Furnace roof vent

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MIKEMCD77

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I have a 6" diameter roof vent for my gas furnace. I want to replace the vent. Is it OK to replace it with a 4" diameter vent? Here is the info on my furnace:

XE-90 gas furnace; Trane (American Standard) “high efficiency.”
Mine is a Downflow and “NON-direct vent”; 74,000 BTUH capacity output; ICS (AFUE) 91.0
Furnace model # TDC080C942B4
 

Dana

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Almost all condensing gas furnaces with 80,000 BTU/hr burners use 2" PVC venting for both the intake combustion air and exhaust.

If vented into a metal exhaust stack it has to be stainless due to the mildly corrosive exhaust condensate. If it was being vented into a 6" galvanized B-vent or single wall galvanized or aluminum stack it probably corroded the stack out(?).
 

Stuff

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Are you talking about the roof cap or the vent pipe itself?

See if the furnace manual is stashed away nearby as it tells you what is allowed.

As an example this is from a different model:
A Trane manual said said:
The recommended system is assembled from 2", 2-1/2", or
3" plastic pipe and fittings (See Table 10, page 16). Where the
system is routed to the outdoors through an existing masonry
chimney containing flue products from another gas appliance,
or where required by local codes, then 3" venting of Type 29-4C
stainless steel must be used in place of PVC material.
 

MIKEMCD77

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Thanks for the replies. On the roof above my furnace, I think there has always been a vent that is about a foot tall, with a "mushroom" cap. However, about 17 years ago when I had a NEW furnace installed, the installer cut a hole in the mushroom cap and then ran the PVC pipe from the furnace up THRU the mushroom cap... and the PVC pipe extends another foot above the mushroom cap, and he placed a U-joint on top of the PVC pipe (to prevent rain from entering it).

So the top of the PVC pipe extends at least 2 feet above the roof! All this vent-work is white, and it stands out against my new charcoal shingles. I wanted to replace it for appearance sake, but have now pretty much decided to just paint the vent-work the same color as the roof.
 

Dana

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Thanks for the replies. On the roof above my furnace, I think there has always been a vent that is about a foot tall, with a "mushroom" cap. However, about 17 years ago when I had a NEW furnace installed, the installer cut a hole in the mushroom cap and then ran the PVC pipe from the furnace up THRU the mushroom cap... and the PVC pipe extends another foot above the mushroom cap, and he placed a U-joint on top of the PVC pipe (to prevent rain from entering it).

So the top of the PVC pipe extends at least 2 feet above the roof! All this vent-work is white, and it stands out against my new charcoal shingles. I wanted to replace it for appearance sake, but have now pretty much decided to just paint the vent-work the same color as the roof.

The hook top is a standard way to do it. If you don't like the shepherd's hook look there are other terminators available at the box stores and elsewhere (some are already black.)
 

MIKEMCD77

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The hook top is a standard way to do it. If you don't like the shepherd's hook look there are other terminators available at the box stores and elsewhere (some are already black.)

Thanks! And if I get one of those, wouldn't it be OK to saw about one-half of the PVC pipe off? (Again, the PVC pipe is at least a one-foot extension above the "mushroom"... and the mushroom sits on top of a pipe that is at least one-foot long.)
 
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