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Boiler pressure: everything you need to know

If you’re lucky, you’ve never had to think about boiler pressure before. But many systems experience issues with pressure at some point.

In this blog, we'll look at:

  • What should your boiler pressure be
  • How to spot if you have a problem
  • And how to fix your boiler pressure if it’s too high or too low.

Most modern boilers usually need little attention beyond an annual service and an occasional glance at the boiler pressure gauge. This tells you the pressure of the water circulating in the heating system.

    If the boiler pressure is too high, there’s an increased chance of leaks developing in the system. But if boiler pressure is too low, the system won’t work as well. So keeping the correct boiler pressure is important to make sure that your system heats your home efficiently.

    Worried about doing this yourself? If you're concerned – or are worried you have a more severe problem – call out a professional to take a look.

    How to check boiler pressure

    On most modern boilers – like a combi – the pressure gauge is a round dial on the front of your boiler or under the control panel. If you’ve got an older boiler, the boiler pressure gauge might be  underneath, amongst the pipework. Have a look in your instruction manual if you can’t find it.

    What should boiler pressure be?

    Normal boiler pressure should be between about 1-2 bars[3]. The ideal boiler pressure is often marked as a range in green on the gauge itself. With red indicating the low and high boiler pressure zones.

    Bear in mind that your boiler may continue to work if the pressure is in these red zones. But if it's shutting down or won’t switch on, you've got a problem.

    Even when your boiler’s working well, it’s a good idea to check the pressure occasionally. And to know how to release boiler pressure when it's too high. And how to top up boiler pressure – when it's too low.

    What should boiler pressure be when heating is on?

    Normal combi boiler pressure is toward the lower end of the green range when the boiler is cool. When the boiler’s hot, you'll see the boiler pressure increasing because water expands when you heat it.

    Don’t confuse the pressure in your boiler with low water pressure… How fast the water comes out of your taps or shower is determined by the pressure of your water. Boiler pressure, on the other hand, is the pressure of the hot water inside your heating system.

    Boiler pressure: low

    Symptoms of boiler pressure loss

    You'll know the boiler pressure is too low in your system, if:

    • There's no heating or hot water
    • Your radiators aren't warming up properly
    • The dial on the pressure gauge has fallen

    If you’re wondering why your boiler pressure keeps dropping, the most common causes are a leak or bleeding the radiators.

    man in cold home with broken heating

    Is low boiler pressure dangerous?

    No – the good news is low boiler pressure isn’t dangerous.

    Sometimes, heating systems have small leaks – a bit like a slow puncture in a tyre. While you might not notice any water escaping, over time the pressure in the system falls until it’s too low.

    If you’ve just bled the radiators, you might also see the boiler pressure dropping. Bleeding the radiators removes any air in the system, which can cause a change in the boiler pressure.

    How to increase boiler pressure

    Want to know how to reset your boiler pressure? The good news is you can usually top up boiler pressure yourself. If you’ve been Googling videos for how to do it, you may have seen this referred to as ‘re-pressurising’ your boiler:

    • Turn the boiler off first.
    • Look for an external filling loop directly under the boiler – this is usually a short length of braided hose linking two water pipes, with levers at both ends to work valves.
    • If you can’t find a filling loop, your boiler may have a keyed filling loop or keyless internal filling loop2instead – consult the boiler’s instruction manual if you can’t find it.
    • Turn both valves so that the levers are aligned with the direction of the braided hose. You should hear water running, and see the boiler pressure gauge going up.
    • At about 1.2 bar, shut off both valves.
    • Provided the pressure stays up, and there are no obvious signs of leaking water, it’s safe to power the boiler back up.
    • After the boiler has run and had the chance to cool down again, check that the pressure hasn’t dropped significantly.

    Ideally, you shouldn’t need to top up your boiler pressure very often[3]. But if the pressure drops more than once or twice a year, check all visible pipes for signs of water leaks. You might also find green deposits near joins in copper pipework, or stains on the skirting boards or ceilings[4]. If you find a leak or your boiler pressure keeps dropping, book a boiler repair.

    Boiler pressure: too high

    Symptoms of boiler pressure being too high

    • The pressure gauge dial has increased or gone into the red zone
    • The system has shut down

    There are several reasons why you might see the boiler pressure rising to 3 bar or above:

    • You could have accidentally put in too much water when trying to correct low boiler pressure.
    • You could have left the filling valves open accidentally – or not screwed them shut tight enough – which lets water carry on flowing in!
    engineer responding to an emergency boiler repair

    Is high boiler pressure dangerous?

    No, it's not normally dangerous[5]. The heating system is protected by a boiler pressure relief valve (PRV). This boiler pressure valve prevents damage by letting water escape – sometimes via a small pipe through the external wall near the boiler – when there’s too much pressure. Most systems also shut down if boiler pressure gets too high.

    How to reduce boiler pressure

    Want to know how to lower boiler pressure in your system by yourself? You can try to self-diagnose the problem in a few simple steps – and lower the boiler pressure:

    • Turn off the boiler.
    • Check that the filling loop valves, or the key or keyless filler, are fully closed.
    • Wait for the system to cool, then release pressure by bleeding each of the radiators[6]. Not sure how? Take a look at this blog. Be careful as it's easy to bleed the radiators incorrectly. So if you're unsure or have a mishap while carrying out the bleed – the pin gets stuck or you can't get it back in the valve, for instance – call out a professional straightaway.
    • When you’ve bled every radiator, check the pressure gauge. If it’s still too high, you’ll need to repeat the process of bleeding one or more radiators.
    • Keep checking until the boiler pressure gauge reads around 1.2 bar – or at least has returned to the green zone.
    • If the boiler pressure ends up too low, follow the instructions above to top it up again. But be sure to close off the filler valves afterwards!

    If your boiler pressure keeps rising into the red, it’s best to arrange a boiler repair. It could indicate a problem with the pressure relief valve – or a fault within the system. A Gas Safe Engineer will be able to diagnose the problem and fix it, so you’re not left without heating or hot water.

    Boiler pressure checklist

    • What does the boiler pressure gauge show? It’s normal for boiler pressure to rise as the boiler heats up, and fall as it cools down. The ideal boiler pressure is between 1 and 2 bars. But so long as the pressure gauge needle isn’t going into the red zones, there’s nothing to worry about.
    • Is boiler pressure dropping? It’s normal for some boilers to lose pressure slowly over several months. Again, it’s nothing to worry about provided you don’t have to top it up more than once or twice a year.
    • Have you experienced rapid boiler pressure loss? A rapid pressure drop is usually a sign of a leak in the system, particularly if it happens when the boiler is on. Look for damp patches, drips around the base of the boiler or water leaks.
    • Is your boiler pressure too high? High boiler pressure can be caused by over-zealous filling. But if it happens spontaneously, there may be a fault in the system. With any rapid or repeated change in boiler pressure, it’s best to call in a Gas Safe engineer to properly diagnose and fix the problem.
    Boiler service engineer with checklist

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