How to Repressurise a Boiler: A Simple Guide

Last updated on January 5th, 2025

If your boiler pressure is too low, it can cause the system to shut down, leaving you without heating or hot water. To repressurise a boiler, you’ll need to add water to the heating system using the filling loop. With over a decade of experience repairing boilers and heating systems, I’ve resolved countless boiler pressure issues.

Repressurising a boiler

The pressure on a boiler should be between 1 and 1.5 bar when the heating is off and the radiators are cold. It rises when the heating is on so you will not get the proper reading until it has cooled down. If your boiler pressure drops below 1 bar you should increase it using the filling loop.

A filling loop is built-in on the bottom of modern combi boilers, but it could also be on the pipework. Usually below the boiler or in the hot water cylinder cupboard if you have one, but an external flexible filling loop could be anywhere on the system.

Video guide

Step-by-step guide

How to increase the pressure:

1. Open one filling loop valve fully (if there are two valves)
2. Open the other valve slowly until you can hear the water
3. When the pressure has reached 1 to 1.5 bar close both valves
4. Reset the boiler (if needed)
5. Turn the heating on

Do not open both filling loop valves fully immediately as the house’s water pressure might be too much for the boiler and cause the pressure to shoot up too high.

Boiler keeps losing pressure

A boiler that keeps losing pressure means there’s a problem that needs to be fixed.

Most common reasons:

• A Leak on the central heating system
• The expansion vessel has lost its air
• Pressure relief valve passing water
• Filling loop open

You should check if the pressure is rising to 3 bar when the heating is on to narrow down the problem. If the pressure is dropping to zero without rising too high first, you most likely have a leak somewhere on the system.

Summary

If your boiler pressure is too low or too high, fixing it is important. Repressurising a boiler is an easy job. It can save you from paying a plumber to do an easy task.

If you have to keep repressurising your boiler, you should get that problem found and fixed as soon as possible.

Have any questions? Drop them in the comments below, I’ll do my best to help! If you found this post helpful, don’t forget to share it with others who might benefit.

FAQs

Do I need to turn my boiler off to repressurise?

No, you do need to turn your boiler off to repressurise it. It is recommended to repressurise when the heating is off and the system is cold as the pressure rises when the heating is on, but I repressurise boilers all the time when the heating is on.

Why won’t my boiler pressure go up?

If your boiler pressure won’t go up it could be because the filling loop is broken or blocked, or the house mains water pressure could be too low or off – If the house’s water pressure is only 0.5 bar then the boiler pressure can not be raised above 0.5 bar.

How often should you need to repressurise?

A system with a pressure gauge is a sealed pressurised system so you shouldn’t need to repressurise it at all. If the pressure drops, the water is getting out which should not be happening. If you need to repressurise your boiler once or twice a year I wouldn’t worry about it as it’s such a small leak that it’s barely noticeable.