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.