How to repressurise a combi boiler

If your combi boiler has stopped and the pressure gauge is below 1 bar, you can usually fix it yourself in five minutes. This is the call Lee takes most often, and most of the time it ends without an engineer.

Check the gauge first

Find the pressure gauge on the front or underside of the boiler. It is either a dial or a number on the display.

Reading (heating cold)	What it means
Below 1 bar	Too low. The boiler may lock out. Top it up.
1 to 1.5 bar	Correct.
Above 2.5 bar	Too high. Do not add water. Call us.

Top up the pressure

1. Turn the heating off and let the system cool for about 30 minutes.
2. Find the filling loop. On most combis it is a silver braided hose with a small valve or tap at each end, under the boiler. Some boilers, including many Worcester Bosch models, use a built-in filling key or lever instead. Check your boiler manual if you are not sure.
3. Open the valves slowly, one and then the other. You will hear water running into the system.
4. Watch the gauge. Close both valves as soon as it reaches about 1.2 to 1.5 bar.
5. Reset the boiler using the reset button, then turn the heating back on.

If it keeps dropping

Water has to be going somewhere. Common causes are a leak on a radiator valve or under the floor, a faulty pressure relief valve dripping outside, or a failing expansion vessel. Topping up again and again hides the problem and can damage the boiler. If you are refilling more than once every few weeks, book a repair.

When to stop and call

• The pressure is above 2.5 bar
• You can see water leaking from the boiler casing
• You smell gas. Leave the property and call the National Gas Emergency Service on 0800 111 999
• You are not comfortable doing it

Call Lee on 020 8050 5913. If he can talk you through it, there is no charge. If you have just bled the radiators, expect the pressure to drop a little and top it up afterwards.