Boilers · Homeowner answer
Why does my boiler keep losing pressure?
Because water is leaving the sealed system. The usual causes are a small leak on a radiator valve or joint, a pressure relief valve letting by, or an expansion vessel that has lost its charge.
Most sealed systems want about 1 to 1.5 bar when cold. Topping up through the filling loop brings it back, but it also brings in fresh, oxygenated water — which is what causes corrosion and sludge over time. That is why repeated topping up is worth fixing rather than living with.
If the pressure shoots up close to 3 bar when the heating is on and then drops, the expansion vessel is the likely culprit. That is a service-visit job, not a DIY one.
What changes the answer
- how fast the pressure drops — hours, days or weeks
- whether it drops only when the heating is on
- whether the pressure climbs very high when hot before dropping
- recent work on radiators or pipework
What you can check yourself
- look for drips or staining under radiator valves and around the boiler
- check the copper pipe running outside from the boiler — dripping there points to the relief valve
- note the pressure cold in the morning and hot in the evening for a few days
What to do next
Topping up gets you through, but if you need to top up more than once or twice a year, the cause needs finding before it damages the boiler.
This page gives general information only. Christy needs to see your system — in person or from your photos — before he can give you firm advice or a price.