Homeowners Professionals

What to do if your boiler pressure is too high

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 (PRV) 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 PRV. Too much air could result in an over pressurised system. It could also result in water coming out from the PRV. This is why it’s important that a Gas Safe engineer works out the cause of your boiler’s high pressure.

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 on. For this reason, you should always check your boiler pressure when the heating is on to get an accurate reading. 

Symptoms of high boiler pressure

As mentioned above, boiler pressure should fall within certain ranges depending on whether the appliance is on or off. So one of the easiest ways to determine if the pressure in your system is too high is simply to check your pressure gauge.  

Another possible sign that boiler pressure has increased too much is that the system shuts down. Although there are other reasons for this to happen, you should consider high pressure as a potential cause in the event of a shutdown.    

In the unlikely event that your boiler’s pressure gauge is faulty, another possible indicator that there is too much pressure in the system is a leak. This usually happens at the weakest spots, for example where the pipework joins the radiators. So, if you notice puddles or other signs of water escape, it may be worth checking the boiler pressure.  

Why is my boiler pressure so high?

There could be a few reasons for the high pressure in your boiler system. Below, we outline 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, high boiler pressure could be down to certain system parts not working correctly. The pressure relief valve 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. 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 3 bar on the pressure gauge. In most cases, the boiler will turn itself off if the pressure goes above 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.

Once any problems like these have been fixed, you can take steps to reduce the pressure in the system. One easy way to do this is 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, radiators can also be bled to remove excess pressure in the system. When you do this, you will need to remove any 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 your boiler pressure continues to creep up and you are not sure why, there may be a fault with the filling loop or the expansion vessel may need to be repressurised. These are not tasks you should attempt yourself. Instead, call a Gas Safe registered engineer to diagnose and fix the problem for you.  


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