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Boiler losing pressure what to do

Boiler losing pressure

Why is my boiler losing pressure?

If your radiators aren’t getting warm and you’re down to a trickle of hot water in the shower, it may well be that you have a boiler losing pressure.

Without the right mix of water and air in the pipes, your central heating system is unable to force the hot water to where you need it to be.

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But is this a situation where you need a new boiler or is it a simple fix that you can do yourself? Well, that depends. If you have an old boiler that’s already unreliable, then this could be the final straw. So, that makes a new boiler the right option.

As for a home repair, well that might be a solution for some boilers that lose pressure, but it is more likely that you’ll need the assistance of a qualified heating engineer.

Now, there are a number of reasons why a boiler loses pressure, but leaks do tend to be one of the main causes. In this guide, we’ll take a look a closer look at what that leak means to the boiler pressure and how it can be fixed. We’ll also consider other reasons for the low-pressure issues.

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What is low pressure?

Boilers are designed to operate within a set range of pressure. You can check the optimum range for your model of the boiler by checking in the handbook or on the manufacturer’s website.

When the boiler pressure falls below what’s needed than first of all efficiency is going to be affected. That’s because your boiler is having to work harder to push the hot water to where it needs to be.

How do I check the pressure gauge?

Another way of checking if your boiler pressure is too low is to check out the pressure gauge which will be located on the front of your boiler. This gauge might be a dial with green and red zones to tell you where the boiler pressure is right now.

On a modern combi boiler, there might be a screen that displays an error code to let you know there’s a problem. A quick check of the manual will then tell you if the error code relates to a boiler pressure issue.

For boilers to operate safely and in the most efficient way, they need a system pressure that’s usually between 0.7 bar & 2.5 bar. Occasional fluctuations in pressure are normal and these can often be sorted out by topping up the water levels to increase the pressure.

If however, the boiler is regularly losing pressure, then that indicates that there is a bigger problem, such as water leaking, that needs to be identified and fixed.

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How do I check if my boiler pressure is low?

If there are signs that your boiler has lost pressure, then the first thing to do is check what it should be. You’ll find that information in the boiler manual.

Then you can check the pressure gauge which is found on the front panel of the boiler. That will tell you if the pressure is low, high or if it’s set correctly.

Most boiler manufacturers now have a control panel on their more modern boiler ranges. That then provides a screen on which an error code will be displayed. Then you can check in the boiler manual to see what the error code means.

Pressure is measured in bars and if the boiler pressure needed is below the minimum needed, then that means that your boiler has lost pressure. If the pressure bar is considered too high, then it has gone beyond the level needed for the normal operation of your boiler. This can then be resolved by bleeding the connected radiators.

Boilers have a built-in safety device that automatically reduces pressure to ensure that your system is never in a dangerous state. However, bleeding the radiators will help the situation until a gas safe engineer can come and assess what’s going on.

Faulty Pressure Gauge

It’s also worth bearing in mind that you might have a faulty gauge. That could mean that the boiler pressure is absolutely fine; it’s just the gauge giving an incorrect reading.

However, we would suggest taking a look at the fault code if your boiler shows them on the screen, and see if that still indicates your boiler losing pressure.

What causes a boiler to lose pressure?

We’ve already mentioned that leaks are the most common reason for a boiler pressure loss, but there can be other reasons too

Boiler leak

Your boiler is full of mechanical parts that are working hard every day to provide you with your hot water needs. However, it also means that there are weak points that can become worn and damaged over time. These include parts such as the automatic air vent, the washers, ring joints, and the diverter valve.

Signs of a boiler leak

This is normally an easy one to spot. That’s because if it’s a larger leak then you’re going to see water coming out of the bottom of the boiler.

When it’s a tiny leak it might not be so obvious especially if the water is dripping internally and then evaporating before you see the issue.

Boiler leak solution

Boiler leaks are not usually something that a homeowner can fix. That’s because that this is a repair that will need the boiler casing taken off and that’s going to need the attention of a gas safe engineer.

There’s strict legislation in the UK as to who can maintain and install gas appliances. That means that you need to be gas safe certified and on the official register of qualified professionals. If you attempt to repair a gas boiler you may well be prosecuted and find that the warranty on your boiler is no longer valid.

In terms of repair costs, that’s going to depend on which part that’s become damaged. Do ask if the engineer offers a fixed price for repairs for common issues so that you know exactly what the cost will be.

If it’s an old boiler that’s becoming unreliable, then this could be the time for a new boiler. Don’t forget that a new boiler is likely to give you much higher levels of efficiency meaning lower energy bills.

Leak in the system

As well as a leak in the boiler causing pressure drops, a leak in the heating system itself can also cause the same problem. This isn’t only the pipework; it can also be things like towel rails and radiator valves.

Signs of a leak in the system

To establish if this might be the cause of the issue, you need to do some investigative work. So, start from the boiler and follow the pipes looking for water escaping from the entire system. Pay close attention to the joints and bends as well as each radiator valve, as these are going to be weak points and so are most susceptible to leaks.

Look out for damp patches on the floor to show you exactly where the leaking pipe is.

System leak solution

Once you know where the water leak is you’ll be able to use the isolation valves to stop any further leaks. But, this does mean no heating. For most people, repairing leaks in the heating pipework is a job that needs the attention of a boiler engineer.

Bled radiators

If you’ve recently bled the radiators then this might be the cause of the boiler pressure loss. Now if you haven’t bled the radiators recently, keep checking down our list for other reasons that your boiler is losing pressure.

Bled radiators solution

This one is an easy fix! So, to fix low boiler pressure because of recently bled radiators, you simply need to re-pressurise the boiler.

To do this you need to find the filling loop. In most modern combi boilers it’s a built-in copper pipe that has two taps. In older boilers, you might have a filling loop that you need to attach to the boiler pipework.

With the filling loop in place, open the first tap connected to the mains water. Then while watching the pressure gauge open the second tap. Now you’ll start to see the water pressure rise but wait until it gets to between 1.0 and 2.0 bars. Once there, turn off the taps and you should now have the correct pressure in place for your boiler to be up and running.

Faulty expansion vessel

When your boiler has a faulty expansion vessel, you get a high-pressure situation that can go all the way up to 3 bars. That’s because the water that’s expanding has nowhere to go and so the pressure increases.

The pressure relief valve is then responsible for letting out all of that excessive pressure. As a result, you’ll see a sudden and significant drop in the pressure gauge reading.  Most boilers now have a specific fault code for a faulty expansion vessel, so do check in the user manual first.

Faulty expansion vessel solution

The repair or replacement of a damaged expansion vessel is usually the option here. So, that’s going to need a gas safe engineer to complete the repair for you. Once the expansion tank is sorted then the boiler can function properly meaning the end to dramatic pressure changes.

This can be a repair that’s undertaken under a fixed price quotation service from a heating repair engineer. This does of course assume that this is the issue and they don’t find any other problems that need fixing.

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Damaged pressure release valve in the heating system

A pressure relief valve does exactly what the name implies, they let out the excess pressure. They’re usually triggered into action once the boiler pressure is over 3 bars.

When there’s a faulty pressure relief valve it will start leaking water at a lower pressure away from the heating system, usually to a pipe that’s located outside of the home.

The solution to a damaged pressure release valve

The only way to resolve this one is for this broken boiler part to be replaced. Again, this is likely to need a callout for a heating engineer to fix the pressure loss issue for you.

Faulty boiler parts

We’ve mentioned some of the boiler parts that cause pressure loss problems but there are many others that can also cause a similar issue. So it may still be a mechanical issue that’s causing your combi boiler to lose pressure


The first thing to do is to check if there’s a fault code displayed on the boiler display panel. This will tell you and the engineer exactly what’s causing the problem. Simply compare the fault code with those listed in the boiler manual to find out which part is resulting in low boiler pressure.

Trapped air

Remember that we said that your boiler system needed the correct balance of air and water to operate correctly? Well, sometimes air can collect within the radiators and towel rails and that creates an airlock that prevents the hot water from flowing.

This problem can be resolved with the use of a bleed key that gets rid of the excess air.

How to bleed radiators and towel rails

  1. Have a rag or a small container to hand to capture any escaping water
  2. Place the key in the vent and turn very slowly
  3. Listen for the air hissing out
  4. When water starts to come out close the vent off
  5. Repeat with every radiator and towel rail in the home

Depending on whether your heating has been on, it might be cold water or it could be very hot that comes out of the radiator so take care when using the bleed key.

What your boiler losing pressure means

Pressure in the boiler goes down when the heating is on

As your central heating fills with hot water, the pipes and radiators expand resulting in an increase in pressure. That then means that your system might not show any leaks when it’s turned off, but when it’s turned on, the water seeps out resulting in low boiler pressure.

Boilers that lose pressure slowly/quickly

If there is a slow loss of pressure then the leak is generally going to be small. So that could be when you can’t see any movement in the pressure dial but when you check it the following morning, there is a loss of pressure.

If on the other hand, you can see a rapid loss of pressure as you watch the gauge, then the leak is going to be significant. If you can’t see water coming from the boiler itself then it’s essential that you check around the home to find out where the leak is to prevent damage to your home.

In this situation, you’ll need to turn the boiler and its water supply off first of all. Then you’ll need to get in touch with an emergency engineer to get the problem resolved.

Fault codes for a boiler losing pressure

Many boiler manufacturers have implemented the use of an error or fault code to let you know when something is causing the boiler to lose pressure. So that could be a combi boiler springing a leak or an increase in the air within the radiators.

By comparing the code to the boiler issues listed within the manual you’ll be able to work out what might be causing your broken boiler and that pressure loss. In some situations, you might be able to then fix the issue yourself or at least be able to provide more information to the heating engineer.

Do bear in mind that each manufacturer has its own list of codes and that they can vary between different models of boiler.

Viessman boiler error codes

C1 = Differential air pressure.

B9, ED, F0, F-E5, F-EC, F-ED, F-EE, F-EF, F-FD, F-FF: Issue with the maximum pressure limiter

F5: The gas pressure switch is faulty

EE and EF = Issues with the air pressure switch

0F = The boiler needs servicing

Alpha boiler error codes

1: Failure of the ignition system

2: Primary system low-pressure problems

61: System pressure is incorrect, may be caused by air in the pump

0A37: More pressure needed, the boiler is switched off into standby mode

0E37 – System pressure too low

Worcester boiler error codes

E9: Problems with the safety limiter.

A1: A dry pump

F0: Internal error

CE 207: Water pressure too low

H07: Reduced performance due to low pressure

1065 B: Faulty pressure sensor

1970 B: Boiler losing pressure rapidly

Is low boiler pressure dangerous?

Unlike high pressure, a boiler losing pressure isn’t usually dangerous. But, when a boiler keeps losing pressure then it’s a pretty strong sign that water is leaving the system somewhere and that could be happening right in your home.

So, that means that a water leak could be seeping into your flooring and ruining carpets. But more worryingly it could also be getting into electrical appliances and that could be incredibly dangerous.

When your boiler begins to lose pressure, it’s not a problem that’s going to go away. In fact, there’s a strong likelihood that it will get worse over time. That then means that with your boiler losing pressure, you will also, at some point, lose your hot water and central heating supply.

And don’t forget that whole your boiler is losing pressure, you’re also losing money. That’s because the efficiency level of your boiler is going to plummet and your bills will become sky-high.

Repair or replace the boiler?

Boiler pressure issues can often be resolved very quickly and at times without the need for calling out an engineer. But when your boiler is constantly losing pressure and you’re without hot water or central heating, then a new boiler could be the best option.

A new boiler could finally allow you to know that you have a reliable source of heating and none of the worry and stress that comes from boiler pressure issues.

A new boiler quote with a fixed price guarantee is a great way of knowing exactly what the cost will be with no nasty surprises once the installation is complete.

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