All sealed central heating systems lose a tiny amount of pressure over months — this is normal. But if you're reaching for the filling loop every few weeks, something specific is causing the loss.
Normal vs abnormal pressure loss
Losing 0.1–0.2 bar over 2–3 months: completely normal. Needing to top up more than once a month: a leak exists somewhere.
Most common causes
Weeping pressure relief valve (PRV): The PRV opens when system pressure gets too high, releasing water. If it's weeping slightly, you'll see drips from the copper discharge pipe (usually exiting outside). The PRV needs replacing — not forcing closed.
Failed expansion vessel: The expansion vessel absorbs pressure changes as water heats. If the air charge inside it deflates, the vessel fails, pressure spikes on heating, the PRV opens, and pressure drops when cool. An engineer can test and recharge or replace the expansion vessel.
Radiator valve drips: TRV and lockshield valve connections can develop tiny drips. Check the base of all radiator valves for damp.
Hidden pipe or boiler leak: Not always visible. A pressure drop test (pressurising the system and monitoring over an hour) can confirm there's a leak when no obvious source is visible — engineering investigation then follows.
What to do
If pressure loss is frequent, book a diagnostic visit. Finding the cause early is far cheaper than the consequential damage from ongoing leaks. Contact us.