July 5, 2022

What should I do if my combi boiler loses pressure?

Although your combi boiler losing pressure is a sign that the boiler is not working correctly. It may be something you can rectify. Firstly, you need to understand how your combi boiler system works. If you have a combi boiler it means that you have a closed central heating system. A closed system is one that is sealed from atmospheric pressure.

How to identify changes in boiler pressure

When the water is cool within the central heating system, the normal pressure is between 1.0 and 1.5 bars of pressure. This should be in the green range on the boiler’s pressure gauge. Low pressure is below 1.bar. If the pressure gauge indicates that the pressure is 0.5 bar or below, this is an indication that the system has lost water, which needs to be replaced by manually re-pressurising the system. Low pressure will show on the pressure gauge in the red area. If your pressure gauge is digital the pressure reading will be in numeric figures.

Similarly, if the system’s pressure is too high, in excess of 2.75 bars, this will be in red. Excess pressure can be removed from the system by bleeding a radiator until the pressure dial returns within the normal range. It may be advisable to call a Gas Safe certified heating engineer if your system is over-pressurised because the water will be extremely hot. Bleeding the radiators, may not have instant results, so calling in the experts is the best solution.

Why is it important that your combi boiler pressure is constant?

Your combi boiler system requires constant water pressure to function at its optimal efficiency level. In most contemporary combi boilers, a constant level of water pressure is achieved, by the flow of cold water drawn from the main water supply through the filling loop. The filling loop is a pipe, which although not part of the boiler is situated close to it. The filling loop connects the mains water supply to the central heating system. It fills the central heating system with water and pressurises the system.

What does a combi boiler losing pressure mean?

If the boiler is losing pressure, it is losing water. This means that there is a problem with the boiler. It is relatively easy to identify if your boiler pressure keeps dropping. Combi boilers usually have an inbuilt pressure gauge. The boiler pressure dropping will be flagged up by a fall in the pressure gauge. Even though combi boilers draw in water via the filling tube, directly from the mains water supply, it will lose pressure once it enters the household’s pipe system. This may mean that the water pressure is much lower when it reaches your taps.

Expansion vessel problems

If your boiler keeps losing pressure it could be from a failure in the expansion vessel. The expansion vessel is also known as an expansion tank. It is a small tank that protects a closed central heating system from detrimental, excessive pressure. Air aids the flow of hot water through your boiler and pipes to the radiators of your central heating system. Water expands and contracts according to whether it is hot or cold. The air in the pipes protects them from damage when the water expands as it heats up. The expansion vessel prevents the boiler pressure relief valve, releasing water to reduce pressure, simply from the expansion caused by the hot water. If the expansion vessel is not working correctly, it damages the boiler pressure relief valve that regulates the water pressure in your boiler system.

Water leaks

There could also be a water leak on or in your boiler, or somewhere in the central heating system.

Examine possible sources of leaks, such as radiators, central heating water pipes, including those to the boiler. You should not open the boiler casing to look for leaks, this should only be done by a Gas Safe certified heating engineer. If there are no signs of leaks on the exterior of the boiler, and the rest of the central heating system, you should call your Gas Safe certified heating engineer.

Recently bled radiators

If you have bled your radiators recently that may be the cause of your boiler pressure dropping. The process of bleeding your radiators removes the excess air from them, which will be replaced by hot water, that radiates the heat. The resultant loss of air from the closed system will lead to a fall in pressure.

How to increase boiler pressure

If the cause of your boiler pressure dropping isn’t a water leak or damage to the expansion vessel, you may be able to manually increase the boiler pressure. This will also work in the short term if the problem is due to water leak or expansion vessel damage. However, you will still need to get the leak or expansion vessel repaired or replaced by a Gas Safe certified heating engineer.

Before attempting to manually re-pressurise the boiler system, you must consult your boiler manual, to see if it is safe to do this. This will also tell you what the correct pressure levels for your boiler are.

For example, if your Worcester boiler pressure is low, consult your Worcester boiler manual, which is available online, if you do not have your paper copy.

With low pressure, if the boiler manual advises that a manual re-pressurise of the system is possible, follow the manufacturer’s process for re-pressurising your system.

The process of re-pressurising a boiler involves drawing in water from the main water supply into your system. This will be facilitated through the filling loop described above.

To re-pressurise your boiler, if it’s losing pressure, follow the simple step by step process:

The boiler must be cool. Switch off. Wait for it to cool.
The filling loop must be attached at both ends. One end to the boiler system, the other to the main water supply.
Both valves on the filling loop should be turned to open. This will allow the cold water from the mains supply into the boiler’s system.
Watch the pressure gauge until it reads 1.5 bar of pressure. If you cannot see the pressure gauge, you may need another person to watch it for you.
Close the valves on the filling loop.
Switch the boiler back on. You may have to press the reset button.
Remove the filling loop by undoing both ends. Be careful to avoid spilling any excess water.
Store the filling loop in a safe place.