Has your Vaillant boiler stopped working just when you need it most? Perhaps the pressure gauge has dropped below 1 bar, your radiators aren’t heating properly or you’ve noticed a low-pressure warning on the display. While these symptoms can be frustrating, they don’t always mean an expensive repair is needed.
In many cases, low pressure can be restored safely in just a few minutes. The important thing is knowing when a simple repressurisation is enough and when repeated pressure loss indicates a fault that needs professional attention.
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What Pressure Should A Vaillant Boiler Be?
Before adding water to your heating system, it’s important to know the correct operating pressure.
Although the exact recommendation varies slightly between models, most Vaillant domestic boilers should read:
- Around 1.0–1.5 bar when the heating system is cold.
- Around 1.5–2.0 bar while the heating is running.
If the pressure falls significantly below 1 bar, the boiler may stop operating correctly or display a low-pressure warning. Equally, overfilling the system isn’t advisable, as excessive pressure can place unnecessary strain on components and may trigger the pressure relief valve.
Always check your Vaillant user manual if you’re unsure of the recommended pressure for your specific model.
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.
Frequently asked questions
Repeated pressure loss usually indicates an underlying issue, such as a small leak, a faulty expansion vessel or a pressure relief valve. If the pressure continues to drop after repressurising, arrange a professional inspection rather than repeatedly topping up the system.
Only occasionally. If you need to repressurise your boiler more than once or twice a year, the heating system should be checked to identify why it’s losing pressure.
Yes. If the pressure falls below the recommended level, many Vaillant boilers will stop operating or display a fault code to protect the heating system until the pressure is restored.
Yes, provided there are no visible leaks or fault codes and you follow the manufacturer’s instructions. If the pressure drops again after topping it up, it’s best to have the boiler professionally inspected.