Combustion Air

Effects of Insufficient Combustion Air on Draft and Heating Systems

Might sound like rocket science but no worries. For the correct operations and venting of gas and oil heating appliances, a good amount of outside fresh clean air must be supplied to the structure to make up for the air loss from venting heating appliances, fireplaces, dryers, fans and other air losses. Insufficient combustion air can cause major or even catastrophic problems for proper draft and operation of both gas and oil heating systems. For years it has been assumed that when a heating appliance was located in an unconfined area, there was sufficient air for both ventilation and combustion. Today, in most cases that is not at all true! With new construction, standards for building insulation and energy efficient windows and doors have reduced the amount of air changes per hour. The combustion and make up air requirements in the codes are based on 1/2 air changes per hour. For newer homes and conversion of electrically heated homes, the air changes could be reduced to 1/3 or less on the coldest days when heat loss is the greatest and there is a chance that windows or doors are closed for an extended period of time. When installing new equipment or troubleshooting problem equipment, the first determination that needs to be made is whether the equipment is located in a confined or unconfined space. According to the NFPA 31 and NFPA 54, an unconfined space is any space whose volume is equal to or greater than 50 cubic feet per 1,000 BTU.

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. If the actual free are of the louvers is not known, wood louvers are assumed to have a 20% to 25% free opening. Metal louvers or grills are assumed to have 60% to 70% free opening. According to NFPA 31 and NFPA 54, an Engineered system such as a CAS-3, 4, 4Jr., 6 or 7 may be used to overcome a confined space.

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