Why does my Boiler Pressure go high to 3 Bar (Red Zone) and then drop?

Why does my Boiler Pressure go high to 3 Bar (Red Zone) and then drop?

If your boiler pressure is rising high to 3 bar and then drops, the cause is a faulty expansion vessel. The expansion vessel is designed to accommodate excess pressure when the boiler is operating but if it is faulty, it cannot perform its job. This triggers the Pressure relief valve (PRV) to open automatically, resulting in boiler pressure drop. The PRV is a safety device which prevents the system from becoming over pressurised.

If the pressure relief valve is faulty, the boiler pressure will remain dangerously high. In this case, the PRV will need to be replaced as well. A faulty expansion vessel and a faulty PRV are often related issues that often need to be replaced simultaneously.

What is an Expansion Vessel and what does it do?

The expansion vessel is designed to absorb excess pressure when the boiler is on and heating up. An expansion vessel should be full of air. A typical boiler should maintain pressure within the range of 1.0 to 1.5 bar when the system is cold. As the water heats up, it expands, and the pressure naturally increases. The expansion vessel absorbs this excess pressure, preventing the system from exceeding safe limits which should be no more than 2 bar. However, if the expansion vessel fails, it can no longer regulate the pressure effectively, leading to the following symptoms:

1. Pressure Rising to 3 Bar – When the boiler heats up, there’s no buffer to absorb the expanding water, causing pressure to rise abnormally.
2. Pressure Drops Quickly – Once the boiler reaches 3 bar, the pressure relief valve (PRV) will open to release excess pressure. The PRV is a safety device designed to prevent the boiler from being over pressurised. If the PRV is working correctly, this is the expected process. However, if the PRV is also faulty, the boiler pressure will not drop automatically, posing a safety risk.
3. Frequent Topping Up – You may yourself frequently topping up your system to maintain the correct pressure of 1.5 bar.
4. Cycle of high and low pressure: Once you top up the pressure and run the boiler, it rises to 3 bar and drops and this cycle repeats. If so, this means you have a faulty expansion vessel with a working PRV
5. Boiler pressure goes high but will NOT go down: Faulty expansion vessel and Faulty PRV

Causes of a Faulty Expansion Vessel

Several factors can lead to the failure of the expansion vessel, including:

• Loss of Air Pressure: The vessel contains an air pocket that helps absorb pressure. Over time, this air can escape, making the vessel ineffective. If the diaphragm is not pierced, expansion vessel can be recharged using a suitable pump
• Rubber Membrane Failure: Inside the expansion vessel, there’s a diaphragm that separates water from air. If it becomes damaged or worn out, it can no longer perform its function which is to absorb excess pressure. When the diaphragm has a hole in it, the expansion vessel will be filled with water rather than air. The vessel should be light and not have any water in it, so that it can absorb the excess pressure.
• Blockage in the Connection Pipe: Sometimes, debris or sludge can clog the connection between the expansion vessel and the system, preventing proper pressure regulation.

How to Fix the Problem

If you suspect a faulty expansion vessel, it’s important to address the issue promptly to prevent further damage to your heating system. Here’s what you can do:

1. Check the Expansion Vessel Pressure – A qualified heating engineer can check if the vessel’s air pressure is correct and recharge it if necessary.
2. Replace the Expansion Vessel – If the diaphragm inside is damaged and the vessel has water in it, it is beyond repair and requires replacing.
3. Inspect the Pressure Relief Valve – Prolonged high pressure can damage the pressure relief valve. The PRV often needs to be replaced alongside the expansion vessel.

Preventive Maintenance Tips

• To avoid pressure-related issues in the future, consider these maintenance tips:
• Schedule an annual boiler service to check the expansion vessel and overall system health.

Regularly monitor your boiler pressure gauge to catch any unusual fluctuations early such as the pressure going up of its own accord.

Conclusion

If your boiler pressure is rising to 3 bar and then dropping, it is due to a faulty expansion vessel. The pressure relief valve (PRV) plays a crucial role in ensuring the system’s safety by releasing excess pressure. If the PRV is functioning correctly, it will open as soon as the pressure reaches 3 bar to dispel the water pressure. However, if the PRV is faulty, the system will remain at a dangerously high pressure, requiring immediate attention. Addressing these issues promptly will prevent more significant damage to your boiler and system. Contact a professional gas engineer to replace these parts immediately.

COSTS: The cost of parts for expansion vessels range from £100-150. A pressure relief valve £25 -£35. The cost of labour to install the above, depends on where the expansion vessel and PRV is situated in your boiler.