How to Repressurise a Boiler: A Simple Guide

Last updated on February 19th, 2024

Having your boiler pressure too low causes the boiler to stop working which means you will have no heating or hot water.

To repressurise a boiler, you must add more water (pressure) to your heating system from the cold water supply via a filling loop.

I have been repairing boilers and heating systems for over a decade, and have fixed many boiler pressure problems.

What Should the Pressure Be?

The pressure on a boiler should be between 1 and 1.5 bar when the heating is off and the radiators are cold.

The pressure rises when the heating is on, so you will not get the proper reading until it has cooled down.

If it’s below 1 bar, you should top up the pressure. If you have to keep topping the pressure up, you have a problem that needs to be fixed.

If the boiler pressure rises too much (above 1.5 when the system is cold), you will need to let some pressure (water) out of the system.

How to Repressurise a Boiler

If your boiler pressure drops below 1 bar you should increase the pressure.

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 straight away as the water pressure in the house might be too much for the boiler and cause the pressure to shoot up too much.

A filling loop will be built in on the bottom of your boiler on most modern combi boilers, it could also be on the pipework.

This is usually below the boiler but there could be an external flexible filling loop anywhere on the system, sometimes hidden away in cupboards or behind boxing.

Boiler Losing Pressure Overnight

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 will need to be found and fixed.

A leak could be anywhere on the full heating system; boiler, radiators, or pipework under floors.

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, it means 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.

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 will need to 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
• Crack a nut on the boiler heating pipes or a radiator valve and catch the water in a tub
• Open the pressure relief valve on the boiler by turning the red plastic head

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 me any questions in the comment section below and I’ll do my best to help.

Please share this post if you find it helpful.

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FAQs

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Why won’t my boiler pressure go up?

If your boiler pressure won’t go up it’s usually because of two main reasons:

1. The filling loop is broken or blocked – This will need to be repaired or replaced.

2. The house mains water pressure is too low – If the house’s water pressure is only 0.5 bar then the boiler pressure can not be raised above 0.5 bar.

What causes a combi boiler to lose pressure?

The most common cause of a combi boiler losing pressure is a leak, this could be on the boiler or anywhere on the central heating system.

Another cause could be the expansion vessel has lost its air and needs to be recharged or replaced. The pressure relief valve could also be letting by. Or, the filling loop could be slightly open or damaged.

Do I need to turn my boiler off to repressurise?

No, you do need to turn your boiler off to repressurise.

It is recommended to do it 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.