Understanding Boiler Room Air Supply

Every combustion system needs a fresh supply of air for optimal operation. When boiler room air supply is set up correctly, it equalizes pressure inside and outside the boiler room and allows for proper combustion. This supply of air must be consistent, or it can cause problems. Process outages, soot build-up, rough combustion, carbon monoxide, operational hazards and maintenance calls just to name a few.

In this article, we will examine how much air your boiler needs and considerations for optimal air flow.

How Much Air Does My Boiler Room Need?A finned metal commercial damper

A good rule of thumb is to provide 4 – 6 square inches of unrestricted airflow for every boiler horsepower. As an example, if you have a 60 HP boiler, you can estimate 240 – 360 square inches of area are needed for optimal operation. 

What if you want to look at a more precise way of calculating combustion air required? If you have a gas or oil fueled package firetube boiler, the following formulas help determine your boiler needs:

Total Air Required (CFM) / Allowable Velocity At Opening (FPM) = Net Free Open Area of Opening 

CFM = cubic feet per minute (cu ft/min)

FPM = feet per minute (ft/min)

BHP = Boiler horsepower (including the total maximum boiler horsepower in the boiler room)

Total Air Required (CFM)

Air for Combustion  = 

BHP x 8 CFM
Air for Ventilation  =  BHP x 2 CFM
Total Air Required (Combustion + Ventilation)  =  BHP x 10 CFM

Allowable Velocity At Opening (FPM)

0-7 Feet Above The Floor  =  250 FPM
More Than 7 Feet Above The Floor  = 500 FPM
Duct From Air Supply To Boiler  =  1,000 FPM 

Note: These calculations are standard for boiler room operations up to 1,000 fasl (feet above sea level). Due to thinner air, it’s important to add 3% air for each 1,000 fasl for accurate estimates.

Furthermore, it is recommended that two louvers are installed for the operation of a single boiler for optimal airflow. If there is more than one boiler installed, it’s important to have at least one louver per boiler. 

The Best Types and Locations

Combustion air vent for boiler roomOnce you’ve determined the square footage you will need for your boiler, it’s important to take into consideration the location of fresh air inlets.

As a general rule, there should be at least two permanent air supply openings in the boiler room walls. If possible, these openings should be on opposite sides of the boiler room and should be no higher than 7 feet off the floor. This setup is optimal because it:

  • Promotes mixing of fresh and warm air already in the boiler room
  • Ensures proper cooling of the boilers
  • Tempers the cooler air prior to entering the burner for adequate combustion

It’s also important to ensure that the air inlets themselves offer unobstructed air flow with protection from the weather, pests, and debris. Some boiler rooms choose a fine mesh-wire screen for airflow. These types of screens are prone to clogging due to dust, paper, dirt, and pest infiltration.

A Simple Test For Boiler Room Air Supply

One of the easiest ways to test if your boiler room has an air supply issue is to open an exterior door (if available). If when you open the door you feel air rushing in and upon releasing the door it slams, then you have a problem. This means your boiler room is under negative pressure.

This can create combustion issues and cause fans to work harder than necessary.

Troubleshooting Centralized Boilers

Not every facility has the most optimal setup for air supply directly from the outside. If the boiler room does not have access to outside walls, there are generally two possible solutions. You can duct a source of fresh air into the boiler room, or duct a source of fresh air directly into the boiler.

Ducting a source of fresh air into the boiler room

For this method, the size of the wall opening in the boiler room can be used to determine the size of the ducting. Additionally, the pressure drop through the duct work must never exceed 0.05″ w.c.

Ducting a source of fresh air directly into the boiler

Direct fresh air boiler ductingThis method of air supply offers some advantages and disadvantages. The ducting actually becomes a part of the boiler system. This can affect the stability of combustion (via wind direction/velocity, humidity, temperature, inclement weather conditions). Consequently, subtle or drastic fluctuations in outdoor temperature can lead to changes in fuel and air mixture, potentially causing a substantial increase in CO production, soot formation, and other unsafe combustion outcomes. However, ducting a combustion air blower to the outside can alleviate issues with negative room pressures, freezing pipes near open air inlets in the winter, or interlocked makeup air heaters. 

The following factors should be adhered to for direct ducting:

  • Every boiler should have its own fresh air ducting and exhaust stack (minimizes combustion problems)
  • Boilers that are directly connected to fresh air ducts must be regularly checked. Generally every 3 months or sooner by a certified boiler specialist. (including proper combustion adjustment). Advanced controls may allow for a longer interval here.
  • Ductwork for fresh air supply should be sized for a maximum pressure drop of 0.05″ w.c. during maximum flow unless the fan design is adjusted to accommodate the additional losses.
  • Fresh air should be tempered by a steam, or glycol heater to at at least 50ºF to prevent condensation or mixing issues.

Need Help With Your Boiler Room Air Supply?

When you need your boiler room air supply set up properly, rely on the combustion experts at Rasmussen Mechanical Services. Our team of experienced experts are here for any of your boiler room needs. We can help with everything from proper air supply, flue gas exhaust systems, fuel delivery systems and everything in between. Call us at 1-800-237-314, chat with a support agent, email sales@rasmech.com, or simply contact us online. Whatever your boiler room needs, we’re here to help!