Why Your Boiler Refuses to Start After Refilling Pressure
It can be frustrating: you’ve topped up the pressure on your boiler in Chawton (GU34), only to find it still won’t fire up. In many cases in Chawton and nearby towns such as Alton, Bordon and Liphook, the culprit is an air lock in your heating system. Air locks occur when trapped air prevents hot water from circulating, leading to cold radiators and a boiler that won’t ignite. This guide explains what an air lock is, how to spot the signs and the straightforward steps you can take to clear it yourself.
Understanding Air Locks in Domestic Boilers
What Exactly Is an Air Lock?
An air lock happens when pockets of air become trapped in your pipework or radiators, blocking the flow of water. Since boilers rely on circulating heated water, these pockets can stop the pump from pushing water through, causing your boiler to go into fault mode or refuse to start.
Why Refilling Pressure Can Introduce Air
When you refill your boiler, you open the filling loop valves to let water in. This process can draw air into the system, especially if the loop isn’t fully seated or if the auto-filling mechanism is worn. Once the pressure is restored, any trapped air remains in high points of the system, such as upstairs radiators or long runs of pipework.
Signs You’ve Got an Air Lock Fault
How do you know if an air lock is the issue? Look out for these common symptoms:
- Some radiators remain cold, particularly at the top.
- Your boiler cycles on error or displays a fault code linked to poor water flow.
- The pump seems louder than normal or doesn’t run smoothly.
- Repeated pressure drops after you refill.
Steps to Clear an Air Lock Yourself
Before calling out a heating engineer, you can attempt a DIY fix. Follow these steps carefully—if at any stage you feel uncertain, stop and seek professional help.
- Turn off your boiler and let the system cool for at least 30 minutes. This prevents hot water burns.
- Isolate the filling loop and close both valves fully.
- Bleed your radiators starting from the lowest level in your home and working up. Use a radiator key to release trapped air until water flows steadily.
- Re-pressurise the system by carefully opening the filling loop valves again until the gauge reads the manufacturer’s recommended level (usually 1.0–1.5 bar). Then close the valves.
- Check pump speed if your model allows adjustment. A higher pump setting can help push air through when you fire up the boiler.
- Restart the boiler and monitor for proper ignition and water flow. Listen for any gurgling in pipework, as this may indicate residual air.
If your radiators warm up evenly and the boiler runs without fault codes, you’ve successfully cleared the air lock. If not, there may be deeper issues such as sludge build-up or a faulty valve.
Preventing Future Air Locks
Regular maintenance goes a long way towards keeping your heating system in top shape. In GU35 areas like Petersfield and Farnham, homeowners schedule annual checks to avoid mid-winter breakdowns:
- Arrange boiler servicing each year to inspect valves, seals and pumps.
- Balance your radiators seasonally to ensure even flow and reduce air pockets.
- Consider fitting an automatic air vent at high points in the system.
- Use a magnetic filter to minimise sludge, which can trap air bubbles.
When to Call in the Professionals
If you’ve tried bleeding and repressurising without success, or if your boiler keeps losing pressure, it’s time to call a Gas Safe engineer. Persistent air lock faults can indicate worn seals, pump failure or corroded pipework. In Whitehill, Liss or Haslemere, our local engineers offer same-day visits to diagnose the root cause, provide repairs and advise on options such as a new boiler installation if your system is beyond economical repair. For peace of mind, explore our boiler cover plans to protect against unexpected heating breakdowns.
Frequently Asked Questions
Q: Can I bleed all radiators to clear an air lock?
A: Yes, but start from the lowest point in your home and work up. This helps push trapped air upward and out through the bleed valves.
Q: How often should I check my boiler pressure?
A: Aim to check your pressure gauge monthly. If you notice frequent drops, schedule a professional inspection to rule out leaks or faulty components.
Q: Is it risky to work on my boiler alone?
A: Routine tasks like bleeding radiators are safe if you follow manufacturer guidance. Anything involving internal boiler components or gas supply should be handled by a Gas Safe engineer.
Q: How can I avoid boiler call-outs in winter?
A: Preventative maintenance—annual servicing, a magnetic filter and proper system balancing—helps reduce air locks, sludge build-up and sudden failures during cold snaps.
If you need help with your heating or boiler, our local engineers are here to help. Call us on (01420) 558993 or book online at embassygas.com/book.
Written by Embassy Gas — your local heating experts covering Bordon, Alton, Liphook, Farnham and surrounding areas.