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insulating a furnace flue - it's heating the attic

Posted by schnuezel (My Page) on
Wed, Oct 29, 08 at 10:22

Hi All!

There was a post similar to this back in December, but it only addressed blocking the airflow around the B-vent, where it comes out of the floor. My problem is a bit different...

I've been working on insulating my attic and discovered that my furnace flue is giving off a lot of heat. This is essentially heating my attic which here in the Utah mountains is contributing to huge ice dams.

My furnace flue is in a b-vent, from what I can tell, except for a 4 foot stretch where there is no B vent. The B-vent comes out of the floor of the attic, then stops. There's a section of B-vent attached to the roof. In between there is just a simple single wall vent. It looks like the flexible vent that comes out of a clothes drier - not sure what it's called. You can see this flexible vent coming out of the b-vent, go up 4 feet and then go back into a b-vent out the roof. It's this 4 foot section that is giving off a ton of heat.

I had the people out who installed the furnace two years ago and they said it's up to code this way. Is it?!?! They got belligerent when I questioned it. I read somewhere that flues in unconditioned spaces *must* go through a b-vent. I would assume that it would mean the *entire* flue.

Even if it is up to code to have part of the flue not in a b-vent, how do I insulate this 4 foot section? The problem is that the b-vent coming out of the floor and the b-vent attached to the roof are offset by about two inches, so the HVAC people couldn't run the b-vent all the way up. Isn't there some kind of flexible, insulating pipe that could be put in to accommodate the offset?

Any help is appreciated!


Follow-Up Postings:

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RE: insulating a furnace flue - it's heating the attic

You don't start, stop and start B-Vent, PERIOD.
They screwed up with the roof jack alignment and it should be corrected. 2" offset is too small for adj 45's though.
There is flexible double wall connectors, usually used at the appliance but without some reroofing perhaps the best thing here is to use one. Some manufacturers pipe are universal and some not, you will have to check your brand.

Here is a link that might be useful: H&C flex b-vent


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