Fan problems can seriously affect combustion efficiency. To fire your boiler at peak efficiency it is necessary to strike the correct balance between fuel and combustion air. These ratios must remain constant throughout the entire firing range so that either fuel-rich or fuel-lean mixtures are avoided.


With an atmospheric burner, air is introduced at the bottom of the burner using natural draft. The fuel/air ratios are then determined by regulating only the gas pressure for the correct mix.

For a full modulating forced or induced draft burner designs, air and fuel ratios are controlled through linkages, fans, dampers and the increase or decrease of gas pressure. As demand is placed on the boiler, the burner will respond by introducing a greater amount of fuel and combustion air. This results in more energy introduced into the heat exchanger.

As a general rule of thumb, it takes about 9.5 cubic feet of air for every one cubic foot of natural gas for ideal combustion to occur. At 10% excess air this ratio will be about 10.5 cubic feet of air to 1 cubic foot of natural gas.

The air and gas must not only be in the correct proportions but also introduced at the proper time to assure complete mixing. Gas pressure is controlled through a pressure regulator and a fan controls the volume of combustion air.

Fan problems can seriously affect combustion efficiency. Here is a list of common fan problems and some possible causes that you may wish to look at.

Fan capacity or pressure is below rating:

• Dampers or variable inlet vanes are not adjusted properly.
• Fan inlet or outlet conditions are impaired.
• Multiple air leaks within the system.
• Damage sustained to the blower wheel.
• Direction of rotation is incorrect.

Fan vibrates or makes noise:

• Worn bearings.
• Unstable foundation.
• Foreign material in the fan causing an imbalance.
• Misalignment of bearings, couplings, wheel or v-belt drive.
• Damaged wheel or motor.
• Bent shaft.
• Worn coupling.
• Loose dampers or variable inlet vanes.
• Speed too high or incorrect fan rotation.
• Vibration to fan transmitted from another source.
• Uneven blade wear.
• Loose or broken bolts or set screws.

Overheated Bearings:

• Improper lubrication
• Poor alignment.
• Damaged wheel or driver
• Bent shaft.
• Abnormal end thrust.
• Dirt in bearings.
• Improper belt tension.

Overload on Driver:

• Speed too high.
• Direction of rotation is incorrect.
• Bent shaft.
• Poor alignment.
• Improper lubrication.
• Wheel wedging or binding on fan housing.