How to Repressurise a Boiler: A Simple Guide

Last updated on October 12th, 2024

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. The filling loop is built-in on the bottom of most combi boilers but some will be fitted on the pipes below or elsewhere. Whatever filling loop you have, it is the same process.

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 much.

Boiler Keeps Losing Pressure

If your boiler is losing pressure overnight and you have to top it up regularly, you must have a leak somewhere on your heating system or the pressure relief valve is letting the water out. To find out which it is, you can tie a carrier bag to the end of the copper blow-off pipe outside.

If your boiler has lost pressure overnight again, you should check the bag to see if it’s wet and if it is, that means the pressure relief valve is letting the water out.

This is most likely an expansion vessel pressure problem, if so, you will need to recharge it or replace the expansion vessel if the diaphragm inside is split.

Leak

If the bag is dry after losing pressure overnight you must have a leak on the system which must be found and fixed. A leak could be anywhere on the full heating system; boiler, radiators, or pipework under floors.

How to Lower the Pressure

Knowing how to lower the pressure on a boiler might save you from some problems. If you increase the pressure too much and cause it to go too high, you must release some pressure.

Here are my go-to ways when out on repair jobs:

• Bleed a radiator and let the water squirt out into a bucket. This can take a while
• Open a drain-off valve on the boiler or central heating system with a hose attached
• Open the pressure relief valve on the boiler by turning the red plastic head
• Crack a nut on the boiler heating pipes or a radiator valve and catch the water in a tub

Summary

It’s good to know for sure what the problem is when your boiler is not working but the first thing you need to check is the pressure gauge.

If the pressure gauge is reading zero (or close to zero) your boiler needs to be repressurised to get it working. 

Feel free to ask any questions in the comments below, and I’ll do my best to help. If you found this post helpful, please consider sharing it.

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.