Boiler losing pressure? What to check before you call

When the heating goes on for the first time in autumn, a lot of boilers show the same problem: the pressure has dropped and the boiler stops, or shows a fault code. Most of the time it is something you can check yourself in a few minutes. Sometimes it is a sign of a leak that needs looking at.

Reading the pressure gauge

Most combi and system boilers have a pressure gauge on the front or underneath, either a dial or a number on the display. With the heating off and the system cold, it should usually read between 1 and 1.5 bar. When the heating is on and hot it will rise a little, which is normal.

• Below 1 bar: the pressure is low. The boiler may stop working altogether and show a fault code.
• About 1 to 1.5 bar (cold): about right.
• Above 2.5 bar or so: too high, which needs an engineer to look at.

Your boiler’s manual gives the exact figures for your model, so check it if you can.

Topping up the pressure

Most combi boilers have a filling loop: a short silver hose, or a built-in lever or key, underneath the boiler. Opening it lets water from the mains into the heating system. With the heating off and the system cold:

1. Find the filling loop and check both ends are connected, if it is a hose.
2. Open the valve or valves slowly, and watch the gauge.
3. When it reaches about 1 to 1.5 bar, close the valves again.
4. Reset the boiler if it is showing a fault, and turn the heating back on.

Every boiler is slightly different, and the manual shows how yours is done. If you are not sure, or the valves will not move, leave it and call us.

Why did it drop?

Pressure dropping once after you have bled the radiators is normal: letting air out lets pressure out too. Top it up and carry on.

Pressure that keeps dropping, every few days or every week, means water is getting out of the system somewhere. Common places are:

• A small leak on a radiator valve or a joint under the floor.
• The pressure relief valve inside the boiler, which lets water out through a small pipe to the outside wall. If that pipe is dripping, the valve may be letting by.
• The expansion vessel inside the boiler, which can lose its charge over time and make the pressure swing up and down.

Have a look around the radiators, valves and any pipes you can see for damp patches, drips or green and white crusting on the joints. Check the small copper or plastic pipe that comes out of the wall near the boiler, too.

When to call an engineer

• The pressure drops again within a few days of topping up.
• The pressure goes too high when the heating is on.
• You can see water dripping from the boiler or its pipe outside.
• The boiler shows a fault code that topping up does not clear.

Anything inside the boiler casing is a job for a Gas Safe registered engineer. Never take the cover off a gas boiler yourself.

If your boiler keeps losing pressure, we can find out why and fix it. It is also worth having it serviced before the winter: a service is a good chance to catch a tired expansion vessel or relief valve before it lets you down.