My basement is finished except for the mechanical room. I have 2 nat gas fired appliances a furnace (75,000 btus) and water heater at (40,000 btus).
Question: am I reading it right, each unit states something along the lines of "Hourly Input Btu's" with the above numbers, I have a combined 115,000 btu's, this is what I have to base my combustion air requirments on correct?
The basement seems too cold, it has 5 registers with no returns and is about 1300 square feet. The mechanical room leaks cold air out from under the door when the furnace is not running, the gap is about an inch or so. I have two six inch flex pipes supplying the room with combustion air, one high one low. That gives me a total of 56.5 square inches of atmospheric combustion air deliverd from the attic for combustion air. According to this calculator (which I have no idea if it is correct) http://www.houseofcraig.net/combustion_air_calc.html
I need 58 square inches of outside combustion air for 115000 btus, like I stated earlier with my calculations (3 x 3) 3.1415 times two pipes equals 56.5 inches squared. If I seal off the bottom of this door with some sort of rubber flap to help keep out the cold, given the requirement of 58" and only having 56.5" will this cause any problems or decrease efficency?
Also, is there an inexpensive mechanical flap that would open and close my combustion air pipes when the appliances are in service or in other words when the room is under a vacuum.