Why Is My Boiler Losing Pressure? Causes and What to Do

A boiler that keeps losing pressure has usually got water escaping from somewhere, or a part that has stopped controlling the pressure properly. Worcester Bosch and Ideal both say a top-up once or twice a year is normal. If you are reaching for the filling loop every month, something needs fixing, and the top-up is only buying you time.

Start with the gauge, with the heating off and the system cold. Worcester Bosch and Ideal both give 1 to 1.5 bar as the normal cold reading. It will rise a bit once the heating is running, by around 0.5 bar according to Ideal. Other makes quote different ranges, so check your manual if the numbers don't look right for your boiler. If you have an Ideal boiler showing F1, that's its low pressure code, and our Ideal F1 guide covers it.

This guide is for sealed systems, the kind with a pressure gauge on or near the boiler, which covers combi boilers and most system boilers.

A leak in the heating system

Leaks are the first thing to rule out. Vaillant says they most often come from radiator valves and system components, and Ideal points to pipe joints near radiators. Go round each radiator and look at the valves and pipework for damp, rust or staining, then check the ceilings and walls for marks.

A small leak doesn't always leave a puddle, because Baxi notes the water can evaporate before you see a drip. A pipe under the floorboards can leak for a long time without showing in your own home, too. If you are in a tenement or an upper flat, it's worth asking the neighbours below whether a damp patch has appeared on their ceiling.

Tracing and fixing a leak on the pipework is plumbing work, and a leak inside the boiler is one for a Gas Safe registered engineer.

The expansion vessel

Water expands as it heats, and the expansion vessel inside the boiler absorbs that extra volume. When it fails, the pressure climbs too high with the heating on, and the extra can be let out through the relief valve. Then the system cools and the gauge reads low.

Watch the gauge from cold to hot. Worcester Bosch says a rise of more than 1 bar suggests the expansion vessel needs repressurising, which is a job for a service engineer. Ideal also suspects the vessel after a sudden drop with no sign of a leak.

The pressure relief valve

This is the boiler's safety valve. It lets water out if the pressure gets too high, usually through a small copper pipe running outside. Ideal says that outlet pipe should be dry, so have a look at it from outside. If it is dripping and your gauge keeps reading low, Baxi says a Gas Safe engineer needs to clean or replace the valve.

Debris can stop the valve sealing properly, and it can also be a symptom of the expansion vessel problem above. That's why the engineer needs to look at the two together.

After bleeding the radiators

Bleeding a radiator lets out air and a little water, so the pressure can drop afterwards. That's normal, and one top-up should sort it. Our guide to a cold radiator explains how to bleed one safely.

How to top up the pressure

Topping up is usually a job you can do yourself, as long as you can clearly identify the controls. These steps are based on Ideal's guidance for combi boilers. Your manual has the exact method for your boiler, and some makes use a different filling arrangement. If the pressure dropped suddenly and you can't see a leak, don't refill it. Ideal says the expansion vessel could need investigating, and that refilling without its protection could cause a failure, so call an engineer.

1. Switch the boiler off and let the system cool.
2. Find the filling controls under the boiler. They may be a flexible silver hose or a ridged connection pipe, shaped like a horseshoe. Don't touch any valve you can't identify.
3. Open the filling valves slowly and watch the gauge, because the pressure can rise quickly.
4. Close the valves fully at around 1 to 1.5 bar. A valve left slightly open can let the pressure keep rising.
5. Bleed the radiators, then check the pressure again and top up if needed.
6. Switch the boiler back on and check it runs normally. Some models need the restart button pressing after a top-up, so check your manual.

If the boiler is a system or heat-only type and the filling loop is somewhere else, or you can't find it, ask your installer or an engineer to do it.

Overfilled it? Worcester Bosch says you can bleed a radiator until the gauge comes back down to 1 to 1.5 bar.

When to call an engineer

Call if the pressure falls to zero or won't rise when you top it up, if you're topping up every month or more, if you've found a leak or a dripping relief valve, or if the gauge climbs by more than 1 bar when the heating comes on. Don't take the casing off yourself. If the boiler is still under its manufacturer's warranty, check the terms before you pay for a repair.

We repair boilers across Edinburgh and the Lothians. Call us on 0131 644 0045 with the make, model, any fault code and how often you've been topping up, and we'll talk through cost and when we can come out. Our boiler repairs page has the other checks to make before you call.