Makeup Combusion Air Vents for Boilers and Dryers
I've done the BTU input measurements of the fuel burning equipment in my basement. My 2 gas dryers, 3 boilers, and 3 water heaters total 532,000 BTU/hr. My basement, where these appliances are located, is 5,288 cubic feet in area. According to my calculations, my basement is only supplying enough air for 79,000 BTU/hr of combustion. I am at 9.9 cu. ft. per 1,000 BTU, and I need to be at 50 cu. ft. Therefore, I need 110 square inches of vent area to the outdoors for intake air, with each square inch supporting 4,000 BTU of combustion.
Now, my basement has four 29x12 basement windows, each with four 7-3/4"x9-1/2" panes of glass. With a 6" round vent hood, each gives me ~28 square inches of intake air. Or, I could remove two panes of glass and screen and hood the openings instead, giving me a total of ~147 square inches of intake air, enough for my appliances.
BUT, my problem is with the air... If I used 4 6" vents, none of them come with flaps (like in a dryer hood, but opening in the opposite direction) inside to stop cold air from coming in when the appliances are not in use. If I used a dryer vent sort of flapper, just turned in the opposite direction, would the negative pressure in the basement from the running appliances be enough to open the flapper? If so, where could I get just a 6" flapper?
Any ideas for satisfying the combustion air while also not letting every cold breeze into my basement when they're not in use?
Here's the boiler/water heater area. On the opposite wall are the dryers, with an identical window above them.: