How to Calculate the Basement Ventilation Needed for a Furnace


Many furnaces are installed in unfinished basements. The empty space allows the furnace the maximum amount of air for combustion. Without this available air exchange, appliances like gas furnaces and hot-water heaters would not be able to light or burn properly. Homeowners finishing out basements are often faced with tough decisions about how to provide ventilation for their furnaces. Calculating the volume of available air required for the furnace to function properly should be the first step to designing the layout of the new basement area.

  • Check your furnace for ventilation pipes that go outside. Furnaces vented to the outdoors do not require indoor combustion air since they pull their combustion air from outside the house.

  • Locate the manual for your furnace to determine the BTU per hour rating. Check the furnace itself for a paper tag with this information if the manual is unavailable.

  • Multiply the BTU-per-hour rating by 50, then divide that number by 1,000 to determine the cubic footage of combustion air required for your furnace in a totally open space. Determine the available cubic footage of combustion air by multiplying the length, width and height of the open-basement area.

  • Enclosed furnaces generally require two openings in the enclosure no smaller than 100 square inches each. Calculate your enclosed furnace's needs by dividing the BTU-per-hour rating by 1,000. Install an upper and lower vent in the structure equal to or greater than this number in surface area. If you will be enclosing the furnace during your remodel, use these figures to determine your ventilation needs.

Tips & Warnings

  • Add 25 percent to the size of the vents if you decide to use vents with metal louvers.
  • Absolutely install both upper and lower vents that meet the minimum size requirement for ventilation. Incomplete combustion can lead to health problems and furnace malfunction.

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