Did you know sufficient air supply is essential for proper boiler operation? The three elements required for combustion to take place are: fuel, heat (ignition) and air. The need for fuel and ignition is obvious. The requirement for air is less obvious and therefore is often overlooked. If one of these three elements is missing the combustion process stops. An adequate combustion air supply is a requirement to minimize the possibility of a furnace explosion.
Installing permanent air intakes is essential to the proper operation of a boiler. When the combustion air supply is closed off, the fire starts to smoke as the air supply is extinguished. Incomplete combustion occurs and carbon monoxide is generated. In many cases, before the flame detection system can close the fuel safety shutoff valve(s), the fire goes out. Other times, the build-up of fuel is re-ignited as oxygen leaks in through cracks and crevices potentially causing a furnace explosion with catastrophic effects on personnel and property.
It’s important to know that two openings leading directly to the outside are required when boilers are installed in a confined space such as a boiler room. One opening should be located high on the outside wall and the second opening should be located close to the floor. If the boiler room is located partially or entirely below grade, a duct should be installed from the lower opening ending at a point equal to the depth of the duct above the floor. An outside window well or areaway can also be utilized. The size of the openings depend on the total fuel input rate of all fuel-burning devices located in the boiler room. The Btu rate per hour of all boilers at maximum burning rate, plus other devices such as water heater are used to size air opening and their associated duct work.
Louvers or grills may protect the outside air opening, but the blocking effect of the louver or grill must be considered, and sizing must be done based on its free area. If dampers are fitted to these intakes for energy conservation or other reasons, they must be interlocked so the burners cannot be fired unless the dampers are in the open position. The use of exhaust fans in boiler rooms is not recommended, especially if the use of the fan places the boiler room under negative pressure. The upper opening provides a means of ventilating the boiler room.
If there were only one air intake opening, heat build up in the boiler room would reduce the density of the air would be lighter than the cold outside air and it would escape through the single opening. This escape would cause a negative pressure condition, which would cause a down in the chimney and breaching adversely affecting combustion. The use of two openings with one located near the floor will help you avoid this situation. Most boiler and burner manufactures have procedures for sizing combustion air supplies included in their installation instructions. These instructions can be followed; however caution is required as local codes may supersede the manufactures instructions.