A burner can't hold its settings if the boiler room doesn't supply enough air. A burner short on air runs rich, produces CO, and drifts from the settings made at its last tune-up.
A common rule of thumb for packaged boilers is 10 cfm per boiler horsepower: about 8 cfm for combustion and 2 cfm for ventilation. Above 1,000 feet of elevation, add about 3% for each additional 1,000 feet. By that rule, a 300 horsepower boiler needs about 3,000 cfm.
The required amount comes from the codes your jurisdiction enforces and the boiler manufacturer's installation instructions. Many states and cities base their fuel gas and mechanical codes on the International Fuel Gas Code and International Mechanical Code, often with local changes. Oil-burning equipment also falls under NFPA 31. Your local inspector has the final say.
A few details cause problems. Louvers are rated by free area, not face size, and a decorative louver passes much less air than its size suggests. Openings work best low in the room and on opposite walls, so outside air moves across the boilers. If a fan supplies combustion air, it must be interlocked so the burners can't fire with the fan off. Exhaust fans that put the room under negative pressure compete with the burners for air.
Signs a room is short on air: CO rises and O2 drops when a door is closed, burners are hard to light or trip on flame failure, flue gas spills at the breeching, or the room runs much hotter than normal.
We tune with the room in its normal operating condition, doors closed and fans running as usual. Settings made with a door propped open won't hold once it's closed.
Related reading: Excess Air and O2, Boiler and Burner Formula Cheat Sheet and What an Out-of-Tune Burner Really Costs. See all of our services on the What We Do page.
Request service, or email service@combustionspecialists.com.
