would be about 4" above the peak of my 4/12 roof , about 6' away. 
if i add the 12"extension ,would i get too much down draft from the west wind coming over the roof ,aimed right at the chimney. or should i go 2 ft.?
also, it appears to be a class A chimney. can i /how do i add the cheaper class B into it?


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## Mitchell Ulrich

jimbo said:


> my funace vent is only 12" above my roof. the pamplet at lowes said 12" was enough for a gas furnace but it's causing my ice dams in the winter by melting all the snow around it. so i want to go higher. at 12",a 4 ft rad touches the roof. a 12" extension would be about 4" above the peak of my 4/12 roof , about 6' away.
> if i add the 12"extension ,would i get too much down draft from the west wind coming over the roof ,aimed right at the chimney. or should i go 2 ft.?
> also, it appears to be a class A chimney. can i /how do i add the cheaper class B into it?


The ice dams are probably NOT from the pipe heating up the snow where it penetrates through the roof. Extending it further up most likely won't fix that issue. The wind would have to be blowing the exhaust fumes down onto the roof at a steady pace to cause enough snow melt to produce your problem. I'd be looking at insulation issues first before investing in a change out of your venting system.

Mitch


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## redshirt32

I believe that code calls for 2 ft. over the peak if your within 10 ft of it. This is just to prevent hot embers from firing your house up. I think I agree with mitch concerning the insulation and the need. Don


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## jimbo

The ice dam is right under the chimney
if i have a 12" snow drift on the roof, & the vent is only 12",
the hot exhaust melts the snow around it,


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## Mitchell Ulrich

jimbo said:


> The ice dam is right under the chimney
> if i have a 12" snow drift on the roof, & the vent is only 12",
> the hot exhaust melts the snow around it,


\


The vent should not be hot enough to melt snow to that extent. Figure out how to eliminate the 1' drift and the ice problem wont exist. 

It seems to me that even if it were 10' over the roof, you would still have the same thing happening.


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## tinmarine

Why would you want B vent over class A? When did you replace the furnace and is this the first year this has happened? It should keep the 10/2 rule for the termination of the pipe.


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## jimbo

it's an old gas heater & it's the hot exhaust blowing out of the chimney melt the snow.
i was thinking of a class B because i think it's about $25 cheaper & B is for gas, were A is for oil & wood.
i'll try a 2 foot class A extension.


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## tinmarine

Make sure when you buy the extension, try and find the maker (American, Selkirk.....). Every maker has a different locking system and/or connection, so you'll need to match them up. You might want to look at a different pipe termination as well. I'm guessing your current one has a domed cap that probably contributes to the exhaust coming back towards the roof. The reason I asked about the change in pipe, is, yes A is classified as a higher temp pipe, it can still be used as a vent for nat gas and is much better than B vent.
Good luck, I hope it works out for you.


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## roger23

as has been said check local codes ,I just put a extension on the neighbors house,it had to be done to sell the house it was not up to code,,it is CPVC for a Hi Efficiency furnace it still has to meet code


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## N M Mechanical

they vented in CPVC?


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## roger23

I stand corrected I was just on his roof with the Bldg inspecter it is PVC,,I guess I had CPVC on my mind


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## Big Reds

My guess would be that the attic space around the pipe is being heated thus melting the snow from underneath causing the ice damming once it refreezes at the eave edge. The snow acts as an insulater for the roof from the frigid temps above the snow pack. Reduce/eliminate the heat from escaping from the pipe in the attic.


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