To correctly exhaust a furnace , water heater located in a unheated area such as a crawl space, garage , the exhaust vent is to be B vent not single wall due to air temperature creating condensation on thje pipe
This has nothing to do what so ever with clearance to combustibles of single wall pipe

Comment please as persons still say ???

Not that simple an answer. 2013 CA PLBG Code 509.10.1.1 covers it. [books.google.com]

i have never heard of that.
clearance for single wall from combustibles is different fron B-vent.
condensation on the outside????

It has more to do with maintaining the temperature of the flue gases so the draft is correct. Otherwise the gases cool down before leaving the flue and "backdraft" back to the appliance. I have had tall flues in an "unheated" building, that I had to "kickstart" with a torch to warm up the flue and get the draft going.



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Thanks for the replies. Read the 2013 plumbing code. It makes sense to have B vent in unheated areas as a single wall pipe is cold and the flue gasses in turn creates condensation where as the B vent does not create the same condensation issue
Any more thoughts regarding this ? Thanks



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if it makes sense to you, please define 'unheated spaces'.
most basements around here are unheated yet single wall vents are run to chimneys all the time.

'unheated space' as in 'cold room'

while a basement may not be directly heated it is connected to the 'heated space' and also picks up heat from the mechanical equipment

a 'crawl space' or 'cold room' will be close to outside ambient

I guess it also depends on how long the vent is

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