Combustion Air Requirements in the Boiler Room

Let Your Equipment Breath! 

Successful, efficient combustion requires three items: heat, fuel, and air. When designing a boiler room, the supply air needed can easily be overlooked. Boiler room supply air is the proper amount of air required for equipment to operate most effectively, including ventilation air. Without adequate air supply sizing, boiler room equipment can become inefficient, and cause accidents.

How much? 

The boiler room inlet air supply must be sized in accordance with the amount of air the equipment in the boiler room requires. Each inlet supply opening shall have a minimum free area of 1 in2 per 2,000 Btu/h. The NFPA recommends that a boiler room have at least two openings directly communicating with the outdoors, although a boiler room can have only one opening. 

Preferred created the Combustion Air Requirements Calculator Based on NFPA 54 and the International Fire Gas Code 304 to help size inlet openings for a boiler room if the BHP or Btu/hr of the equipment is known.  

Combustion Air Opening Requirements Tool

Note: The information expressed above is general in nature and must be verified against specific local code requirements.

Things to consider 


The NFPA 54 recommends that each boiler room has two openings within one foot of the ceiling and one foot of the floor. Having both inlets allows for airflow into the boiler room even if one of the openings is blocked and gives the boiler room natural ventilation. According to code, each opening must be at least 1ft 2 in area.  


If a boiler is over 1,000 feet above sea level (fasl), then a correction factor of 3.5% per 1,000 fasl must be added to the total required combustion air due to less dense air in higher altitudes.  

Mechanical ventilation 

Adding a mechanical fan to an opening increases the movement of supply air into the boiler room. Mechanical fans, when installed, must have an interlock to verify the operation of the fan before starting the boiler. If mechanical ventilation is being used, 0.35 CFM of air per each 1,000 BTU/hr of gas burning equipment must be added. 

Louvers and Screens 

Because large holes in the side of a building are not recommended, a combination of louvers/grills and screens are used to keep vermin and sometimes people from entering the boiler room. Because both items block the flow of air slightly, there is an additional factor that must be used to calculate the size of the new opening with metal louvers and wood louvers if the manufacturer doesn’t give a K factor. Check ASME CSD-1 Controls and Safety Devices for Automatically Fired Boilers, and CG-260 Combustion Air before installing motorized dampers in your boiler room. 

Per the International Fuel Gas Code, section 304.10 Louvers and Grills 

Metal Louver: 75% free area 

Wood Louver: 25% free area 

Mesh screen size minimum: ¼” 


Calculating how much combustion air needs in the boiler room is not as difficult as it seems. And, it is essential to properly size the inlet airways for the supply air into the boiler room, and make sure that they are regularly maintained for maximum equipment efficiency. 


  • NFPA 54 – National Fuel Gas Code, 1992, section 5.3 Air for Combustion and Ventilation.
  • NFPA 31 – Installation of Oil Burning Equipment, 1992, section 1-5 Air for Combustion and Ventilation.
  • ASME CSD-1- Controls and Safety Devices for Automatically Fired Boilers, 1992 with addendum 1a 1993. section CG-260 Combustion Air.
  • BOCA – National Mechanical Code, 1990, article 10, Combustion Air.
  • SBCCI- Standard Mechanical Code, 1991, section 305 Combustion and Ventilation Air.
  • American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Handbook – Fundamentals, 1993, Chapter 15, page 15.9 Air For Combustion.

Combustion Air Design  

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