One of the most often asked questions I receive is in regards to combustion air requirements. With today’s modern construction techniques more and more emphasis is being placed on insulation, Tyvek style vapor barrier materials, triple pane glass, weather stripping and caulking. While this does help conserve energy there is a point at which the house becomes so tight that you have insufficient combustion air. For the proper operation and venting of oil heating appliances it is essential to supply sufficient air to the structure to make up for the air lost from venting heating appliances, fireplaces, clothes dryers, exhaust fans and other building air loses.

Insufficient combustion air causes major operational problems for oil heating appliances. Problems ranging from an inability to achieve a clean fire, to sooting boilers, to odors in the house, to random control lockouts all can be attributed to insufficient combustion air. For years it has been assumed that when the heating appliance is located in an unconfined area there will be sufficient air for both combustion and ventilation. Today, in most cases, this is NOT true. The combustion and make up air requirements in the codes are based on ½ air changes per hour. For newer homes and conversion of electrically heated homes the air changes could be reduced down to 1/3 or less air changes per hour. Insufficient air problems are most noticeable on the coldest days of the year when heat loss is greatest and there is less of a chance of windows or doors being open.

When installing new equipment, or troubleshooting problem equipment, the first determination that must be made is whether the equipment is located in a confined or unconfined space. In accordance with NFPA 31 and NFPA 54 an unconfined space is defined as follows:

An UNCONFINED SPACE is any space whose volume is equal to or greater than 50 cubic feet per 1,000 BTU (or 20 BTU/Cubic Feet). This is calculated on the sum of the total input ratings of all fuel burning appliances installed in that space. Only areas connected to the space that have no doors or with fully louvered doors can be considered part of the unconfined space. NOTE: If the actual free area of the louvers is not known: wood louvers are assumed to have a 20% to 25% free opening, metal louvers and grills are assumed to have 60% to 70% free opening.

EXAMPLE: Lets take a room 25 feet by 25 feet with an 8 foot ceiling height.

Calculated Space: 25′ x 25′ = 625 sq. ft. x 8 ft. (ceiling height) = 5,000 cu. ft.

Maximum BTU Heating Appliance Allowed: 5,000/50 = 100,000 BTU

The Maximum Total BTU Input Rating of all Fuel Burning Appliances = 100,000 BTU (for this example room size).

If the space were calculated as a confined space (less than 50 cubic feet per 1,000 BTU) provisions to the room would have to be made to allow a greater volume of usable air. To add air from an adjoining room, two openings between the rooms could be made 12 inches above the floor and 12 inches below the ceiling. The size of the opening is based on 1 square inch per 1,000 BTU input. Outside air intake kits are also available and are designed to be piped directly to an air intake box on the burner. Please remember a UL listed outside air intake kit must always be used in these instances.

Special thanks go out to Kelly Barger of Field Controls LLC for his help in compiling this information.


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