To reduce pressure in a boiler, switch off the heating and allow the system to cool, then carefully bleed a radiator using a radiator key until the pressure drops to the recommended level, usually around 1–1.5 bar when cold. Check the boiler pressure gauge afterwards and stop once it reaches the correct range. If the pressure keeps rising or repeatedly becomes too high, contact a qualified heating engineer, as there may be an underlying fault.
If the boiler pressure rises above 1.5-2 bar, turn off the system and let it cool down. A simple way to lower the pressure is to bleed a radiator. Place a container beneath the radiator valve, use a bleed key to open it slightly, and release until the pressure reaches around 1-1.5 bar; then close the valve firmly.
When your boiler starts heating water, the water expands, leading to a rise in the pressure gauge reading. The expansion vessel helps the boiler manage these pressure changes. Therefore, a higher-pressure reading is generally normal when the central heating and hot water are both in use.
In this blog, we will discuss the common causes of high boiler pressure, different ways to reduce it, and when to call a professional.
Common Causes of High Boiler Pressure
Temperature Issues
Very high boiler temperature settings can make the water expand more, which may put too much pressure on the expansion vessel. Decrease the boiler flow temperature to around 60 to 65°C instead of keeping it at the maximum setting.
Internal Blockage
Rust can build up naturally inside the radiators and pipes, which may restrict water flow and cause the pressure to rise. A professional power flush can remove these deposits and improve water circulation. Installing a magnetic filter can also help prevent further buildup.
Faulty Pressure Relief Valve
The PRV is designed to release extra pressure when it becomes too high. If it gets stuck or blocked, it may not work properly. A capable engineer may need to clean or replace the PRV.
Expansion Vessel Issues
The expansion vessel allows for the natural increase in water volume as it heats up. If it becomes full of water or loses its air pressure, it may not be able to handle the expansion properly. A qualified engineer may need to recharge the expansion vessel with air or replace it if necessary.
Faulty Filling Loop
The most common reason for high boiler pressure is a filling loop that is not fully closed or is faulty. If the valves are left open after adding pressure, water can keep entering the system. Make sure both valves are completely closed. If the pressure still rises, the filling loop may need replacing.
How to Reduce Pressure in 2 Different Ways
Method 1: Release Air from your Radiator
- Prior to bleeding the radiator, turn off the boiler and let the heating system cool down.
- Make sure the filling loop or pressure the filling loop or pressure relief valve is fully closed so no extra water enters the system and raises the pressure.
- Locate the small bleed valve at the radiator’s top, typically positioned on one side.
- Position a cloth or small container beneath the valve to catch any water drops.
- Insert the radiator key and slowly turn it anti-clockwise. You may hear a hissing sound as trapped air escapes.
- Allow the air to escape until water starts coming out. Keep an eye on the boiler pressure while doing this.
- Turn the key clockwise to close the valve securely.
- Check the boiler pressure to make sure it has returned to the recommended level.
Method 2: Drain Valve or Filter Valve
- Turn off the boiler and allow the heating system to cool completely.
- Find the drain-off or filter valve, typically located near a pipe or a filter.
- Place a bucket beneath the valve to collect the water.
- Slowly turn the valve counterclockwise to let water out of the system. Release it slowly to control the pressure.
- Watch the boiler pressure gauge and stop draining once it hits the recommended level, usually about 1-1.5 bar when cold.
- Turn the valve clockwise to close it securely once the pressure is at the correct level.
When to Call a Professional for Boiler Pressure
Contact a Gas Safe registered engineer if:
- Your boiler pressure keeps rising within hours or days.
- The pressure relief valve releases water on a regular basis.
- Strange noises occur when the pressure rises.
- The boiler displays pressure-related error codes.
- You are unaware of how to lower the pressure securely.
Conclusion
Boiler pressure management is important not only for system efficiency but also for workplace safety and equipment lifespan. Keeping the pressure within the recommended range helps maintain steady performance, lower energy use, and reduce the risk of breakdowns.
If the pressure is too high, you need to let the system cool down and then carefully bleed the radiator or release a little water to reduce it to the suggested level.
Check your boiler pressure frequently, particularly after repressurising or doing upkeep. If the pressure is constantly rising, rapidly goes back up after lowering, or if there are leaks or other issues, you may need to reach out to a professional.