Why Your Boiler Is Losing Pressure (EXPLAINED) + What To Do

Losing pressure is one of the most common problems that can arise with your boiler. It is also something you are often told you can fix yourself. This is true sometimes but not always. In this guide, we explain the most common reasons why your boiler might be losing pressure, when there is a safe DIY fix available, and when you need to seek professional help.

Most people won’t even look at their boiler, never mind try and fix it if there is a problem. But when a boiler loses pressure, a lot will think it is a simple issue they can probably handle themselves.

Sometimes they are right. But sometimes it can be very dangerous to go fiddling with your boiler if you don’t know what you are doing.

In this guide, we will outline the 7 most common reasons why your boiler might be losing pressure and explain the best way to fix each issue. But to begin with, we will share a great DIY fix that will deal with many pressure loss problems.


A DIY Fix For When Your Boiler Loses Pressure

If your boiler is losing pressure, the chances are that the cause is a leak in either your boiler or your heating system. In our experience, this accounts for around 99% of all the pressure loss incidents we have looked at.

The good news is that if this is the problem, there is a simple DIY fix that could save you hundreds of pounds on call-out fees for a qualified engineer. The solution is a DIY leak sealant such as Fernox F4 leak sealant.

Leak sealants will travel around your heating system, including inside your boiler, and work their way into any little cracks and gaps that might have developed. This plugs the gap permanently and stops the leak. In doing so, it should also solve the pressure loss issue and get your boiler working as normal again.

Fernox is a well-known leak sealant brand and if you do call out an engineer, there is a good chance it is what they will use to solve the problem.

So, why not buy some yourself for a few pounds from most good DIY outlets. F4 leak sealant should work within 24 hours and solve your problem quickly, easily, and cheaply.

What If Your Boiler Is Losing Pressure But There Are No Leaks?

While a leak is a cause for the overwhelming majority of boiler pressure-loss issues, there are a few other issues that can result in your boiler losing pressure.

Here is our rundown of the top seven issues we have encountered:

#1 – Faulty Pressure Relief Valve

The majority of household boilers operate between 1.0bar-1.5bar. But if there is a problem, they can go as high as 3bar.

Your boilers release valve will notice any abnormal change in system pressure such as this and releases it. But if your boilers PRV has failed, this will cause the boiler to lose pressure.

To resolve this issue, you will need to call out a qualified heating engineer.

#2 – Auto Air Vent Leaks

Over time, all central heating systems are likely to collect air pockets. This is especially common after they have been refilled.

Usually, this problem can be solved by bleeding your radiators. But in new combi boilers, the auto air vent feature can also help tackle the problem.

But if the auto air vent has a problem, this could result in a loss of pressure and you will need a heating engineer to fix this problem too.

#3 – Leaking Radiators And Towel Rails

Leaks can come from anywhere in your heating system, but it is units like radiators and towel rails that are most susceptible.

If your boiler has lost pressure, it is a good idea to do a visual check of all your radiators, towel rails, and any exposed pipework and joints to see if there are any signs of water. Even a small leak can impact your boiler pressure, so look closely.

While looking, it is worth checking any exposed connections. These can work themselves loose over time and result is pressure losses too, so tighten these up firmly but carefully and see if this helps the problem too.

If this doesn’t solve the problem and you have tried a leak sealant too, you might need to get your radiator or towel rail repaired or replaced. In our experience, replacement is often the most cost-effective option in the long run.

#4 – Air In Your Radiators

Getting air into your radiators is easily done, especially if you have work done to your boiler or heating system. But excess air in radiators can affect the whole heating system and lead to a drop in boiler pressure.

This is a problem that is easy to fix yourself by bleeding the affected radiators. To do this, all you need to do is get your bleed key (you can buy these from any DIY store if you don’t have one) and a bit of kitchen roll.

Open the vent just a little bit with the key. A quarter of a turn is usually enough. You’ll hear air hissing out and when it has gone, a little water will start to come out. Use the key to close the vent quickly and clean up the water with the kitchen roll. Do this to all your radiators and towel rails to make sure every one is free of air.

#5 – Expansion Vessels and Schraeder Valves

Sometimes, the expansion vessels will need to be repressurised. If this is don’t done, they can start to affect the pressure of the boiler and cause a loss of pressure in some extreme circumstances.

Problems with the Expansion Vessel itself are rare but a more likely cause of boiler pressure loss is the Schraeder valve on the expansion vessel leaking. A Schraeder valve looks much the same as the valve on your car inner tube

If the diaphragm on the Expansion Vessel has degraded, which can happen over time, this can also result in a loss of pressure.

You can try repressurising the Expansion Vessel to see if this solves the problem. If it doesn’t, the chances are that the whole unit will need replacing. Your heating engineer will advise you on the best course of action.

#6 – Leaks From Your Soldered Joints

System leaks are the most common cause of boilers losing pressure and one of the most vulnerable areas is joints on your piping or boiler connections. These are generally soldered and the combination of pressure and hot water will wear away at these over time. Leaks are extremely common.

If you can find a leaking joint and are adept at using a soldering iron, you can probably fix it yourself. If you are not good at soldering or the leak is a serious one, you will need to call out a heating engineer to do the job for you.

One the leak is repaired, you will then need to use the filling loop to bring the boiler back up to the correct pressure and check that the repair has worked and there are no other leaks in the system.

#7 – The Pressure Gauge is Faulty

This problem is the least likely of all the possible causes of boiler pressure loss but we have seen it happen, so it is worth mentioning it as a possibility.

Pressure gauges, which are generally located on the front of most boilers, can sometimes not be reading correctly.

This can be a major issue especially if you are taking the reading as correct and trying to top up the boiler pressure when it doesn’t need it.

If there is a problem with the gauge, there are usually two ways to tell. One is that the gauge falls back to zero, which is extremely unusual. The other is that you top up the pressure but there is no change to the gauge reading.

If you experience either of these, it is advisable to leave your boiler well alone and call out a qualified heating engineer to take a look.

Different Types of Pressure Loss

Pressure Loss When The Heating Is On

When you switch on your heating system, the pressure in the system increases and the heating pipes, radiators and other metal fittings expand.

This means that a heating system might not leak when it is switched off but will reveal a leak when it is switched on.

Fast Pressure Loss

If your heating system loses pressure quickly, this means that the leak is likely to be a big one. This can mean a major problem, a big water leak, or a major problem with your boiler. Whichever it is, it is highly advisable to get the problem fixed as quickly as possible.

Slow Pressure Loss

If your boiler pressure drops down slowly, so you can’t actually see the gauge moving but over a period of hours or days it definitely drops, this means your leak is probably a smaller issue. Nevertheless, it is best to get it looked at sooner rather than later to ensure it doesn’t develop into something more serious.


If your boiler is losing pressure, the first thing to do is not panic. This is a common issue and the most likely problem is a small leak which can be easily fixed.

But it could be a symptom of something more serious too, so it is important to assess the issue and make the appropriate fix in a timely manner.

In this guide, we have detailed a DIY fix that will solve most leaks. We have also outlined seven common issues that can cause boiler pressure loss and advised you on how to deal with these problems.

Have you had a problem with pressure loss on your boiler? How did you deal with it? Did you try our DIY fix or did you need to resort to a heating engineer?

It is always helpful to hear the advice and experiences of other readers, so please do share yours with us today using the comment box below.

Leave a Reply

Your email address will not be published. Required fields are marked *