Combustion Air Calculator Form Please enable JavaScript in your browser to complete this form.Please enable JavaScript in your browser to complete this form.Furnace type *80% furnace90% furnaceChoose the appliance type you are checking.Total appliance BTU input *Add the input BTUs for all fuel-burning appliances in this space: furnace + water heater + dryer, etc. Enter numbers only.Combustion air required (cu ft)Calculated from the total BTU input.Measure the room where the appliances are installed. Room width (ft) *Use feet. Decimals are OK.Room length (ft) *Use feet. Decimals are OK.Ceiling height (ft) *Use feet. Decimals are OK.Room volume (cu ft)Calculated from width × length × height.BTU capacity of this roomAdditional BTU capacity neededIf this value appears, the room does not have enough combustion air by this calculation.Result: Enough combustion air for this 80% furnace. Result: Enough combustion air for this 90% furnace. One vent pipe may be used by this calculation. Result: Not enough combustion air. For a 90% furnace, install two vent pipes: one fresh-air intake and one exhaust vent. Alternative option Add combustion air to the room using transfer grilles to another space or a fresh-air opening from outdoors. If using transfer grilles, install two openings: one high and one low. Required free area for grilles (sq in)Use the manufacturer free-area rating. This is not the overall grille size.Additional connected-room volume needed (cu ft)Additional room floor area needed at this ceiling height (sq ft)Submit
