- Combustion Air = HP* x 8 CFM/HP =
- Ventilation Air = HP* x 2 CFM/HP =
- Total Air Required = HP* x 10 CFM/HP =
Acceptable Air Velocities in a Boiler Room 0-7 ft. above floor 250 FPM Above 7 ft. high 500 FPMWhen sizing an opening to the outside, it should be a minimum of one square foot. Care should be taken to ensure that no water, oil or steam lines are run in the direct path of cold fresh air entering from any of the outside air opening. Heated heavy oil lines should be protected from cold air and they should be electrically or steam heat traced and insulated. What about air ducting? In some applications the boiler room is located in a building such that it has no outside walls. Many of these applications do not have sufficient excess makeup air in the factory to allow for combustion air requirements. In these cases there are two solutions: The first is ducting fresh air to the boiler room. Where this is required, the general rules for the size of wall opening for fresh outside air can be used. The duct size to the outside and its free open area inlet must never be smaller than the wall opening in the boiler room. In addition, the pressure drop through the duct at maximum flow must never exceed 0.05″ w.c. The second is ducting fresh air directly to the boiler. In general, this method of air supply should be avoided whenever possible. The disadvantages of this type of system far exceed any perceived advantages. If used, the ducting becomes a part of the boiler system and can effect the stability of combustion due to varying weather conditions, wind direction and velocity, humidity and temperature. An outside temperature variation of -10EF in the winter to 80EF in the summer (many areas of the country are wider) can cause a burner adjusted for 15% excess air combustion on the coldest winter day to be 5% short of air on a warm day. This can lead to massive CO production, soot formation, plus unstable and unsafe combustion. If direct ducting must be used, we suggest the following minimum steps be followed:
- Each boiler has its own, completely separate fresh air ducting and exhaust stack. Shared air supplies and exhaust stacks will lead to combustion problems and unsafe operating conditions.
- Boilers directly connected to fresh outside air ducts must be checked for proper combustion adjustment and operation every three months by a certified package firetube boiler specialist.
- The duct work supplying the fresh air to the boiler must be sized so that it has a maximum pressure drop at maximum flow of 0.05″wc.
- The fresh air supply duct should have an electric, hot water, or steam heater to temper cold outside air to at least 50EF.
- If the application is utilizing a low emission with flue gas recirculation, do not use direct ducted outside air. The potential problems associated with a standard burner are intensified with a low emission burner.
Total maximum HP = 300 + 800 = 1,100 HP Total air required = (1,100) (10) = 11,000 CFM Altitude correction = (11,000 CFM) (1.03) = 11,330 CFM
11,330 Net free open area required = = 45.32 sq. ft. 250The boiler room will require a minimum of two fresh air opening of 22.66 sq. ft. (45.32 sq. ft ) 2) net free open area. NOTE: For all applications, the above is a general minimum requirement for fresh air supply. Always consult local codes which may supersede the above recommendations.
(article from Johnson Boiler Company)
Combustion Problems? It May Be Your Fresh Air Supply