Thread: Single wall or B Vent?

A gas furnace is in the crawl space.
Horizontal 4" single wall vent pipe from furnace to vertical 6" asbestos pipe.
Should the 4" be double wall pipe?

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Rick,

I see single wall in foundation crawlspaces frequently. Particularly on old floor furnaces. I believe that single wall is not allowed in attics and this would extend to foundation crawlspaces as well. In addition, if this is a newer 80%, the old transite pipe should be replaced with b-vent.

What does the code say?
Bob

Rick; How horizontal is the pipe and what is the horizontal run? Is the pipe within 6" of anything that can catch fire? All things to be considered.

You can always recommend higher standards to improve the safety of the vent. Nothing wrong with that.

Bob
I did not find anything in the 2006 IRC

David
6" rise in 8' run
not closer that 6" to anything.

Thanks all

Originally Posted by Rick Cantrell

A gas furnace is in the crawl space.

Originally Posted by Bob Harper

What does the code say?
Bob

Rick,

Location: Columbus GA

From the IRC. (underlining is mine)

- G2427.7 (503.7) Single-wall metal pipe.

Single-wall metal pipe vents shall comply with Sections G2427.7.1 through G2427.7.12.

- - G2427.7.2 (503.7.2) Cold climate.

Uninsulated single-wall metal pipe shall not be used outdoors for venting appliances in regions where the 99-percent winter design temperature is below 32ºF (0ºC).

The 32 degree line for 97-1/2% winter design temperature is at Jacksonville, Florida - from Appendix D, Table D101, IPC - which is what goes into Table R301.2(1) of the IRC (see Winter Design Temperature and note 'e').

Columbus, Georgia is shown as 24 degrees for the 97-1/2% winter design temperature.

While I cannot find the 99% winter design table, the freeze line would move somewhat north of the 97-1/2% line - with Columbus being at 24 degrees on the 97-1/2% table, I suspect it would be at or below 32 degrees on a 99% table (but I don't know for sure).

That would mean that single wall metal pipe is not allowed 'outside the building's thermal envelope', i.e., not allowed in the crawlspace or in the attic.

Jerry
So single wall vent pipe is not allowed in the crawl space, because it might freeze. OK
Thanks

Originally Posted by Rick Cantrell

So single wall vent pipe is not allowed in the crawl space, because it might freeze.

Rick,

Not because it might freeze, but because the cold will keep it from heating up and creating the upward flow and draft like it should (keep it from creating the 'chimney effect' as much as it should have).

Now that makes sense.

I can't quote you chapter and verse on the code in your respective area but as a Plumbing contractor, I have always gone with "no single wall pipe in unconditioned spaces".
In addition to the drafting issues, the flue gases from a gas appliance have a significant amount of moisture content. When these flue products contact cold, single wall pipe they condense. This condensate will cause damage to the metal parts of the flue system and on back to the furnace or water heater they are serving. (assuming they are properly graded) So...in the attic, under the house or exterior shed, I always call for B-vent replacement if single wall is found.

Drafting
condensation
frequency of inspection
warranted listed product

So many reasons not to use single walled in inaccessible, outdoor, unconditioned spaces. Don't forget in attics you 75% offset rule: for every 3 feet of horizontal offset, you need 4 feet of vertical and that's a minimum std.---does not guarantee performance. Also, use listed supports and components. If the chimney is unsuitable, it must be relined with a listed liner or, if allowed by mfr, B-vent inside chimney.

Bob