Spotting and fixing a central heating leak

Think your central heating system may have a leak? Discover how to spot leaks and, if possible, how to fix some of them yourself.

Safety first

This article offers advice on how to perform some leak checks by yourself. However if you don’t feel confident carrying them out, we recommend calling a Gas Safe engineer to come out and examine your central heating system for leaks. 

Find and fix a leak in your central heating system

Central heating leaks can lead to a breakdown. If your boiler has been losing pressure or is making hissing noises, then you may have a leak in your central heating system. These issues are more common during winter months due to increased usage as well as damage caused by frost. Identifying the cause and location of the leak can help in resolving the issue much quicker and possibly save you repair costs down the line. We'd always recommend arranging a Gas Safe engineer to inspect a potential leak, but if you feel confident enough you can follow these steps.

Visually inspect underneath and around the boiler

The most obvious sign of a central heating leak is dripping around the boiler itself. Thoroughly check the boiler for any signs of water damage, being careful not to check pipes when they are hot and check all the boiler’s fittings. If there are puddles of water underneath the boiler or coming from pipework surrounding the boiler then it’s recommended to call out an engineer to take a look at the leak to prevent any further issues being caused by attempting to investigate it further yourself.

Use coloured paper

Place a sheet of coloured paper underneath the boiler and wait for water drops, follow where the drops are coming from to see where the leak is coming from.

Check for faulty valves on the radiators

Sometimes the valves on your radiators can become faulty, which can be tricky to spot as water will flow down the parts of the radiator and through to underfloor piping. Radiator valves can usually be found at the bottom left or right of the radiator and will be connecting the pipes to the radiator. One way to check if the radiator valves are leaking is to tie some tissue around piping underneath the valve. Leave it for a day or so and then check to see if the tissue is wet. If it is, then you may need to replace them.

Check for faulty Thermostatic Radiator Valves (TRVs)

The ground around the TRV will be wet and water will be noticeable around the pin when the head of the valve is removed.

Check radiators for pinhole leaks

Radiators corrode over time and can cause pinhole leaks that are difficult to spot, if you notice rust anywhere on your radiator this can be a tell tale sign of a leak. The heating must be on for water to be pumped through it, and water can evaporate quickly due to the radiator being hot, use tissue or a towel to dry the radiator surface and check to see if you can locate the area the leak is coming from. Never scratch or disturb a rust patch on a radiator as this is normally due to the radiator leaking and can make the leak worse.

Check for wall damage

If you have any wall damage that has resulted in wet spots or mould, this could be a sign of a leaking pipe.

Using a leak sealer

If you’ve successfully located the leak on a radiator, for example a pinhole leak, you may be able to fix it yourself with a leak sealer specifically designed for radiators. In the situation that you think the leak is coming from pipework within the central heating system, you can also use a leak sealer to help form a barrier around the leak.

Leak sealers can be seen as a short term solution to a long term problem. If your central heating system is old and has wear and tear issues, it can often be more cost efficient in the long run to replace the central heating system altogether with a new boiler.

How to fix a pinhole leak in a central heating radiator

Leaks in a central heating system commonly come from the main body of the radiator itself, its pipework, valves or radiator valve nuts. Upon locating the pinhole, you can then add an internal leak sealer to the system. There are many types of leak sealers available on the market, we suggest following the instructions on the bottle packaging of sealant. Usually, you will simply open the radiator valves and pour in the sealant. Another, more long term solution is to replace the radiator altogether to prevent long term leaks and costly repair jobs. 

How does central heating leak sealer work?

Leak sealers are designed to repair and prevent leaks within your central heating pipework. They work by seeking out tiny internal micro leaks and forming a barrier to seal the leak. Leak sealers can be a viable option for smaller leaks within the pipework, as they can be a cheaper and easier fix than rerouting pipework, removing floorwork or calling out an engineer and possibly footing the cost of a repair job. 

How to add leak sealer to central heating system

Instructions vary depending on the type of leak sealer you are using, we strongly recommend checking the packaging of your leak sealer and following the instructions. Whilst these instructions are not exhaustive, we’ve provided a general overview on how to add leak sealer to your central heating system to help seal small leaks: 

  1. Make sure your central heating header tank is almost empty and pour in the leak sealer.
  2. Fill 5 -10 litres of water from the drain cock (usually located at the bottom of the radiator, boiler, or on a pipe running at the lowest position in your central heating system) and put it back into the header tank to make sure leak sealer treated water is thrown away.
  3. Repeat step 2 until you’ve drawn all the leak sealer into the circulating system, don’t leave the leak sealer sitting in the header tank.
  4. Turn on the circulating pump while also applying a little bit of heat to reach an even distribution.

If you're not confident in using a leak sealer, are having trouble locating the leak, or are unable to fix it yourself, we recommend instructing a Gas Safe engineer to inspect your central heating system to identify and fix the problem. It may be a case of the best option being to replace your central heating system altogether. If you want to replace your central heating system, we offer a range of A-rated new boilers to suit any household. 

When is it time to call the professionals?

Leak sealers are generally intended for use on small leaks and can sometimes be used as a long term solution. However, using leak sealers within your central heating system may affect your warranty, so it’s worth checking this with your manufacturer beforehand. For larger leaks, a leak sealer may not be effective as a long term solution, in this case we suggest calling out a Gas Safe engineer to determine the severity of the leak and decide what options you have. If you’re unsure where the leak is coming from and/or how to fix it, we recommend calling out a professional to help.

If you currently do not have any type of central heating cover, we’ve partnered with HomeServe to offer central heating cover from just £7.99 per month which covers central heating breakdowns and boiler repairs as well as leaks and annual services.