First: what “losing pressure” usually means
A sealed heating system is meant to hold pressure like a closed bottle. If the needle is drifting down, one of two things is happening: either water is leaving the system, or the system is “relieving” pressure because something is off (and then you’re topping it back up, repeating the cycle).
Little real-world moment: we’ll arrive at a flat in Stratford or Islington, and the tenant says “I top it up every morning, it’s become part of my routine.” That routine is a clue. It tells us it’s not a one-off bleed — it’s a fault that’s been allowed to run for weeks. The sooner it’s sorted, the less chance you end up with bigger damage (or a soaked kitchen unit).
Cause 1: A small leak somewhere in the system (often not obvious)
Radiator valve weeping, a pinhole in pipework, a slow drip under a sink cupboard, staining on a skirting board. Sometimes it’s so slow it evaporates before it puddles.
Cause 2: Pressure Relief Valve (PRV) letting by
The PRV is a safety valve. If it lifts, it dumps water out through a copper pipe (often outside). If it’s lifted a few times, it can fail to reseal properly — then you lose pressure quietly.
Cause 3: Expansion vessel problem (pressure swings)
The expansion vessel is basically the “shock absorber” for pressure when water heats and expands. If it’s lost its air charge or the diaphragm has failed, pressure can rise sharply when hot, then the PRV dumps water, and later you’re low again.
Cause 4: Filling loop not fully closed (slowly topping up or letting out)
If the filling loop valves are left slightly open, pressure behaviour gets weird — and it can mask the real problem. We also see worn washers allowing tiny leaks at the loop.
Cause 5: Recently bled radiators (one-off drop)
This one’s normal: bleeding releases air, pressure drops a bit. You top up once, and it stabilises. If you’re topping up repeatedly after that — it’s not “just from bleeding”.
- Note the pressure cold and again after the heating’s been on for 30–60 minutes.
- Check visible radiator valves, tails and accessible pipe joints for damp or staining.
- Look at the PRV discharge pipe outside (if you have one) for drips or water marks.
- Confirm the filling loop valves are fully closed after topping up.
- Take photos: boiler pressure gauge (cold/hot), any damp patches, any discharge pipe evidence.
- Don’t keep refilling every day — you can make a small leak worse and trigger PRV issues.
- Don’t ignore water under the boiler “because it’s only a little”. Route to the leak page.
- Don’t keep hammering reset if it locks out — use the fault-code guide instead.
- Don’t push pressure high “to be safe”. Most systems don’t want it sat at 2 bar cold.
What an engineer will want to know (so diagnosis is faster)
If you want the call-out to be efficient, these details genuinely help (and they’re easy to gather):
- Boiler make/model (photo of the front is fine).
- Pressure cold vs hot (e.g. cold 1.1 → hot 2.6 → next day 0.6).
- Any fault codes (even if they disappear after a reset — write them down).
- Any signs of water (inside cupboard, under boiler, outside PRV pipe).
- Recent changes: radiator bleed, new radiator, recent work, or a freeze event.
If the boiler is also switching off and showing errors, it may be a lockout triggered by pressure or sensors. Use: lockouts & error codes.