Combustion Air: Theory vs. Reality

by Jim Davis

It’s often said that under steady state or ideal state conditions, prescribed methods of supplying combustion air to an appliance should be adequate. Unfortunately, despite traveling the country for the past 25 years, those are two states I’ve yet to find.

There are two methods of supplying combustion air: passive and forced. The first provides adequate openings for the air to reach the appliance, without measured or guaranteed results. The second is controlled and measured for 100% guaranteed results.

The National Fuel Gas Code, International Fuel Gas Code and NFPA (National Fire Protection Agency) 54/ANSI 2223-1-1999 list the recommendations to follow to provide passive combustion air.

First it must be determined if we’re dealing with a confined space or an unconfined space.

In a confined space, the volume of the equipment room is less than 50 cu. ft. per 1,000 Btuh. input of all appliances installed in area. In such a case, additional combustion air is required.

In an unconfined space, the volume of the equipment room is at least 50 cu. ft. per 1,000 Btuh input of all the combustion appliances installed in area. When this is the case, no additional combustion air is required.

Under either condition, it’s assumed that no other source uses air from this room, such as duct leakage or normal building ventilation. This assumption is fine if you are located in the State of Ideal or Steady. But what about the rest of us?

Passive Combustion Air

In a confined space, the proper openings for passive combustion air are as follows:

  • Grilles with direct access to outside = 1 of free area per 4,000 Btuh input
  • Ducts with direct access to outside = 1 of free area per 2,000 Btuh input
  • Grilles with access to attics and crawl spaces = 1 free area per 2,000 Btuh input
  • Grilles with direct access to adjacent rooms = 1 of free area per 1,000 Btuh input.

In each of the guidelines listed above, there should be a high and a low grille or duct of equal size installed: one to bring air in and one to take air out — or at least that’s one explanation. Another explanation for the high opening is to provide venting if the flue becomes blocked. Somehow it doesn’t make sense to have one vent bringing air in, and two openings (the high vent and the flue) taking air out. It seems like we’re depending quite a bit on inanimate objects and Mother Nature to understand our objectives.

Forced Combustion Air

Forced combustion air, on the other hand, is air supplied by a mechanical fan or blower. It provides a measured volume of air in and lets the vent or flue take air (or, in our case, flue gas) out.

Actual combustion air requirements for forced combustion air are:

  • 10.5 cu.ft/hr. air per 1,000 Btuh of input
  • 4.5 cu.ft./hr. excess air for complete combustion
  • 15.0 cu.ft./hr. air for drafthood dilution (if drafthood appliance)
  • 30.0 cu.ft./hr. air total.

The formula for this is:

Total Btuh Input X 30 = CFM of Fan 60 minutes

This formula for forced combustion air doesn’t take into consideration any duct leakage or additional building ventilation, but it does guarantee a certain amount of measured air whenever it’s needed.

Proper combustion testing and flue interference testing will verify if what we’ve have done to provide combustion air is adequate. Because we’re controlling the combustion air, outside factors are less likely to influence it.

It’s our responsibility to verify that our method of providing proper combustion air works under all conditions, not just when we’re on the job. It’s up to us, as the system designer or installer, to guarantee that something actually will work, or to make the proper modifications to ensure that it does. n

Jim Davis is senior instructor at National Comfort Institute (NCI), a national training, certification, and membership organization. Davis can be reached by phone at 440/949-1850, or by e-mail at [email protected]. For more information on NCI, visit

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