How Do I Fix Low Pressure On My Vaillant Boiler?

Are you facing the frustration of lukewarm water or chilly indoor temperatures, accompanied by the ominous appearance of an F22 fault code on your Vaillant boiler’s display screen? The culprit could very well be low boiler pressure, a common issue that disrupts the comfort of your home. To diagnose this problem, start by examining the display panel of your boiler. There you’ll find either a physical or digital pressure gauge, depending on your boiler’s model. This gauge, indicated by a dial or bar, provides insight into the pressure level within your system, helping you to pinpoint the root cause of your heating woes.

 What Should The Pressure Be On A Boiler?

Typically, boiler pressure should be around 1 – 1.5 bar.  Any number less than 1 indicates that your boiler pressure may need to be topped up (repressurised)Boiler pressure tends to drop gradually over time but, if the boiler drops very suddenly, it may indicate that a major leak has occurred and you will need to call a professional heating engineer right away.  

What Causes A Boiler To Lose Pressure?

There are several causes to a boiler losing pressure over time. Common culprits include small leaks in the system, such as damaged seals or valves, which allow water to escape. Bleeding radiators or releasing water from the system for maintenance purposes can also lead to pressure loss. Additionally, fluctuations in temperature can cause the expansion vessel to malfunction, resulting in pressure issues. Faulty pressure relief valves or incorrect installation may further exacerbate the problem.

 Is Low Boiler Pressure Dangerous?

Low boiler pressure isn’t inherently dangerous, but it can indicate underlying issues within the boiler system that could pose risks if left unresolved. For instance, prolonged low pressure could lead to the boiler shutting down, resulting in a loss of heating and hot water. Low pressure might also indicate leaks in the system, which could cause water damage to the property and even pose a risk of electrical hazards if the leaks affect the boiler’s components. While low boiler pressure may not directly endanger individuals, addressing it promptly is crucial to prevent further complications and ensure the safe and efficient operation of the boiler system.

A sudden drop in pressure could indicate a major leak has occurred. This is an urgent matter and should be dealt with immediately by a professional.

Solutions For Low Pressure On A Vaillant Boiler

If your boiler pressure has been dropping gradually over time, then repressurising the system should fix the problem. We always recommend engaging a professional heating engineer to repressurise a boiler, but if you feel confident and want to try topping up the pressure yourself, these are the steps a heating engineer would take:

How To Repressurise A Vaillant Boiler

Every boiler is unique, so always make sure you check your boiler manual for the exact instructions for your model. The typical steps for repressurising a boiler are:

  1. Switch off the boiler and let it cool down. 
  2. Locate and inspect the filling loop and pressure gauge. 
  3. Open the filling loop valves to allow water to flow into the boiler. 
  4. Close the valves when the pressure reaches between 1 and 1.5 bar. 
  5. Check for leaks, restart the boiler, and monitor the pressure.  

Let’s look at the details:

Turn Off The Boiler

The entire heating system needs to cool down before work can be carried out on it. This is primarily to ensure safety and prevent potential damage. When the heating system is hot, the water inside is under pressure and can expand. Attempting to repressurise the boiler while the system is hot could lead to excessive pressure buildup, which may damage components such as valves, seals, or the expansion vessel. Allowing the system to cool down ensures that the water returns to a stable state, reducing the risk of overpressurisation and safeguarding the integrity of your boiler and heating system. Additionally, working with a cooled-down system makes it easier to accurately measure and adjust the boiler’s pressure levels.

Inspect The Boiler Filling Loop

The filling loop is usually a flexible hose or a silver-coloured pipe with a valve at each end. One valve typically connects to the mains water supply, while the other connects to the boiler. It’s important to ensure proper attachment at both ends of the filling loop to prevent leaks, which are a common cause of lost boiler pressure. Checking for secure connections at both ends can help maintain the system’s pressure and prevent potential issues.

 Open The Filling Loop Valves

Open the first isolation valve by turning it ninety degrees anti-clockwise. The isolation valves are open when the valve head is in line with the pipe (if the valve doesn’t have a lever, you can use a flat head screwdriver to do this): 

Before opening the second valve, look at the main display. While closely watching the display, slowly open the second valve:

You should hear the water entering the boiler to repressurise it. Monitor the pressure gauge on the boiler to ensure it reaches the recommended pressure level, typically between 1 and 1.5 bar.

Check The Pressure Gauge And Close The Valves

Once the pressure gauge reaches the desired level, close the second valve on the filling loop to stop the flow of water:

Then close the second valve. Ensure both filling valves are completely closed to stop the flow of water. The valves are “closed” when both valves are in this position: 

The boiler should now be repressurised.

Check For Leaks

After repressurising the boiler, inspect the filling loop and surrounding areas for any signs of leaks. Ensure that all connections are tight and secure.

Restart The Boiler

Once the boiler is repressurised and you are certain there are no leaks, you can safely turn the boiler back on and resume normal operation.

Monitor The Pressure

Periodically check the boiler’s pressure gauge to ensure that the pressure remains within the recommended range. If you notice any significant drops in pressure or recurring issues, it may indicate a larger problem that requires professional attention.

By following these steps carefully, you can effectively repressurise your Vaillant boiler and restore it to optimal working condition. However, if you’re unsure or encounter any difficulties, it’s always best to consult a qualified heating engineer for assistance.

What Shall I Do If The Boiler Doesn’t Repressurise?

If you have followed all the steps above but your boiler pressure levels drop as soon as you shut the valve, your boiler is likely suffering a major leak or there is a fault with the boiler’s pressure release valve. We don’t recommend trying to resolve these issues yourself, always contact a professional.


It is important to note that you shouldn’t have to repressurise your boiler more than once or twice a year. If it is consistently happening, you’ll need to get it professionally investigated. If you are in our catchment area, please feel free to give our fully qualified team of heating engineers a call on 020 8879 7372 or contact us online at your convenience.