Combi Boiler Pressure Keeps Rising?

When it comes to your boiler, there are a number of small issues that can disrupt its working efficiency. The best thing you can do is identify the problem and ensure it’s fixed before it becomes more serious (and more expensive) down the line. 

Ultimately, for a boiler to run at maximum efficiency, a lot of things have to be just right – including the pressure. If the water pressure inside your boiler is too low or too high, the heating system simply won’t work as it should.


What is Boiler Pressure?

Boiler pressure refers to the balance of water and air running inside your heating system and it’s measured by the pressure gauge in the panel or underneath your boiler. 

Simply put, for a boiler to heat the water efficiently, the pressure has to remain at the ideal level. Low pressure can cause the system to shut down, whilst high pressure can overwork the system and cause it to fail.  For this reason, it’s crucial to check pressure levels and keep them regulated.

Checking Boiler Pressure

Boiler pressure should be checked regularly to ensure everything is at the correct level. Most combi boilers have a built-in pressure gauge that can be easily accessed and checked. For some models, the gauge will display the pressure levels using a dial, but for newer models the display may look more like a digital watch that can also show additional information such as temperature and pre-set timers. The display will vary from model to model, so take a look at your boiler manual for more specific information.

What’s the Ideal Boiler Pressure?

For combi boilers specifically , the ideal pressure levels are roughly as follows:

  • When the heating is turned off, the pressure gauge should read between 1 and 1.5 bars.
  • When the heating is switched on, the pressure should ideally be around 2 bars.

As mentioned, any number of small issues may cause fluctuations in boiler pressure which can go on to impact your whole heating system. One of the most common issues we hear about is rising pressure. But remember this, an increase in boiler pressure isn’t an immediate cause for concern! The boiler pressure should change depending on if the heating is switched on – in fact, a small rise in pressure between when a boiler is switched off and when it is heating up is a sign that the boiler is working perfectly.

However, a warning sign to look out for is if the boiler pressure rises above the levels stated above. If you spot your boiler pressure rising outside of these ideals, it’s normally a sign of an underlying issue that can be fixed either by a qualified engineer or, depending on the problem, by yourself at home.  

High and Rising Pressure

So, you’ve checked your pressure gauge and have spotted the levels rising. You’ve also noticed the boiler keeps cutting out abruptly after the pressure increases. This is unfortunately a sign that something’s not right. 

High and continuously rising boiler pressure is not inherently dangerous, but can lead to expensive damage down the line. If pressure gets too high, more often than not, your heating system will simply cut out due to the PRV (pressure release valve) in place. In this case, your boiler efficiency will be severely compromised. Continuously high and rising pressure will put your boiler under a lot of stress and can damage vital components such as valves, which leads to leaks. In these situations, those components may have to be replaced – and in more severe cases an entirely new boiler may have to be fitted.

The good news is, if you isolate the cause early enough, the fix can be fairly straightforward.

High and rising pressure can be caused by a variety of factors. Here are some of the most common, alongside some solutions you may be able to carry out yourself.

Too much water

If a boiler has experienced low pressure in the past the fix has most probably been to repressurise by topping up the water level. However, if the gauge has now started to display high pressure, chances are the boiler has been overfilled. In this situation, the solution is an easy one – you need to drain some water from the system.

How do you do this? Luckily this is one of the quickest solutions. Here’s how to drain some water from your heating system:

  • Bleed the Radiator – If you’ve ever read up on excess boiler pressure, you’ll find this to be one of the most recommended methods of draining water from your heating system. The first step is to switch off your boiler and simply locate the valve on the radiator. Using a radiator key, turn the valve anti-clockwise. When doing this you should start to hear air escaping before you see a small amount of water being released- there shouldn’t be a massive amount, but it still might be a good idea to make sure you have something to catch or wipe this up with afterwards.


  • Open the Filter Valve – Some boilers may not have one of these, but they are common in a majority of models. Heating systems often have a filter that cycles through water and removes any impurities. This filter and its valve can also act as a drainage system. If you suspect you have an overfilled boiler, open the filter valve to drain some of the water out and stabilise the system’s pressure.


If, after using these suggestions, you find the pressure stabilises and remains within the ideal limits, then the problem’s been solved! However, if you see the pressure starting to creep up again, this may be the sign of a different issue that might need to be addressed by a boiler professional, including:

Blocked and Damaged Pipes

All closed systems (i.e. a combi boiler) have an expansion vessel to allow for excess water levels. In these cases, there is a pipe leading from the regular boiler to the tank. Unfortunately, these pipes can become clogged by dirt and debris, which results in a pressure build up within the system. To check this, turn off your heating system and look to see if there is a build up in the pipe. If so, clean it out and put it back in place. If this was the cause, you should see pressure levels return to normal. In some cases, the pipe may have gotten damaged which is causing a blockage. If this is the case, an engineer will have to be called in order to fix or replace the pipe in question.

The Expansion Vessel

One of the prime reasons why a boiler’s pressure may keep rising is a fault in the expansion vessel itself. As touched on above, an expansion vessel is there to maintain pressure due to the expansion of gas and water whenever the system is heated. If there’s a fault with the expansion vessel, the pressure in the system will not be regulated. So, if you’ve checked the valves and bled your radiators and still find the pressure increasing. In this case, it’s most probably the expansion vessel. However, it’s incredibly important not to attempt work on this yourself. If you suspect an issue with your expansion vessel or damage to a boiler component, call in a qualified engineer to take a look.