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Witten by Bernard Morgan (Engineer).
First Published: June 14, 2019. Updated: October 16, 2023

Boiler Pressure Too Low – What It Means And How To Respond

Boiler pressure too low can be a scary thing to deal with.

After all, how does it actually work inside that big, white boiler? The designs of combi boiler systems are made to be economical, functional and reliable. But like any piece of equipment, it might need a checkup to ensure it runs efficiently. We’re here to talk about that.

How to repressurise your boiler

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’s currently too low.

To learn how to correct this, watch the following short video at the top of this page.

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 very 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 90 degree position.
  5. Turn the boiler back on. rushing noise (going into  the system) 

If you’re not industry-savvy, you may fall victim to easy persuasion by anyone looking to make a quick buck. Don’t be scared into thinking your only option is to get a new boiler. This doesn’t have to be the case.

However, there is one question you’ll need an answer for – Is your boiler pressure too low? Firstly, don’t panic. This doesn’t necessarily mean there’s anything wrong with the boiler.

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 real damage can occur.

But what does low boiler pressure mean? How can you deal with it? And how can you prevent it from happening again? Fortunately the response ts a very simple and easy response that won’t cost you extra money or stress.

More importantly, we’ll look at how you can solve the problem of low pressure without any hassle.

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For more information on combi boilers, take a look at the following posts:

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Boiler pressure too low – How can you tell?

Look at the control panel at the front of your boiler. In almost every modern unit, there’s 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. The red area tells you when the pressure is too high, but watch out for it falling below 1 as well. In many modern boilers, if the pressure drops below around 0.5, there is a low pressure cut-off switch that can save you a lot of trouble.

For the purpose of this article, we’re going to look at what happens if the pressure drops too much and how you can respond. If you’ve had to deal with an old boiler constantly breaking down or regularly dropping in pressure then you may be looking at getting a new boiler. If this is the case, then continue reading – you might find a way of fixing the problem which could save you a lot on your annual energy bills.

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. Take a look at radiators that are higher up in the property and pay attention to how much they’re heating up. Do the tops stay cold? 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 for magnetic sludge will mean you’ll need to have 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. A leak could be anywhere – including what isn’t readily visible. Look for water on the pipework or any stains and swelling on the skirting board. 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, so keep your eyes peeled!

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 to have a look.

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 of your central heating system. 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, then 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. It’s unlikely to damage the boiler in short terms but be wary – your boiler’s efficiency and lifespan could still be effected if nothing is done.

If you’re the sort of homeowner who’s conscious of these sorts of issues, then you need to address the problem directly. If you’re already familiar with the filling loop (which we’ll talk about a little further on as well), be sure to never leave it on the boiler. Otherwise the boiler could overpressurise and blow it off!

Look for any cold patches at the bottom of any radiators. If you find any, then you’ll likely need a professional to carry out a chemical or power flush to get rid of any of remaining sludge.

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Leaks are checked and radiators are bled – Why is boiler pressure too low still?

Well for one, you don’t have to feel hopeless. Even if you’ve checked for leaks and haven’t performed any radiator bleeds, there are still other reasons for your pressure too low. It might be a problem with your boiler, or worst of all, there may be a leak on the internal heat exchanger but if it’s one of these two reasons, you should definitely consult a professional. Besides these causes, you can also repressurise the system if the needle on your pressure gauge is still between 0 and 1. When this happens, you’re letting water back in through the filling loop. If you’re confident enough to do it yourself, you can do so by following some very simple steps.

How to repressurise the system

  • Find your filling-loop and pressure gauge. If you’re repressurising, the needle on the pressure gauge should still be pointing below 1.
filling loop

An open filling loop looks like this. Note the black handle is facing the same direction as the pipe meaning this is ‘open’ and letting water through.

For the filling loop, the handles need to be at a 90 degree angle to the flow of the pipe.

  • For best results, turn your boiler off before repressurising. It should be fine, but you can always wait for the unit to cool down before going any further.
  • Turn the filling loop handle(s) to match the direction of the pipe. You should immediately hear water flowing through the pipe and back into your system. Keep it open until the pressure gauge stays within the green area (usually between 1.5-2)
  • Once it’s reached the green area, return the filling loop handle to the 90 degree position. You should hear the water stop straight away.
  • Turn the boiler back on. You may also need to press the reset button. If this is the case, it’s because the low pressure safety device on your boiler has been activated.

Continue checking the pressure gauge. If the pressure gauge stays in the green then you’ve done it correctly and everything is back to working order! The number one rule to remember at this stage is to never leave your filling loop on. This is because it might allow too much pressure to build in the system and cause a lot of damage.

When the problem persists…

If you find that over time the pressure has dropped again, then there’s likely another issue that would need a professional assessment. You can always try repressurising the system again or carry out another radiator bleed but whether you’re dealing with a Viessmann boiler, Vaillant or an Ideal boiler your next move should probably be to speak to an expert.

If you’re topping up the pressure in your system twice a year, you can consider that relatively common. However, if it’s more like once a month and no leaks are found, there may be a problem directly with your boiler. At this point, you should seek expert advice.

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 alternatively, get a free online quote by entering your details below!

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