how to repressurise a combi boiler

How to Repressurise a Combi Boiler Made Simple

To repressurise a combi boiler, you need to add more water to your heating system from the cold water mains supply via a filling loop. You will need to open the two valves at either end of the filling loop to let the water in, then close both valves when it has reached the correct pressure.

A filling loop will be on the bottom of your boiler on most modern and small combi boilers, but it could also be on the pipework. This is normally directly below the boiler, but could be an external flexible filling loop anywhere on the system, sometimes hidden away in cupboards or behind boxing.

What Should the Pressure be on a Combi Boiler

The pressure on a combi boiler should be between 1 and 1.5 bar when the central heating is off and the radiators are cold. The pressure rises when the heating is on, so you will not get the true reading until it has cooled down.

If it is below 1 bar, you should top the pressure up. If you have to keep topping the pressure up, then you have a problem that will need to be fixed.

If the pressure is above 1.5 when the system is cold, then you will need to let some pressure (water) out of the system. You can do this via a drain-off on the boiler or radiator valve or pipework if you can find one.

Or you can bleed a radiator from the bleed nipple into a bucket. You will need to open the bleed nipple with a bleed key and let the water stream into a bucket until the pressure has dropped back down.

Combi Boiler Losing Pressure Overnight

If your combi boiler is losing pressure overnight, then you will 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 end the end of the copper blow-off pipe outside. If your combi 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 problem, if so, you will need to recharge it or replace the expansion vessel if the diaphragm inside is split.

If the bag is dry after your combi boiler loses pressure overnight, then you must have a leak on the central heating system, which you will need to be found and fix. A leak could be anywhere on the full system, boiler, radiators, or pipework under floors.

How to Repressurise a Baxi Combi Boiler

To repressurise a Baxi combi boiler, you will need to find the filling loop under the boiler on the right-hand side.

There should be one of these types of filling loops under your Baxi combi boiler:

  • Two black knobs – These will need turning anti-clockwise like taps to open, and fully clockwise to close.
  • Two black or blue lever-type handles – These will need turning a quarter turn each to open and close.
  • Baxi Easy-Fill green handle – This will simply need to be pulled and then released to stop filling.

The two knobs or handles will need to be opened until you have reached the pressure you want, and then both closed.

How Often Should You Need to Repressurise a Combi Boiler?

If you need to repressurise your combi boiler often, then you have a problem that you need to get fixed. A combi boiler 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 combi boiler once or twice a year, then I wouldn’t worry about it, as it’s most likely such a small leak that it would barely be noticeable.

Combi boilers are sometimes undersized and are not big enough for a large house. This means the expansion vessel on the combi boiler might not be big enough, which causes problems with the pressure.

Getting the best combi boiler for a large house is very important, but adding an external expansion vessel that is large enough for the central heating system is also an option rather than upgrading to a bigger boiler.

How to Release Pressure on a Combi Boiler

Knowing how to release pressure on a combi boiler might save you from some boiler problems. If you repressurise a combi boiler too much and cause the pressure gauge to go too high, then you will need to release the pressure.

There are a few ways to release the pressure on a combi boiler. Here are my go-to ways when out on combi boiler repair jobs –

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

How to Repressurise a Combi Boiler FAQs (Frequently Asked Questions)


Can you Repressurise a Boiler by Yourself?

Yes. You can repressurise a boiler by yourself, and you should definitely be repressurising your own boiler.

What Causes a Combi Boiler to Lose Pressure?

There are many causes for a combi boiler losing 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 needs to be recharged or replaced. The pressure relief valve could also be letting by. Or, the filling loop on the combi boiler could be open or letting by.

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