Boiler pressure dropping: likely causes and safe checks
Quick summary
Boiler pressure slowly dropping usually means either a small leak in the system or water escaping through the pressure relief route. The key is to identify where the water is going before repeatedly topping up the system.
If pressure drops only after heating cycles, expansion vessel or pressure relief valve behaviour may be involved. Start with the checks below before topping up again.
Safety first
- If you smell gas or suspect carbon monoxide, leave the property and call 0800 111 999.
- Do not keep topping up pressure daily — this can damage the system and hide the real fault.
- If water is leaking near electrics, isolate power only if safe to do so.
Full checklist: Safety guidance
What to check (in order)
- Record the pressure cold. Note the pressure when the heating is off and the system is fully cooled.
- Check pressure after heating runs. Turn the heating on for a while and check the pressure again once the system is hot.
- Look for visible leaks. Check around radiator valves, pipe joints, and ceilings below pipe runs for damp patches or staining.
- Check the pressure relief pipe outside. Most boilers have a copper pipe exiting outside. Look for dripping or white staining on the wall or ground.
- Confirm the filling loop is fully closed. A partially open filling loop can cause pressure fluctuations or slow loss.
What the result means
- Pressure drops steadily all the time → likely a small system leak or a filling loop/check valve passing.
- Pressure rises when hot then drops again later → possible expansion vessel issue causing the pressure relief valve to lift.
- Water from the outside discharge pipe → pressure relief valve has opened and should be checked professionally.
What you can safely do
- Check the system pressure when cold before topping up.
- Inspect accessible radiator valves and joints for slow leaks.
- Ensure the filling loop valves are fully closed.
- If topping up, do so slowly and only to the manufacturer’s recommended pressure (often around 1–1.5 bar when cold).
- Monitor the pressure over the next day to see if it drops again.
When to call a professional
- Water is coming from the pressure relief pipe outside.
- No visible leaks are found but pressure continues to drop.
- The boiler needs frequent topping up.
- Pressure rises significantly when heating runs.
Engineer notes
Differentiate leak versus discharge behaviour. Confirm peak pressure when hot versus cold resting pressure. If pressure spikes during heat-up then drops, suspect expansion vessel charge, connection, or PRV seat issues. Also confirm filling loop or check valve is not passing.
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