How to repressurise your boiler

Posted on 8/1/2019

Have you turned up the radiators but the house still feels chilly? Or perhaps your shower wasn’t piping hot this morning. Low pressure in your boiler could be to blame so take a look at the pressure gauge. You’ll find it on the front of your boiler.

Modern boilers work best when they’re at a constant pressure, usually around 1.5 bar. Many pressure gauges will show you the minimum and maximum operating pressure on the dial. But if yours doesn’t and you’re not sure what it should be, check the boiler manual.

If you can’t find it, you can download a new one from your boiler brand’s website. Just put in the model name or number, which you should find on the boiler casing.

The causes of low boiler pressure and how to fix them

There are three main causes of low pressure in your boiler and heating system. And luckily the most common one is the easiest to fix.

  1. Radiators

    Bleeding your radiators will cause the pressure in your system to drop, sometimes quite dramatically if you let a lot of air out.

    But repressurising the system is pretty easy, it’s just a question of letting in more cold water. All boilers are different, so take a look at your manual. It will show you exactly what to do, in a few simple steps. You shouldn’t need any tools (or expertise) to do it. Another good place to look is your boiler brand’s website, which may well have a video showing you just how to do it.

  2. Make sure the power to the system is turned off before you start to repressurise the boiler.

  3. Check for leaks

    A water leak in your heating or hot water system could be another reason for a drop-in pressure – especially if you’ve tried topping it up with cold water and the pressure soon drops again. Leaks can be very hard to pinpoint because pipes are often hidden behind skirting boards, under floorboards or behind bath panels. So unless you can spot it easily (try the valves on your radiators first) it’s probably worth getting a qualified tradesperson in.

    When you examine the connections on your radiators, they can sometimes feel wet. But this can often be condensation due to changing temperatures. To check, wipe the pipe dry with some kitchen paper, and put a tray or a piece of paper underneath the joint to see if it starts dripping.

    Check the radiators' connections by wiping the pipe dry with some kitchen paper, and look underneath the joint to look for dripping

  4. Boiler fault

    Failure of the expansion valve or another boiler fault can cause your heating system to lose pressure. So it might be time for a service by a qualified heating engineer, especially if it keeps losing pressure even after you’ve topped up the system (see point 1).

All work needs to be carried out by a certified engineer, ideally one who is local and can pop round quickly to give you a quote. They’ll also know the most common faults that happen in your area. For example, if you live in a hard water area that can cause a build up of limescale in your boiler and central heating system.