The Combustion Fan & Air-Pressure Switch Explained
The fan draws air for combustion and pushes flue gases out; the pressure switch proves it is working before gas flows. Here is how this safety pair works.
Before a modern boiler is allowed to light, it has to prove that air can flow through it and flue gases can get out. Two components handle that: the combustion fan (or inducer) and the air-pressure switch. Together they are a key safety interlock — and a frequent source of "won’t start" faults.
What the fan does
The combustion fan pulls in fresh air for the burner and pushes exhaust out through the flue. On a sealed, room-sealed boiler it controls the precise air-to-gas ratio that makes clean, efficient combustion possible. The control varies its speed to match the firing rate.
How the pressure switch proves airflow
Before opening the gas valve, the control runs the fan and watches the air-pressure switch. The switch only closes if airflow and flue draught are adequate. If it does not prove, the boiler will not light — a deliberate block against firing into a blocked flue.
Common causes of a no-prove fault
- A blocked or restricted flue or air intake (debris, ice, nests).
- A failing fan that no longer reaches the needed speed.
- A cracked sensing tube or condensate blocking the pressure switch.
Key takeaways
- The fan supplies combustion air and removes flue gases at a controlled rate.
- The pressure switch must prove adequate airflow before gas is allowed to flow.
- A blocked flue or intake is a common and important cause of no-prove lockouts.
Frequently asked questions
What does an air-pressure or fan fault code mean?
It means the boiler could not confirm safe airflow before firing. Causes range from a blocked flue or intake to a tired fan or a blocked sensing line. Because it protects against firing into a blocked flue, it should be diagnosed promptly, not repeatedly reset.
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