Boiler pressure low? How to check and top up a combi boiler

Combi boilers and system boilers run on a sealed heating system, which has to hold water at a set pressure to work. If the pressure falls too far, the boiler stops firing to protect itself, and you lose the heating and, with a combi, the hot water too.

If you have a regular (heat-only) boiler with a small feed and expansion tank in the loft, the heating is usually open-vented and tops itself up from that tank, so there's normally no pressure to adjust. Your boiler's manual will tell you which type you have.

Look for a dial or a digital reading on the front or underneath the boiler, marked in bar. With the heating off and the boiler cold, most combi boilers should read roughly 1 to 1.5 bar. It rises a little when the heating is running, which is normal.

If the reading has fallen below 1 bar, or the display shows a low-pressure fault code, the system probably needs topping up. A reading at or near 3 bar is too high, and the boiler's pressure relief valve will open to let water out.

Most combi boilers are topped up through a filling loop. That's either a silver braided hose with a valve at one or both ends, usually under the boiler, or a built-in lever, key or tap on the underside. Designs vary a lot between makes, so read the manual first. These are the general steps:

1. 1Turn the heating off and let the boiler cool. The reading only means something when the system is cold.
2. 2Find the filling loop and check both ends are firmly connected. If yours is a removable hose, the manual shows where it attaches.
3. 3Open the valve slowly. If there are two, open one fully and then the other slowly. You'll hear water flowing into the system.
4. 4Watch the gauge, and close the valves as soon as it reaches about 1 to 1.5 bar, or the figure in your manual.
5. 5Check every valve is fully closed. If your loop is the removable kind, the manual will usually tell you to disconnect it afterwards, so have a cloth ready for drips.
6. 6If the boiler is showing a fault, reset it as the manual describes, then turn the heating back on and check the reading again once it's warm.

A sealed system shouldn't lose much water. Needing a top-up straight after bleeding radiators or work on the heating is normal, but if the pressure keeps falling, water is escaping somewhere. The usual causes are:

• A leak on the heating system, such as a weeping radiator valve, a joint under the floor or a pipe in a wall. Look for damp patches, stains or green crusting on copper fittings.
• A pressure relief valve that's letting water through. It releases water when the pressure gets too high, through a small copper pipe that usually runs out through the wall near the boiler. If that pipe drips, or there's a wet patch under it, the valve may need attention.
• An expansion vessel that has lost its air charge. Without it, the pressure surges as the water heats up, the relief valve opens, and the pressure falls again when the system cools.
• A leak inside the boiler itself, which only a Gas Safe registered engineer should investigate.

Topping up through the filling loop and pressing the reset button are the only things most manufacturers expect you to do yourself. The relief valve, the expansion vessel and anything else inside the casing are a job for a Gas Safe registered engineer, because a gas appliance that isn't put back together correctly can leak gas or carbon monoxide.

Stop using the boiler and have it checked if you notice soot, scorch marks or a lazy yellow flame. If a carbon monoxide alarm goes off, or you smell gas, open the windows, leave the property and call the gas emergency number.

When you need help, we can send an engineer to find out why your boiler is losing pressure and fix the cause.