A small increase in pressure is completely normal in your heating system, however, sometimes a system can become over pressurised.

As your boiler heats up the water for your radiators and taps, the water expands. This is why you’ll notice that your boiler pressure is different when the heating is off compared to when it is on. A pressure relief valve is able to control this jump by releasing the additional pressure from the system automatically.

Boiler pressure is controlled by the expansion vessel, the balance of air versus water in your central heating system. Too little air in your expansion vessel and the pressure may become too high. resulting in the water coming out of your pressure relief valve. Too much air could result in an over pressurised system, this could also result in water coming out from the pressure relief valve. This is why it’s important that the installer work out the cause of your boiler’s high pressure.

Radiator with airlock

What pressure should my boiler be?

Your boiler should have a pressure of approximately 0.5 to 1 bar when the heating is off, and 1 to 1.5 bar when the heating is one. For this reason, you should always check your boiler pressure when the heating is on to get an accurate reading. 

Why is my boiler pressure so high?

There could be a few reasons for the high pressure in your boiler system. Below, we’ve listed some of the most likely causes.

If you’ve recently repressurised your boiler because the pressure was too low, you may have overestimated how much water the system needed. This is a very common mistake. The pressure will naturally increase when the boiler is on so, if you’re adding water to the system while the heating is off, don’t add too much. When repressurising your boiler, it’s important to keep an eye on the pressure gauge so that you know when you’ve added the right amount of water.

If you don’t think there’s too much water in the system, boiler pressure could be down to certain system parts not working correctly. The pressure relief valve (PRV) releases water when the pressure in the system becomes too high. Sometimes, the PRV can have a leak or the valve may not close properly, this could gradually reduce your boiler’s pressure. Alternatively, a filling loop (which is opened to allow water in when your boiler’s pressure is too low) may have been left slightly open, meaning that new water is constantly entering the system. This could be a reason for the increased pressure.

What happens if boiler pressure is too high?

High boiler pressure isn’t dangerous, even if it’s showing as three bar on the pressure gauge. In most cases, the boiler will turn itself off if the pressure goes about a certain level and a working PRV should successfully control the pressure, preventing it from getting too high. However, you shouldn’t just leave the boiler, a qualified heating engineer will be able to advise you.

How to reduce boiler pressure

Before the boiler pressure is lowered, you need to find the root cause of the problem. Start by checking that the filling loop is definitely closed and isn’t allowing any water to enter the system. Your heating engineer might also want to make sure that the PRV isn’t stuck.

You can easily reduce your boiler pressure by bleeding the radiators. Radiators are usually bled to remove any excess air from the system. This air can cause cold spots in your radiators and might make your heating system less efficient. However, when you’re bleeding a radiator to reduce the pressure in the system, you’ll need to remove this air as well as some water. Therefore, you’ll need a dry cloth or towel as well as a bucket or container to catch the water.

Before attempting to bleed your radiators, you should ensure that the boiler is off completely and has cooled down. Otherwise, the water that comes out of the radiators will be very hot and you could scald yourself.

Once you’ve released a small amount of water, you should return to the boiler unit to check the pressure. If you think that you need to remove more water, do so and then check the gauge again. Repeat this step until you’re happy that your boiler’s pressure is at the correct level.

Over the following days or weeks, you should keep an eye on the pressure. If it continues to creep up and you’re unsure why, you may need to call a Gas Safe registered engineer who could look at your boiler in more depth to determine the cause of the problem. 

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