Boiler Air Pressure Switches: How They Work & Why They Fail
Air pressure switches are critical safety components found in all modern boilers. Their job is simple but vital: to confirm that flue gases are being safely expelled from the boiler before ignition begins. If the switch doesn’t confirm safe airflow, the boiler won’t fire—and for good reason. In this guide, we’ll explain what air pressure switches do, how they work, why they fail, and how to test and replace them. Whether you’re a heating professional or a savvy homeowner, understanding this component is key to reliable boiler performance.
What Is an Air Pressure Switch?
An air pressure switch (APS) is a safety device that ensures the fan is pushing combustion gases out of the boiler and into the flue correctly. It prevents the boiler from igniting if there’s a risk of dangerous gases recirculating into the home.
The switch works by detecting air pressure changes generated by the fan. If the correct pressure is detected, it closes an electrical contact and signals the PCB to proceed with ignition. If not, the boiler will lock out.
How Air Pressure Switches Work in a Boiler
- When there is a demand for heat or hot water, the boiler initiates the fan.
- The fan creates air movement through the combustion chamber and flue.
- This airflow changes the pressure inside a small chamber connected to the APS.
- If pressure is within safe limits, the switch activates and sends a signal to the PCB.
- The PCB then allows the boiler to fire up safely.
Without this safety confirmation, the boiler cannot and should not ignite.
Why Air Pressure Switches Matter
Air pressure switches are one of the key safety features in condensing and non-condensing boilers. Their benefits include:
- Prevents ignition if the flue is blocked or fan has failed
- Reduces the risk of carbon monoxide leaks
- Improves system diagnostics with fault codes
- Ensures combustion safety before gas flow begins
Common Signs of a Faulty Air Pressure Switch
- Boiler shows ignition fault or fan fault codes
- Fan runs but boiler doesn’t fire
- Repeated lockouts, especially in windy or damp conditions
- Intermittent ignition when demand is present
- Clicking or no response from the APS during fan operation
These faults can be frustrating and often mimic issues with other components, such as the PCB or fan. That’s why pressure switch testing is part of every good diagnostic process.
Causes of Air Pressure Switch Failure
- Diaphragm wear – the internal membrane that detects pressure becomes brittle
- Blocked flue or condensate pipe – causing abnormal pressure levels
- Faulty fan – not creating enough airflow to trigger the switch
- Condensation ingress – water reaching the switch via poorly insulated pipework
- Loose or damaged wiring – interrupting the switch’s signal to the PCB
- Age-related failure – older switches degrade and drift out of calibration
How to Test an Air Pressure Switch
What you’ll need: Multimeter, manometer (optional), boiler manual
- Isolate the boiler power
- Remove the front cover and locate the air pressure switch
- Inspect for damage or condensation in the tube or casing
- Set multimeter to continuity or resistance mode
- Check terminals while the fan runs to see if the switch activates
- Confirm activation pressure using a manometer (if available)
Always consult the boiler’s manual for pressure tolerances and wiring details. Never bypass an air pressure switch—this is dangerous and illegal.
When to Replace the Air Pressure Switch
Replace If:
- No continuity when fan is active
- Switch rattles or shows moisture damage
- Resistance values are out of range
- Pressure readings fail to reach the manufacturer’s spec
Test First If:
- You suspect a flue or condensate blockage
- The fan sounds weak or inconsistent
- There’s visible corrosion but the switch still functions intermittently
How to Replace a Boiler Air Pressure Switch
- Isolate power and remove boiler casing
- Disconnect the electrical terminals and tubing from the APS
- Unscrew or unclip the switch from the boiler frame
- Fit the replacement switch and reconnect all leads and tubes
- Power on and test operation
Always use a like-for-like part or compatible match. Use our Air Pressure Switch Finder for model-matched parts.
Maintenance & Prevention Tips
- Keep flue terminals clear of debris, leaves or snow
- Lag external condensate pipes to prevent backflow into APS
- Check tubing for splits or condensation during every boiler service
- Replace older switches every 5–10 years, especially if in damp areas
Expert Insight
“The air pressure switch is a go-to check when a fan runs, but nothing else happens. Nine times out of ten, it’s either a worn diaphragm or blocked flue.” — Gas Safe Engineer, Leeds
“Always inspect for condensation build-up. In winter, we see loads of switches damaged by backflow from frozen condensate pipes.” — Technical Team, NBS
FAQs
How long does an air pressure switch last? Typically 5–10 years, but lifespan can be shorter in damp or coastal environments.
Can I bypass a faulty switch to get the boiler running? No. It’s a critical safety feature. Never bypass it.
Do all boilers use the same pressure switch? No. They are model-specific. Always check compatibility before ordering.
Can I test an APS with just a multimeter? Yes, you can confirm continuity—but to check pressure tolerance, use a manometer.
What happens if the switch fails mid-cycle? The boiler will shut down as a safety precaution and likely show a fault code.
