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How to get combustion air into a hot water closet from the atic above ?

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My hot water heater closet is located in the upstairs hall way, and has a louvered door so combustion air comes from the living area.  Recently the AC / heating unit was  installed in the atic with the return air located in the same hall way as the water heater.  The thermostat is on the wall next to the Hot water closet.  The warm air from the heater closet is being drawn out and in my opinon is keeping the air temperature around the thermostat higher thus preventing it from cycling off.  So I need to seal off the hot water closet, and get the combustion air from the atic. The closet measures 29 & 1/2 " W X 24" D X 7' H.  with a 7&1/2" X 6" X  7" H space in the back corner that houses the flue from downstairs where the heating unit use to be. How should I go about this?  The Hot water heater is rated at 38000 BTU / Hr input, which computes to 1900 cf of air needed.  What is the best way to do this?  Help!  Thanks, Themailman.
asked in Gas Water Heater by themailman (120 points)
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1 Answer

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Hi TheMailman,
In order to use attic as a source of combustion air for your water heater you have to provide two openings between the WH closet and ventilated attic area. One of those openings shall end 1 foot above the floor (air inlet) and the other must be installed at the ceiling level (terminate within 12” from the ceiling).
Both openings must be sleeved (metal pipe usually serves as a sleeve) and extend a minimum of 6” above the ceiling joists… I’d make it 6” above the insulation level if that one is higher. Screens are prohibited on combustion air vents leading to the attic and the minimum required air flow area of each vent is 1sq inch per 4K BTU’s. 
For 38K BTU’s you’d need approximately 10sq inches inlet and outlet (3” diameter pipe has 7.065 sq inch, 4” pipe has 12.560 sq inch air flow area) so you need two 4” diameter galvanized pipes.
Let me know if you have any additional questions,
answered by darekrudy (21,730 points)