All sealed central heating systems lose a small amount of pressure over time — this is normal. But if you're re-pressurising more than once every few months, there's an underlying issue.
Normal vs abnormal pressure loss
Losing 0.1–0.2 bar over 2–3 months: normal — air and very minor seepage is expected. Needing to top up monthly or more: abnormal — there's a leak somewhere.
Most common causes of pressure loss
Pressure relief valve (PRV) weeping: The PRV releases pressure when the system overheats. If it's slightly open, it drips water continuously. Look for a copper pipe from the boiler (usually exiting outside) that's damp. A weeping PRV needs replacing.
Radiator valve leaks: Tiny drips at TRVs and lockshield valves can lose significant pressure over time. Check around the base of all radiator valves.
Expansion vessel failure: The expansion vessel absorbs pressure changes as water heats up. If it fails, pressure spikes on heating and triggers the PRV. A qualified engineer can test and replace the expansion vessel.
System leaks: Joints, connections, and radiator seals can weep. A pressure test will identify the location.
What to do
If you're topping up more than once a month, book a diagnostic visit. Finding and fixing a small leak early prevents larger water damage. Contact Corby Boiler Installations.