For the proper operation and venting of gas or oil heating appliances, sufficient outside air must be supplied to the structure to make up for the air lost from venting heating appliances, fireplaces, clothes dryers, exhaust fans and other building air losses. Insufficient combustion air can cause major 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 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 air changes per hour. Air problems are most notable 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.
In accordance with NFPA 31 and NFPA 54, an unconfined space is defined as follows: Any space whose volume is equal to or greater than 50 cubic feet per 1,000 BTU (or 20 BTU/Cubic Foot).
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 area 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.
To add air from an adjacent room, two openings between the room could be made 12 inches above the floor and 12 inches below the ceiling. The size of these openings is based on 1 square inch per 1,000 BTU input.
To add air directly from the outside of the structure, two openings could be made. The size of these openings is based on 1 square inch per 4,000 BTU input. The above requirements are based on guidelines in NFPA 31 or NFPA 54. Alternately, if operating in a confined space, additional air may be added by a duct to the outside, sized on 1 square inch per 5,000 BTU input.
For a room 30' x 20' x 8' ceiling height and a heating appliance rated at 140,000 BTU/hr.
To determine the maximum total input firing rate allowable in this room: