Why Is My Boiler Leaking Water? How to Stop Boiler Leaking

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Last updated: October 25, 2023

If you’ve discovered a puddle of water by your boiler or suspect it has been slowly leaking for some time, the most important thing to do is not ignore it. Depending on where the leak is, and what has caused it, it doesn’t necessarily mean a disaster, but you do need to get it fixed. Ignoring a boiler leak will only make things worse over time and can even cause something as serious as a short in the boiler’s electrical components. And the longer you leave it, the more expensive it is likely to be to put it right.

So, to help you avoid an eyewatering repair bill and prevent any further damage, we take a look at the reasons why your boiler is leaking water and what you can do to stop it.

What Causes A Boiler To Leak?

With modern heating systems sealed and pressurised, boilers shouldn’t leak, so any leaking water is a sign that something is not right with your boiler and needs further investigation. However, if you do suspect a leaking boiler, it is important you get a gas safe registered engineer out as soon as possible to carry out a boiler repair. To help you understand what may be going on, here are the main reasons why your boiler is leaking water:


In many cases, corroded pipes and hot water cylinders are the cause of boiler leaks as they can weaken, allowing water to escape over time. Corrosion can also cause the metal to become brittle and eventually break if not caught in time. As it tends to be older boilers that get to the stage of corrosion, this cause of boiler leaks could be an indication that it may be time for you to get a boiler replacement. But if the corrosion is slight and localised and has been caught before it spreads, it is possible to just get the corroded pipes, pressure release valve or joint replaced – but only by a registered engineer.

Too much pressure

If your boiler pressure is too high, then this can cause a leaking boiler as it releases some of the excess pressure inside the unit. The pressure release valves (PRV) in boilers are designed as a pressure relief, operating as a safety precaution to prevent further damage.

If you suspect your boiler leak is being caused by an increase in pressure, then you need to check the gauge on the pressure relief valve. The pressure gauge needle needs to be in the green section on the dial, which means it is set at around 1 bar of water pressure and in the safe range. If the needle is in the red section – so anywhere above 2 bar – this shows that the pressure is too high and the boiler is over pressurised, tripping the PRV and causing the leak.

To bring the boiler pressure down, your boiler needs to be re-set by ‘bleeding’ which sounds gruesome but actually means you reduce the pressure inside the boiler to within its safe limit. If you have never bled your boiler – you may also need to bleed the radiators – then always consult with a gas boiler engineer.

Faulty heat exchanger

The heat exchanger in your combi boiler is one of the key components, as it enables the water held inside the boiler to heat up sufficiently before it is pumped into your radiators and your central heating system. With sustained use, a heat exchanger can crack or fail over time, causing the combi boiler to leak out water. Heat exchanger components can be expensive, so it is important that only a gas safe registered engineer attempts to fix or alternatively replace this part in your boiler.

A compromised pump seal

The pump in your boiler works to pump all the water around your central heating system and so is put under constant pressure. And this means its components, including the vital seals are also working hard and can decay or split over time. A damaged seal on your boiler pump will inevitably lead to your boiler leaking water and should it actually fail, then your boiler will leak pretty rapidly, and you will know about it.

The pump seals should only ever be replaced by a gas safe engineer who can also advise if the damage caused by leaking water is too great, especially if it’s on an older boiler, as a new boiler system may be needed.


Your boiler will also be fitted with a temperature valve – also known as a TCV – which constantly monitors the water temperature inside your boiler to ensure it doesn’t get too high. But the TCV can also spring a leak, and so may not be working properly, which can lead to a raised temperature inside the boiler, and dangerously hot water coming out of your taps. Obviously, this needs rectifying as soon as possible, so if you suspect a broken or leaking temperature valve, call out your gas safe registered engineer pronto for a boiler repair.

Badly installed

There is also a chance that your leaky boiler has simply been caused by poor installation, leading to water leaking from the pipe fittings and joints which have not been fitted or tightened correctly. So, if you find your newly or relatively newly installed boiler is leaking water from the bottom, and you can’t see any other possible cause, then contact the gas safe engineer who did the original fitting to get the installation checked out and if necessary, rectified.

General wear and tear

Just like almost everything else in your home, boilers aren’t built to last for ever and with everyday use, can start to wear out over time. Natural wear and tear on the components such as joints, seals and pipe fittings and connections can lead them to loosen or deteriorate, allowing water to gradually leak through. The continual action of water expanding as it heats up, then contracting as it cools within the boiler will also put a strain on your gas boiler, potentially leading to water dripping, as well as cracks and leaks over a prolonged period of time. As a DIY fix, joints can be tightened, and seals replaced but eventually there is only so much you can do, especially if your boiler is more than 10 years old. If wear and tear is the main reason your boiler is leaking, then it’s probably the right time to install a new boiler for your heating system that is well-constructed and much more energy efficient too.

Cropped shot of male plumber repairing

How to Fix a Boiler Leaking Water

When it comes to your boiler leaking water, prevention is better than cure so the easiest way to fix leaking water is to try and prevent it in the first place. And it is essential that you only use qualified and registered heating engineers instead of attempting your own DIY repairs.

But if you do suspect a leak, whether it is from the pipes, valves or leaking water from the bottom, then there are a few steps you can take to minimise the problem:

Conduct a visual check – carefully looking over your leaking boiler is going to help you to pinpoint the exact source of water leaking as well as identify the potential causes for the boiler leak. Look where the water is leaking from and also check the pressure and temperature gauges to ensure they are both working well and not showing a reading that could be a cause for concern.

Minimize the leak – you don’t want all that water to cause any more damage than it already has, so place a bucket or bowl under the source of the leak to protect your floor from all that water. If there is no obvious leak source, then use old towels or cloths to surround the boiler and absorb as much of the moisture as you can.

Correct the pressure – when visually inspecting your boiler’s pressure gauge, it may have read above 2 bar and so in the red zone. This is too high, so you may need to bleed the boiler or radiators to reduce the pressure and bring it back to the green. Always consult your boiler’s manufacturer’s instructions or call in a gas heating engineer if you don’t know how to do it. Which leads us neatly onto our final step…

Call in a heating engineer – if you suspect or know there is a leak in your boiler, minimise the impact by following the above but don’t attempt to fix a leaking boiler yourself if you are not appropriately skilled or qualified. A gas safe heating engineer will be able to fully assess your boiler and make the required repairs or, if your boiler is an older model advise whether it is time for an upgrade. Using a professional will also bring you peace of mind that your heating system is in full working order.

How to Prevent a Boiler Leakage

Dealing with a boiler leaking water is a messy business and can also prove to be an expensive one. To prevent a leaking boiler and avoid the potential damage and disruption all that leaking water can cause, it is important to keep your boiler in tip-top condition.

Top of your ‘how to prevent a boiler leakage’ list should be a yearly boiler service by a qualified gas engineer who will ensure your boiler and heating system is running safely and identify any potential issues before they become a full-on leaking problem. Nipping any potential water leak, corrosion or faulty parts in the bud will save you time, money and stress in the long run.

An annual gas boiler service will also keep your boiler running as efficiently as it can and help to keep those energy bills as low as possible. And replacing your old combi boilers when it is time is another practical way to prevent a boiler leakage by installing a new boiler that is less likely to leak and will also help to boost the energy efficiency of your home.

Can I use my boiler if it’s leaking?

A leaking boiler should never be ignored, even if it is only small amounts of water as the damage to both the boiler and your home will only get worse over time. If you know your boiler is leaking, especially if the pressure has dropped or you suspect a faulty heat exchanger, it is wise to switch off the water supply and your central heating and contact a gas engineer to arrange a prompt call out.

The basic rule of thumb is to never be tempted to ignore a leak and continue using your boiler but call out a heating engineer who can fully assess the issue and make the necessary repairs. And the quicker you act, the less the likely damage, to both your home and your bank balance.

How dangerous is a leaking boiler?

A leaking boiler can be potentially dangerous as left to its own devices, the leak will continue to corrode the inside of the boiler, potentially causing a short in the electronic circuitry. And the leaking water can also damage your home, not least your flooring, furniture and personal belongings.

Does a leaking boiler cause low pressure?

We have already looked at how high pressure in your boiler can trip its pressure relief valve, which can lead to leaking from the bottom. And leaking water can also cause low boiler pressure, as a result of reduced pressure in its water system. Low pressure can be identified by the pressure gauge, when the gauge needle drops below the green zone, putting the boiler into the low pressure ‘danger’ zone.

Low boiler pressure is unlikely to cause damage to your boiler, but it will have a marked effect on its efficiency as it has to work harder to keep your home warm. However, the source of the low pressure – the water leak – can eventually harm your boiler so if you suspect your low pressure issue is the result of a leak then call out a gas safe engineer.

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  1. Is your boiler leaking water? Here’s what to check – Viessmann