Vaillant Boiler Pressure Too Low or Too High? Complete Guide

Last updated on October 28th, 2023

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 a Vaillant 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 come across many boiler pressure problems.

Here I’ll give you my best advice for Vaillant boiler pressure problems based on my experiences.

What Should the Pressure Be on a Vaillant Boiler?

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.

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.

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 Pressure on a Vaillant Boiler

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 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.

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 increase 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.

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 Losing Pressure Overnight

If your Vaillant 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.

If your Vaillant 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 Vaillant boiler loses 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.

How Often Should You Need to Repressurise a Vaillant Boiler?

If you need to repressurise your Vaillant boiler often 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 Vaillant 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 bet a Vaillant F83 fault code on some boilers when the pressure is too low.

Expansion Vessel Problems

Vaillant 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 on the Vaillant boiler 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.

How to Release Pressure on a Vaillant Boiler

Knowing how to release pressure on a Vaillant boiler might save you from some boiler problems. If you repressurise a Vaillant 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 on a Vaillant boiler.

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

• Open a drain-off valve on the Vaillant 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 Vaillant boiler by turning the red plastic head.

• Crack a nut on the Vaillant boiler heating pipes or a radiator valve and catch the water in a Plumb Tub or similar.

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.

Repressurising a Vaillant boiler is an easy job. It can save you from having to pay a plumber to do such an easy task.

If you have to keep repressurising your Vaillant 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.

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FAQs

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Can you repressurise a Vaillant boiler by yourself?

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

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

What causes a Vaillant boiler to lose pressure?

There are many causes for a Vaillant boiler to lose pressure.

The most common cause of a Vaillant 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 on the Vaillant boiler 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.