Fault guide
Boiler pressure keeps dropping, why, and how we fix it
If you are topping the boiler up every week, the water is going somewhere. In practice it is almost always one of four things, and each one is provable rather than guesswork.
The four usual causes
We work through these in order, because they get progressively harder to reach.
- A leaking automatic air vent on the boiler or in the airing cupboard, often dripping quietly onto pipework.
- An expansion vessel that has lost its charge, so pressure swings hard between hot and cold and the relief valve lifts.
- A passing pressure relief valve, usually spotted as a steady drip from the small pipe outside the wall.
- A genuine leak on the system: a radiator valve, a joint under a floor, or a towel rail that weeps only when hot.
Simple checks first
Look at the pipe outside where the pressure relief discharges. If it is wet or dripping while the heating runs, you have found your answer without lifting a floorboard.
Then check under every radiator valve and around the boiler case with a dry tissue. Slow weeps evaporate off warm pipework and leave nothing but a faint stain, so touch is more reliable than eyesight.
What we do about it
We test the expansion vessel charge, check the relief valve, inspect the air vents and then, if needed, isolate and pressure test the system in sections to narrow down where water is escaping.
Once it is fixed we refill with inhibitor, because the constant topping up will have diluted whatever was in there and left the system corroding faster than it should.
Questions we get asked
- What pressure should my boiler be at?
- Roughly 1 to 1.5 bar when the system is cold, rising a little when the heating is running. Well below 1 bar and the boiler will usually lock out.
- How often is it normal to top up?
- A sealed system should hold pressure for months. If you are topping up weekly, water is going somewhere and it needs tracing rather than refilling.
- Is losing pressure dangerous?
- Not dangerous, but it is doing damage. Every refill brings fresh oxygenated water into the system, which accelerates corrosion and sludge, and running dry stresses the boiler.
- Can you find a leak under the floor?
- Yes. We pressure test and isolate section by section rather than lifting floors at random, so we open up in one place instead of five.