how to repressurise a Vaillant boiler

How to Repressurise Vaillant Boiler Pressure

Last updated on June 8th, 2024

When your Vaillant boiler pressure is too low the boiler will stop working which means you will have no heating or hot water.

To repressurise it, 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 Vaillant boiler should be between 1 and 1.5 bar when the central heating is off and the radiators are cold.

pressure on a Vaillant boiler
Correct boiler pressure

The pressure rises when the heating is on, so you will not get the proper 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, you have a problem that needs to be fixed.

Having the boiler pressure too high (above 1.5 when the system is cold), means you will need to let some pressure (water) out of the system.

You can do this via a drain-off valve. A drain-off valve can be on the boiler, a radiator valve or anywhere on the central heating pipework.

how to drain a central heating system
Radiator drain off valve

Or, you can bleed a radiator from the bleed valve into a bucket.

You will need to open the bleed valve with a bleed key and let the water out and into a bucket until the pressure has dropped back down.

How to Check the Pressure

To check the pressure on a Vaillant boiler you must press some buttons under the digital display. Annoyingly you have to use a digital display to read the pressure on Vaillant boilers.

They do have manual pressure gauges also but they are useless to most people as they don’t have any numbers on them.

Vaillant boiler pressure gauge
Vaillant boiler pressure gauge

Vaillant is the only boiler manufacturer that I know of that doesn’t have numbers on the manual pressure gauges.

It’s meant to be the grey zone is safe but the bottom of the grey zone is so much different to the top.

To check the pressure of a Vaillant boiler on the digital pressure gauge you typically have to press the top right button and then the bottom left button below the screen.

You might have to press the same button twice on some models.

How to check pressure on a Vaillant boiler
Checking pressure

Checking the pressure on a Vaillant boiler can be very frustrating sometimes, especially when repairing the boiler or working on the heating system.

Sometimes it doesn’t stay on long enough and you have to keep fiddling about with buttons trying to get it back on.

How to Repressurise a Vaillant Boiler

If your Vaillant boiler pressure drops below 1 bar you will need to repressurise it by increasing the pressure.

To increase the pressure on a Vaillant boiler you must:

  1. Open one valve fully
  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 Potterton boiler pressure to shoot up too much.

How to repressurise a Vaillant boiler

A filling loop will be built in on the bottom of your boiler on most modern and small combi boilers, but there could also be an external filling loop 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.

There should be one of these types of boiler filling loops under your Vaillant combi boiler, if not then it will be an external filling loop.

Vaillant boiler filling loop
Vaillant boiler filling loop closed

Losing Pressure Overnight

If your boiler is losing pressure overnight 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 the end of the copper blow-off pipe outside.

Boiler Leaking Water From Overflow Pipe
Boiler blow-off pipe

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 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 losing pressure overnight you must have a leak on the central heating system which will need to be found and fixed.

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

radiator leaking boiler losing pressure
Leaking radiator valve

How Often Should You Need to Repressurise?

If you need to repressurise your boiler often you have a problem that you need to get fixed.

A system with a pressure gauge is a 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.

low pressure on a Vaillant boiler
Low pressure on a Vaillant boiler

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.

You might get a Vaillant F83 fault code on some boilers when the pressure is too low.

Expansion Vessel Problems

Boilers are sometimes undersized when fitted and are not large enough to cope with a large house’s central heating system. This means the expansion vessel might not be big enough and can cause 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.

External expansion vessel fitted
External expansion vessel fitted

How to Release Pressure

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

There are a few ways to release the pressure.

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

  • Open a drain-off valve on the boiler or central heating system with a hose attached and let the pressure drop.
Drain off valve on a Vaillant boiler
Drain off valve on a Vaillant boiler

  • 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 boiler by turning the red plastic head.
Pressure relief valve on a Vaillant boiler
Pressure relief valve

  • Crack a nut on the boiler heating pipes or a radiator valve and catch the water in a tub.


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

Feel free to ask me any questions in the comment section below and I’ll try my best to help.

Please share this post if you find it helpful.


Can you repressurise a boiler by yourself?

Yes. You can repressurise a boiler by yourself, and you should learn how to repressurise a boiler if you don’t already know.

Some boilers can be tricky to repressurise but most are very simple.

What causes a boiler to lose pressure?

The most common cause of a 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.

You should ensure the heating is turned off before repressurising the boiler as it will sometimes fire up as soon as it reaches the minimum working pressure.

This can cause the pressure gauge to move around so you don’t get an accurate reading but it isn’t a big deal.


  • Steven Reid

    I am a full-time plumber and Gas Safe registered engineer. I incorporated Housewarm Ltd. in 2011 to provide heating and plumbing services to homes in Newcastle upon Tyne, England. I now blog about what I've learned over the years to help DIYers and plumbers.

    View all posts
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9 thoughts on “How to Repressurise Vaillant Boiler Pressure”

  1. Charlie

    My valiant boiler often seems to lose pressure more often lately ,by that I seem to be re pressurising the boiler about half dozen times a year, my home is wet underfloor heating, no evidence other than the loss of pressure in that no signs of a water leak? I have been re pressurising the system as required and boiler starts up ok.
    Due a yearly boiler check this week is there anything I can ask the engineer to check that might identify the pressure loss?
    Thanks in advance.

  2. Mohammad Aslam

    I have Valiant combi ecoTech
    I noticed boiler was closing down often.
    I thought pressure is low so I represurised by using water loop underneath the boiler.

    Switched on and it showed high
    Pressure over 3 bar.
    I tried bleeding radiators .
    Pressure have come down but not enough.
    I am waiting for boiler and radiators
    to cool down and replied again.

    As a lay person I don’t know what else I can do to fix this problem.


    I am 80 years old retired pensioner
    and trying to save money.
    My boiler is five years old.

      1. Hi my pressure gauge is in the red but comes down when I let water out of radiators but later on goes up again to 3 is it the boiler

        1. Steven Reid

          Hi Denise,
          It sounds like the filling loop is not closed fully causing the pressure to keep rising. If the pressure only goes up when the heating is on it’ll be because the expansion vessel needs to be recharged or possibly replaced.

  3. I currently have heating but no hot water. Pressure gauge is showing at zero. I have tried following your guide to repressurise my boiler. When opening both valves, there was no change to the gauge at all.
    Any help would be appreciated

  4. Hi, is a heat only boiler for central heating or does it heat the water as well.

    1. Steven Reid

      Hi Andy, a heat only boiler heats the hot water cylinder as well as the central heating.

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