Why is my boiler gaining pressure?

6 December 2023

Boilers are robust appliances, and they do have quite a wide range of operating pressures, usually in the range of 1 to 2 bar (but preferably close to 1 bar). However, the pressure in the system determines how effective and efficient it is, so if you notice that it keeps creeping up, it’s definitely something to take note of.

If anything, boiler pressure dropping is much more likely, as it’s a closed system with no way for new pressure to enter it under normal circumstances. However, faults in the construction, maintenance or operation can cause the pressure to start building up.

The simplest way to check if your pressure is high is to look at the readout on the gauge. In modern boilers, this is usually a digital readout, although you might have to go through a menu to find the pressure reading – check your boiler’s manual for details. You might also have an analogue gauge, like a clock with a needle or pointer. It should indicate the normal pressure range.

Note that it is possible for the gauge or pressure sensor to become faulty, so look out for other signs of high pressure. They include:

  • excessive noise
  • leaking from the boiler or radiator
  • water coming out of the pressure relief valve (check the outside wall behind your boiler)
  • the boiler switching itself off for no apparent reason 
  • the boiler’s pressure gauge showing no change or changing erratically when you bleed radiators or fill the system via the filling loop.

With all that in mind, here are the most likely reasons for a boiler gaining pressure.

1. Overfilled boiler system

The commonest and simplest cause of high boiler pressure is overfilling. This usually happens when the homeowner or an inexperienced person fills the boiler without monitoring the pressure levels accurately.

To fix this, you need to release excess water from the radiators through the bleed valve until you achieve the correct pressure. Always monitor the pressure gauge while doing this. If you overshoot the ideal pressure so it becomes too low, it’s not a big problem – just charge it up with fresh water using the filling loop.

2. Faulty safety relief valve

The safety relief valve is a failsafe in your boiler system that keeps your boiler’s pressure beneath the maximum allowable by discharging water when the pressure is too high. A faulty relief valve can make the system accumulate pressure without losing excess water.

To solve this problem, it’s highly recommended you call a heating engineer to repair or replace the valve. DIY attempts can further damage the system and might make it unsafe.

3. Defective expansion vessel 

An expansion vessel in a boiler is designed to accommodate the extra water volume that is produced when water is heated up. If it’s defective, it can’t do its job, potentially leading to high pressure. This issue is most likely to occur in a sealed system.

Again, this would need professional fixing from a heating engineer who may have to replace or repressurise it.

4. Heating system imbalance

An out-of-balance heating system can also contribute to high boiler pressure. This is sometimes due to radiators not heating up equally or at the right time, causing undue strain on the boiler.

Resolution can be sought in balancing the heating system, or sealing a leak, which should be done by a professional (notice a pattern emerging here …?).

5. Sealed system leak 

Leaks in a sealed heating system can cause boiler pressure to increase. This happens because the system attempts, and often fails, to compensate for the lost water. The more the boiler tries to replace the lost water, the higher the pressure gets. 

The best solution for this is to get a heating engineer to locate and seal the leak, and then ensure the boiler is running at a safe pressure.

6. Faulty filling loop

A filling loop is a short piece of pipe or tubing that is only used when refilling the boiler with water. The rest of the time it should be closed or even completely disconnected. If your boiler’s pressure is continually high, the filling loop may be letting in water because the valves are faulty or it hasn’t been closed properly. 

In this case, you’ll need a professional to replace the filling loop or ensure it’s correctly closed.

7. Excessive limescale buildup

Excessive limescale buildup can also cause elevated pressure in boilers. This most often happens in areas of the country with hard water. That’s most of the East, South, South-East and Midlands of England. The North-West and much of eastern Wales have moderately hard water too. Limescale can block pipes, effectively raising the pressure. 

Such an issue needs a professional to clean and flush your boiler system. They might also recommend a limescale reducer or water softening solution to prevent recurrent problems.

8. Incorrect installation 

Incorrect boiler installation doesn’t commonly occur but it can lead to pressure issues. If your boiler was not installed correctly, it might contribute to a range of problems, including high pressure. In this situation, the boiler may be wrongly sized for your home or improperly connected to the water supply, leading to a pressure imbalance.

The resolution to this issue would generally lie in consulting with a professional heating and plumbing engineer who can conduct a thorough system check and rectify the installation errors. If your installation is new or still under the installer’s warranty, get in touch with the engineer who installed it.

9. Defective pressure sensor

A final possibility is a faulty pressure sensor. The pressure sensor within the boiler triggers the system to relieve pressure when it becomes too high. If the sensor is defective, it may incorrectly measure the pressure or fail to signal the system to reduce pressure, leading to the boiler operating at overly high pressure levels. 

This situation requires an immediate check by a heating engineer. They can replace the defective sensor and ensure the returning pressure is safe for operation.

In all cases, it’s essential to remember that high pressure in your boiler can lead to severe damage to your property or your boiler, as well as potential safety issues. Taking prompt action at the first sign of an issue can prevent more significant problems from taking hold, and will ensure your boiler remains functional for longer.

It’s also vital never to attempt to repair a gas appliance yourself; instead, always use the services of a registered Gas Safe engineer. This will ensure all work is conducted safely to the necessary standard, keeping your home safe and warm.