How To Fix A Leaking Boiler

A leaking boiler is perhaps one of the more alarming boiler issues you might encounter - but don’t panic! Heres how to fix one.

Boilers are durable, robust workhorses that we can usually rely on to heat our homes year after a year. 

After years of humble service, boilers will likely show evidence of wear and tear and their performance will eventually falter. 

While any boiler problem is likely to cause some alarm, many issues are fairly benign and can be sorted out with a minimum of fuss. On other occasions, it’s worth getting professional advice as soon as possible. 

A leaking boiler is perhaps one of the more alarming issues you might encounter - but don’t panic! Not all leaks are the sign of a major fault, but time is of the essence - don’t ignore a small leak and let it develop into a bigger one!

This is a guide to leaking boilers and how to fix them. 

How do I know if my boiler is leaking water?

The telltale signs of a boiler leak are:

  • Drips coming from the boiler itself, including the cover.
  • Drips emanating from nearby pipework 
  • Unexplained damp or moisture near the boiler 
  • Rapidly corroding pipes in and around the boiler 

You’re probably most likely to find drips coming from the pipework at the top or bottom of the boiler, depending on what boiler it is and where it’s installed. 

Is a leaking boiler an emergency?

A water leak isn’t an emergency in the same sense as a gas leak, unless the water is pouring out of your boiler! 

Whether or not a leaking boiler is dangerous or not depends on the boiler itself. If it’s a newer combi or system boiler and the water leaks through the electronics inside the boiler, the water may cause an electrical short. This could be dangerous, though boilers are designed to prevent anything like this from happening.

If you spot even a small leak in/from your boiler, it’s wise to shut the boiler off immediately. If you have a newer boiler that contains electrical components, be careful when touching it if it’s wet. Shut off the electricity at the switch or breaker if possible. 

Can I use my boiler if it's leaking?

While using an older conventional boiler while it’s leaking a little isn’t dangerous, the problem won’t go away and will eventually lead to much greater problems down the line, such as widespread corrosion. 

If you have a newer combi or system boiler with electric components, then you should shut your boiler’s power off at the mains and stop using it immediately. 

Regardless of what boiler you have, if the leak is serious (e.g. the water is gushing out and not just dripping), then shut off your water at the stopcock or the stop valve in the road. Thames Water has a guide on finding and using your outdoor stop valve here.

Does a leaking boiler cause low pressure?

If you have a modern boiler (e.g. a combi or system boiler), then a leak anywhere in the system will cause low boiler pressure. You’ll be able to identify low pressure from the pressure gauge on the boiler itself. Most boilers require a pressure of 1 to 2 bar. 

If you have a conventional or traditional gravity-fed system, then a small leak will still impact your central heating performance, but not to the same extent. 

How dangerous is a leaking boiler?

A boiler leaking a small quantity of water is still a high priority fault, but not an emergency. The only exception is if the boiler is a combi or system boiler and the water leaks through the main electrical components. In this case, short-circuiting of the electrical components is possible. 

White boiler with multiple dials
Above: Boilers with electrical components are vulnerable to water damage

Modern boilers should automatically shut off if they detect a leak or other issue, though. Here are some leaking boiler error codes to look out for:

Boiler Brand Possible error codes

  • Worcester Bosch - A1, E9, CE207, HO7
  • Vaillant - F.22, F.24, F.13, F.73, S.41, S.53
  • Ideal - F1, L1, FD
  • Baxi - 117, 118, 125, E78, H.02 – 06

In any case, it’s sensible to isolate a leaking boiler from the electricity. To safely isolate a leaking boiler, switch the entire off at the mains. You should be able to find a breaker switch. Failing this, check your main breakers. If you still can’t find how to switch it off, switch off the main switch and call one of Gas Safe engineers to fix the problem as soon as possible. 

Why is my boiler leaking water?

Boilers can leak for several reasons. The source of the leak will provide some clues, but can often be difficult to identify. 

1: Corrosion in pipework 

If the leak emanates from the pipes under on top of your boiler, it might be caused by corroded pipework. Leaks tend to start small and develop over time.

white pipes with red screws underneath white sink on blue and white tiles
Above: Most leaks come from underneath a boiler

Small leaks (also called pinhole leaks) can result from a build-up of sludge and grime inside the pipe. These are relatively easy to fix if the pipe can be isolated and repaired. However, it’ll be much harder to repair if the corrosion spreads to other components. 

2: Corrosion inside boiler 

If corrosion penetrates the boiler itself, it can affect practically every component inside. Removing the boiler’s case will reveal whether or not there’s a leak inside, and whether it’s causing any corrosion. 

While Gas Safe does say it’s safe to remove the case of most boilers, so long as you follow the manufacturer’s guidelines and isolate the electricity, it’s advisable to call our Gas Safe engineers if you’re not confident. 

3: Broken or leaking seals

Seals are there to prevent leaks, so if they fail, leaking is inevitable! Virtually any connection between two pipes or components will have a seal. Corrosion or high boiler pressure may cause the seals to fail over time. 

4: Loose joints

Similarly to the above, loose joints can cause leaks. 

Joints may loosen due to vibrations in and around the boiler, especially if it’s placed near a washing machine or other electrical appliance. Corrosion may also seep through the joints and provide a means for water to escape. 

5: High pressure

Combi or system boilers (and some older boilers) will come equipped with a pressure gauge. Boilers commonly operate between 1 and 2 bar, which will be marked by a green zone on the gauge. If the boiler pressure is too high, your system becomes more vulnerable to leaks. Leaks can quickly develop in a system operating above its recommended pressure range. 

Excessive boiler pressure is often caused by a build-up of air in the system or an airlock. 

6: Poor installation or repairs 

Leaks may result from shoddy boiler installation or repairs. If your boiler has been serviced, replaced or repaired recently, and you later find a leak, you should call the installer as soon as possible. 

Often it’s just a case of re-tightening a joint that has shaken loose during standard operation. 

7: Wear and tear

Boilers wear over time. Leaks resulting from general wear and tear might take years to develop, but boilers older than 15 to 20 years likely need more regular and intensive servicing to prevent leaks and other issues from developing. Corrosion is a common issue, especially when the boiler is installed in a damp environment already, like a garage. 

8: High temperature 

One of the valves installed in a boiler is the temperature control valve (TCV). If the TCV wears, corrodes or develops some other fault, it might leak water while simultaneously failing to regulate the system’s water temperature. This is one of the more serious faults, as it’ll cause your hot water to approach scalding or even boiling temperatures. This can include water within the hot water cylinder, read our guide for more information.

How to fix a leaking boiler?

In almost all of these situations, you’ll need to contact a Gas Safe engineer to properly diagnose the problem, with a few exceptions: 

If the leak is coming from a join or seal

If you can see that the leak is coming from a joint or seal, then you can try tightening the joint with a spanner yourself. Resealing the joint with plumbers tape is also an option, but you’ll have to isolate the section from the water supply or drain the system first. 

Tightening white pipes with pressure meter with silver spanner
Above: Sometimes you’ll just need to tighten the leaking pipe 

If you’ve recently depressurised your system 

If you’ve recently depressurised your boiler or fiddled with the filling loop tap underneath the boiler, ensure the black taps on the loop are closed properly. If they aren’t closed properly, your boiler pressure will remain too high. 

Silver flexible hoses with black stops
Above: Boiler filling loop

If your boiler pressure is too high

If your boiler pressure is consistently too high, start by bleeding your radiators. You’ll need to move through your home systematically, bleeding each radiator at a time. It shouldn’t take much longer than half an hour or so. If you notice any leaking from your radiator, read our guide here.

Bleeding white radiator into glass, whilst wearing plastic gloves
Above: Bleeding radiators can fix a number of central heating issues

How to prevent a boiler leakage

A well-fitted modern boiler is extremely unlikely to develop leaks for many years, providing it’s well-serviced. The older the boiler and central heating system in general, the more likely leaks become. 

1: Service your boiler

Servicing your boiler is the best way to keep it working smoothly year in, year out. The standard advice is to service a boiler every year, before winter - so around late summer. 

The Gas Safe engineer will also be able to check for faults and proactively replace worn parts. The older your boiler is, the more important servicing is. But, any and all boilers benefit from yearly servicing. 

2: Keeping an eye on corrosion 

Corrosion is the primary cause of tiny leaks that gradually worsen. Keep an eye on your boiler and its surrounding pipes - you should address any tiny leaks accompanied by corrosion as soon as possible before the issue spreads. 

3: System flushing and magnetic filters

An corrosion-ridden system can be cleaned with a chemical flush. Fitting magnetic filters to the boiler also extends its life when the rest of the system is quite old.

Summary: Boiler Leaking Water – How To Fix A Leaking Boiler

Leaking boilers are generally not an emergency, unless the water is leaking into internal electrical components. In this situation, most modern boilers should switch off automatically, but you can’t take that for granted. 

If you suspect that water has leaked inside your boiler’s electronics, shut it down at the mains breaker. 

In most situations, a leaking boiler warrants professional attention - a Gas Safe engineer will be able to diagnose and repair the fault safely. Even small leaks shouldn’t be ignored, as they’re bound to get worse over time. Call one of our Boiler service engineers if you are not sure. If you are covered by our boiler and heating insurance contact us to have an engineer investigate your issue

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