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Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing). Published: January 23, 2024. Updated: May 10, 2024

Boiler Pressure Low – What to do if your boiler pressure is too low

The pressure on your combi boiler or system boiler should be anywhere from 1 to 2 bar. If the boiler pressure is too low the boiler may not work.

This can be worrying, especially in winter, or for vulnerable children and adults.

How to repressurise your boiler

An external filling loop | Boilerhut

An external filling loop.

Take a look at your boiler’s pressure gauge. It should have a green area highlighted that covers between 1 and 1.5 – if your boiler pressure falls below this, then it is too low.

For more detailed instructions, you can also take the following steps:

  1. Turn your boiler off.
  2. Locate your filling loop.
  3. Turn the filling loop handle(s) to match the direction of the pipe. If there are two, turn one all the way. Then carefully, and slowly open the second.

  4. Watch the pressure gauge – when it returns to the green area, return the filling loop handle to the closed position.

  5. Turn the boiler back on.

It’s not ideal that your boiler pressure is too low. However, it’s even worse if it’s too high, as that’s where damage can occur.

Boiler pressure too low – How can you tell?

Look at the control panel at the front of your boiler. Almost every modern boiler has a dial with areas filled in green and red.

The green area that you see in the image above is the standard operating pressure – which usually covers between 1 and 1.5 to 2 bar. The red area tells you when the pressure is too high, but watch out for it falling below 1. In many modern boilers, if the pressure drops below around 0.5, there is a low-pressure cut-off switch.

For this article, we’re going to look at what happens if the pressure drops too low. If you’ve had to deal with an old boiler breaking down or dropping in pressure then you may be need a new boiler.

Are there other symptoms for boiler pressure too low?

Take a look at how your radiators function – do they fail to heat up the way you’d expect? Radiators will always struggle to provide heat if your boiler pressure is low. Feel the radiators that are upstairs. Do they struggle to get warm? If they do, then low pressure will likely have something to do with it.

Keep this in mind before making any purchases – the pressure gauge gives you all the clues you need.

The big question – Why?

Low boiler pressure is typically a result of a water leak or recent radiator bleed. These reasons are simple enough, but now you can go through a process of elimination to find out specifically why it’s happened.

A build-up of sludge will require a chemical or a power flush, but it won’t be the reason for your low pressure, so it’s worth making the distinction.

If it’s a water leak – how can you find it?

The first thing you need to do is conduct a visual examination of the pipework and radiators. A leak could be anywhere – including what isn’t visible. Look for water on the pipework or any stains and swelling on the skirting boards.

Joints and bends are commonplace for leaks, but nowhere more so than radiator valves. This is where leaks can occur most frequently but it often overlooked.

Be aware of the difference between condensation and leaks – the former happens more frequently in colder rooms. If you’re sure it’s a leak but are unable to find the source, you can always call in a professional.

Will bleeding a radiator affect my boiler’s pressure level?

If this is something you’ve done recently to improve the performance of your central heating system, then it will directly affect the pressure. Radiator bleeds are performed to remove air that has built up over time, causing portions of the radiator (usually the top) to remain cold. If this is the case, the pressure level won’t return to normal by itself, you’ll have to top it off manually.

If you’re looking for detailing instructions, please read our ‘How to bleed a radiator’ article.

Are there any risks with having boiler pressure too low?

Don’t worry if your boiler pressure is too low – it’s usually a result of something minor and you’ve got nothing to worry about. Often, normal operation can cause pressure to go up and down. It is common to have to top up the boiler pressure a couple of times a year.

It’s unlikely to damage the boiler in short term, but be wary – your boiler’s efficiency and lifespan could still be effected if nothing is done.

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Leaks are checked, and radiators are bled. Why is my boiler pressure still low?

It might be a problem with your boiler, or worst of all, there may be a leak on the internal heat exchanger. If it’s one of these two reasons, you should consult a professional.

When the problem persists…

If you find that the pressure has dropped again quickly, then there’s likely an issue that would need a professional assessment. You can always try pressurising the system again. Or carry out another radiator bleed.

But whether you’re dealing with a Viessmann, Vaillant or Ideal boiler, your next move should probably be to speak to an expert.

If you have any questions about your central heating pressure, or are looking into purchasing a new boiler, you can call Boilerhut on 02920 099898. Or you can get an instant free online quote by entering your details below!

Get Your FREE Online Quote Today!
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