BoilerFaultCodesWhy does my boiler keep losing pressure?
If you're topping up your boiler every few days and the pressure keeps dropping again, you've got a leak somewhere — the question is where. This guide walks you through the six most common causes, ranked by how often we see each one in the fault data.
Quick check first
Normal pressure is 1.0–1.5 bar cold, rising to 1.5–2.0 bar hot. If you've never used the filling loop, start with our how to repressurise guide. If you've already done that and pressure's dropping again, carry on reading.
The six usual suspects
Work through these roughly in order — the first two account for most cases we see.
Recently bled radiators
When you bleed a radiator you release air, and that drop in volume shows up as lost pressure on the boiler. Top up via the filling loop and the problem is gone — unless you're bleeding the same radiator repeatedly, which points to a bigger issue.
A small leak somewhere on the system
The number one cause of persistent pressure loss. Check every radiator valve, under carpets near radiator pipes, and around the boiler itself. Look for rust stains, white lime residue, or damp patches. Even a pinhole leak drops pressure over days.
Faulty pressure relief valve (PRV)
The PRV opens to vent water when pressure gets too high. If it's stuck partially open, your system quietly loses water through the pipe outside. Check for dripping from the overflow pipe on an outside wall — it's usually copper and leaves a lime trail.
Failed expansion vessel
The expansion vessel absorbs pressure changes as water heats up and cools down. When the internal bladder fails, pressure swings wildly — high when hot, low when cold. A Gas Safe engineer can recharge or replace it.
Leaking auto air vent
Located on top of the boiler or pump, the auto air vent releases trapped air automatically — but if it sticks open, it can also release water. Usually accompanied by a damp patch or drips on top of the boiler.
Cracked heat exchanger
The most serious cause. A cracked heat exchanger means water is leaking internally, and on many boilers it makes replacement more cost-effective than repair. Usually accompanied by visible condensation inside the casing or water under the boiler.
Fault codes for low/dropping pressure
Call a Gas Safe engineer if…
- › You have to top up the boiler more than once a week
- › The pressure drops below 0.5 bar in under 24 hours
- › You can see water leaking from the boiler
- › The overflow pipe outside is dripping when the heating is on
- › You've checked all radiators and can't find the leak