how much 3 inch B-vent (double wall) for water heater can be above the roof? Leave a comment

The natural gas venting tables talk about lateral versus vertical rise and pipe diameter, plus the 2 and 10 ft rule for height above the roofline and outdoor vertical walls but they never mention how much of the B-vent can be above the roof versus inside the attic or conditioned space.

In my case Im moving a 40 gallon natural venting gas water heater into the unconditioned attic with only 3-4 feet of headroom before I need to penetrate the roof at which point I must be 2 to 2.5 ft above the roof for a 9/12 pitch roof. That means I will have maybe 7 feet of total vent (straight up).

The attic isnt conditioned but its not freezing cold in winter either. Will 2 feet of b-vent exposed to ice cold winter air cause issues on a short run of 3 inch B vent?

Maybe its irrelevant since its an unconditioned attic to start with….

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