Type B Vent Cap Height For Wall Furnace

[turkeyvulture]

I'm remodeling an older home in California with no central air, so there will be two gravity-flow natural gas wall furnaces with Type B venting up through the top plate and roof. Question is, how high above the roof should these terminate?

The vent manufacturer (Amerivent) says 1 foot above the roof.

The furnace manufacturers (Cozy, Williams, and Empire) say 2 feet above anything 10 feet away (like a chimney). Then they also want a minimum of 12 feet from the bottom plate, (I guess this is for draft purposes), also they mention minimum 3 feet above the roof.

Which of these is right, anyone know? Looking at other houses, I don't see anything vent pipes conforming to the 10 foot over/2 foot down rule. They probably don't have wall furnaces, but they do have water heaters, using B vents, and isn't it basically the same thing? The water heater in my own house vents only a foot above the roof too, for that matter so does the central furnace.

[hj]

Ideally, the vent should be higher than the roof's ridge, but that is often impractical unless the heater is near the center of the house. 1' to 2' is usually adequate except in some unique situations. A chimney within 10' should not create enough turbulence to require modifying the flue's height.

[jimbo]

The National Fuel Gas Code is invoked in California. The rules for B-vent are slightly different that for chimneys and single wall metal pipe;

For B-vent: the height of the lowest discharge opening of the cap, above the roof, varies with the slope of the roof. For a flat roof, H is 1.0 ft. for an 8/12 roof, H is 1.5 feet, and for a 12/12 roof H is 4.0. The chart covers from flat to over 20/12 ; you get the idea. There are mininum heights above highest connected flue cap, and other specs. Special specs for parapet walls. Etc. And as always, the manufacturer's installation instructions always apply, but generally you would not find any to conflict with the code.