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Why Does My Boiler Pressure Keep Dropping?

Pretty much every homeowner will have experienced their boiler’s pressure dropping at some stage in the past, often presented as your radiators not warming up properly, a lack of hot water or your boiler turning itself off and displaying an error code. 

But whilst determining the problem is often as simple as checking the pressure gauge on your boiler, there’s a good chance you’re not going to know what’s causing this to happen (and that’s why you’re here, right?). 

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And in this guide, we’ll walk you through the most common causes, which of these problems you can (easily) fix yourself and when you need to call out a gas engineer. 

Specifically, we’ll look at:

Is a Boiler That’s Losing Pressure Dangerous?

Whenever you discover that there’s a problem with your boiler, however small it may be, there’s always going to be some element of concern over whether or not it’s safe to continue using and if it’s in any way dangerous.

You’ll be pleased to know that, in most cases, low boiler pressure isn’t dangerous or something that should cause you to panic, although it will prevent your boiler from heating your home and water as it should be doing.

If your boiler’s pressure drops too low, you’ll usually find that it turns off and displays an error code, and this can help you to quickly diagnose that there’s a problem and take the necessary steps to resolve it.

If you’re experiencing low pressure, don’t panic. It’s a common problem and one that’s usually simple to fix. 

What Are The Signs that Your Boiler Pressure Has Dropped? 

Whilst boiler pressure is something that you can check yourself; the reality is that most people don’t do this until they’ve discovered there’s a problem with their central heating or hot water. 

But what are the signs that your boiler’s pressure is low? If you’re experiencing any of the following, go and take a look at the pressure gauge, as there’s a good chance that it’s the cause of the problem:

  • You run the hot tap, but the water doesn't warm up
  • Your radiators aren’t heating up when your central heating is turned on
  • Your boiler has turned itself off, and an error code (learn more about relevant error codes below) is showing

Keep reading to learn how to check your boiler pressure and some suggestions on what’s causing it to drop.

How To Check If Your Boiler Pressure Is Low?

It is incredibly easy to check your boiler pressure and takes just seconds. 

In fact, this is something you can do on your own and that we recommend you check regularly.

You can check your boiler pressure by:

  1. Locating the pressure gauge on your boiler. This looks a little like a clock face with coloured bars and numbers typically from 1.0 to 4.0. 
  2. The needle in the green zone means your boiler is at the correct pressure
  3. The needle in the red zone means either low pressure (before the green zone) or high pressure (beyond the green zone)
  4. Some more modern combi boilers or system boilers do not have a visible pressure gauge and will display on the front digital display instead - refer to your user manual if unsure, as this should be something that is easily checked. 

What Should Your Boiler’s Pressure Gauge Read?

Normal boiler pressure is between 1 and 1.5 bar when your boiler is turned off and cool, and slightly higher when it’s in use.

You’ll see this shown as the green zone on your boiler’s pressure gauge. 

Is It Normal for Boiler Pressure to Drop?

It’s not uncommon for your boiler to lose a little bit of pressure slowly, and this can usually be resolved very quickly by repressuring the system.

But a rapid drop in pressure, or a repeated drop after you’ve repressurised, usually means that there’s a problem somewhere.

What Causes A Boiler To Lose Pressure?

To help you diagnose the reason why your boiler keeps losing pressure, you need to know what the most common causes are. 

There could be numerous reasons why your boiler pressure keeps dropping, but the most common ones are:

  1. A recently bled radiator
  2. A leak in your central heating system
  3. A leak in your boiler

Knowing the cause helps you to determine whether you can resolve the issue yourself or need to call out a gas engineer, and gives some indication as to whether you should expect to need to have your boiler replaced.

But when it comes to finding the problem, it helps massively if you understand what exactly boiler pressure is.

Understanding Boiler Pressure a Bit Better

Modern boilers use a filling loop to bring in cold water and then run the water through a network of pipes and radiators throughout your home. However, if the system only used cold water, it wouldn't be a heating system.

Your boiler heats the water. Just as with a pressure cooker on the hub, boiling water in an enclosed container creates pressure. And, just like cooking, that pressure is a good thing.

But, pressure also needs to be stable and constant. 

Boiler pressure moves the hot water effectively and efficiently through your home’s central heating system and supplies hot water to your taps.

A loss of pressure means the water moves slower or not at all, that no heat is being dispersed, and that the water cools down faster. This results in the boiler working harder and costing more to operate.

To put it simply, the boiler needs constant and stable heat and pressure to work at its best and run most cost-effectively.

Let’s now take a look at what the most common causes of dropping boiler pressure are.

IMPORTANT: We cannot stress this enough; always inspect an installed and operating boiler with caution. Boilers are very safe appliances but remember, a gas safe engineer and expert can find the cause of a boiler losing pressure and inform you of the best solution. Customers cannot remove the case on most boilers and should not attempt to do so.

1. Recently bled radiators

If you’ve recently bled your radiators, then this is likely to be the reason why your boiler has lost pressure.

This is because when you bleed a radiator, air is released from your central heating system, which lowers the pressure. 

After bleeding a radiator, be sure to check the pressure gauge and top up the pressure if needed.

2. A leak in your central heating system

This is the most likely reason for a boiler losing pressure, but locating the leak can be tricky. The boiler operates with a network of pipes and radiators, so the leak could be anywhere in your home throughout the pipework, with the potential of being within a wall or under the floor. If the leak in the system is not found and fixed, the boiler will continue losing pressure.

How to find a leak in your central heating system

The good news is that a leak in a central heating system tends to happen at the most vulnerable points; the joints and fixtures. So that is where the investigation should always begin.

The connections to radiators are one of the most common points for leaks. 

Depending on the type of boiler you have, the network of pipes, fixtures, and radiators creates a closed or sealed circuit. So, if you can see or feel water accumulating near a joint or fixture, this can be evidence of a leak, but it can also be a normal result of condensation.

Look for tell-tale signs of a leak around radiators and pipes, such as:

  • Discolouration
  • Staining
  • Paint bubbling or flaking
  • Softened or rotting wood
  • Rust marks
  • Swelling
  • Bulging or lifting of the skirting board
  • Other obvious water-related damage

If you suspect there is a leak, you can also place a bowl underneath the joint or fixture and see if water begins to pool. If that happens, it's most likely an obvious sign of a leak and thus a reason for why the boiler is losing pressure.

If you cannot find any evidence of leaks in these areas, any possible leak could be inside a wall, under the floors, or inside the boiler itself.

How to fix a central heating system leak

You shouldn’t attempt to fix a leak in your central heating system yourself, but call a gas engineer who will be able to resolve the problem far faster and give you the reassurance that everything has been done safely, as well as preventing further water damage.

If you’ve found a leak, lay down towels and place bowls under the leak to reduce the chance of further damage.

3. A leak in your boiler

If you cannot spot leaks from your radiators or pipes, it’s worth knowing that a boiler losing pressure can also result due to boiler leaks inside the unit itself. Over years of use, parts inside the boiler can become corroded or loosen, resulting in low boiler pressure.

One of the more common leaks among parts in your boiler can be a PRV letting by, and this often results in a wet patch outside your boiler where the pressure relief valve terminates, usually on the other side of the wall. 

Some boilers that aren't on the outside wall could have long runs or they may terminate internally, so they may be more difficult to identify (if your boiler is in the airing cupboard with a flue through the roof, for example). 

This could include:

  • A faulty pressure relief valve
  • A damaged expansion vessel
  • The filling loop
  • A diverter valve

How to find a boiler leak?

The most obvious evidence that the leak is at the boiler unit is if there is water dripping from the boiler or accumulated around the boiler. 

If you see this, your first step is to switch off the boiler to prevent further damage and any potential short-circuiting.

However, the boiler unit must be opened to find the specific location of a boiler leak. 

You should not attempt this yourself. 

Call a gas engineer to arrange for a boiler repair.

How to fix a boiler leak?

Again, you should never open your boiler yourself. 

This is a job for a trained expert. It’s time to call in a gas safe engineer to fix these issues.

Fixing the boiler leak vs replacing your boiler

Fixing a leak inside a boiler is arguably a little more complicated, in many instances, than fixing a leak in your central heating system.

And whilst there’s every chance that a repair or replacement part will solve the problem, there’s also the possibility that a new boiler is going to be the best solution, depending on how much it’s going to cost to fix the issue and how much water damage has been caused to other components This is especially the case on older boilers where an expensive repair doesn’t normally make financial sense.

If you find yourself needing to consider between a repair or replacement boiler, having had a gas engineer diagnose the problem, think about:

  1. How old is your boiler? The average boiler lasts around 10 years, and if yours is older than this, you may be best with a replacement.
  2. Can the part be replaced / can the problem be repaired?
  3. If yes, does it make sense to invest more money into your current boiler pending its age?
  4. If yes, is it cheaper to repair your current boiler compared to buying a new boiler?
  5. Go with the most cost-effective solution.

Common Boiler Error Codes for Low Pressure


Sometimes you’ll find that low pressure has caused your boiler to turn itself off and display an error code. Here are the most common codes associated with low pressure:

Worcester Bosch Error Codes - Low pressure


E2, A1, CE 207, H07, 224 V, 1017W, 2970 B, 2971 B.

Baxi Error Codes - Low Pressure


E28, E119, 118, 119, H.02 – 06, E.02 – 7, E118

Vaillant Error Codes - Low Pressure

F.24, F.12, F.22, F.73, F.75

Ideal Error Codes - Low Pressure

F1, ‘Water Pressure’

How To Repressurise Your Boiler

Repressurising your boiler is a simple process and something that you can do yourself without needing to call out a gas engineer. 

It is recommended that you consult your owner's manual or review the process on the manufacturer's website, but it’s a simple process that shouldn’t take too long to do.

But before you do anything else, make sure you have turned the boiler off and allowed enough time for all the water to cool. You want to make sure that you’re repressuring based on the pressure reading on a cold system.

Here’s how to repressurise your boiler: 

Step 1 - Locate the filling loop

Your boiler’s filling loop is the hose that connects the boiler to the cold water supply. 

The location will differ depending on the brand and model, but you’re looking for something that looks like this:

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(Source: Worcester Bosch)

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Check your boiler’s manual to find out exactly how to use the filling loop for your boiler or take a look on their website.

You’ll also find that many manufacturers post tutorials on how to undertake common tasks like this on YouTube, such as this example from Worcester Bosch:

Step 2 - Increase the pressure

Increase the pressure by opening one or two taps (depending on your boiler) to 45 degrees, letting cold water into the system. Be sure to do this slowly to avoid adding too much pressure.

Continue filling with cold water until the pressure rises to between 1 and 1.5 bars, visible in the green zone on the pressure gauge.

Step 3 - Once in the green zone, close the taps

Once the pressure has reached the ideal level, close both valves.

At this point, you can turn the boiler back on. You may need to press a reset button, but this varies from boiler to boiler.

You’ll want to keep an eye on your boiler’s pressure gauge over the next couple of days to make sure the pressure doesn’t keep dropping.

Can You Fix Low Boiler Pressure Yourself?

This depends upon the reason behind the boiler losing pressure.

If the problem can be fixed by repressuring the system, then yes, this is something that you can do yourself. 

If you’re a dab hand at DIY, you might feel comfortable replacing a leaking radiator valve, but it’s recommended that a gas engineer undertakes most fixes. 

If the boiler is losing pressure because of an internal leak, then this should only be handled by a professional.

What To Do If Your Boiler Keeps Dropping Pressure After Being Repressurised

If repressuring your boiler doesn’t stop your boiler from losing pressure, there’s without a doubt a bigger problem, such as those that we’ve outlined above.

Arrange for a gas engineer to come and diagnose and fix the problem for you. 

FAQs

How often should boiler pressure drop?

It’s common for boiler pressure to drop or even fluctuate.

It is normally nothing to worry about if your boiler pressure drops between 1 to 2 times a year.

A boiler losing pressure more often than this means a potential problem, and you should look for leaks and call in a gas engineer. 

Will bleeding radiators help a boiler that keeps losing pressure?

No, bleeding a radiator is likely to reduce the pressure in your boiler and will not help fix the problem when your boiler is losing pressure.

How often should you need to repressurise a combi boiler?

The frequency of having to repressurise your boiler is the same as the normal frequency of the pressure dropping.

The boiler and heating system are sealed units, so loss should be minimal.

Thus, you should only be repressurising your boiler once or twice a year.

More often than that, it is a sign there may be a problem.

Final Thoughts

A boiler that’s losing pressure is a common problem and one that most homeowners will experience more than once, but it’s usually not something that should be overly concerning, expensive to fix or that causes too much inconvenience.

Your boiler pressure will need topping up from time to time, that’s completely unavoidable, but problems, most commonly leaks, can cause a more rapid drop and something that needs your attention.

Depending on the specific cause of the problem, you might be able to fix it yourself, but if you have even the slightest suspicion that it’s something more than the expected slow drop over time, it pays to call a gas engineer to get things sorted. 

An annual boiler service is a great way to prevent unexpected problems and give you the reassurance that you’re not about to encounter any (potentially expensive) problems unexpectedly.

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