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Top 5 causes of a leaking combi boiler (and how to fix them)

If you're reading this article, you've probably just discovered a puddle of water below your boiler, or it's been leaking for a while and you've finally decided to stop ignoring it.

Boiler LeakDepending on what's leaking, it could be easy to fix or something a little more severe which will require an engineer's attention.

The first thing you need to know is - a leaking boiler is not good.

Imagine life without your boiler, a cold house and no hot water is truly miserable.

The first question you should ask is - should you repair or replace your boiler? It's also probably a good idea to research the cost of a new boiler.

You can find out what boiler you need along with costs using our quick questionnaire (it only takes 60 seconds).

With that said, here are some of the most common causes of a leaking boiler, along with potential fixes: 

Top 5 causes of a leaking combi boiler

#1 Water leaking from underneath? Poorly installed or corroded pipework

Boiler Leaking Water Underneath

The pipes immediately underneath your boiler are going to be your number one culprit for the leaking boiler. The most common cause of this pipework causing a boiler leak is corrosion of the pipes.

Over time, the water, combined with the metallic debris within the system will naturally corrode the copper piping eventually leading to small gaps which water is escaping from - leaking.

Should you repair or replace your boiler? Find out what's best for you in our quick help guide

And if you're just curious to see how much a new boiler will cost complete this quick questionnaire to find out for free.

If, on the other hand, your boiler is particularly new – perhaps even just installed – then clearly this isn’t going to be the case and it is more than likely some joints in the new boiler pipework that haven’t quite been fitted correctly.

Cowboy Boiler InstallerDon’t panic – you haven’t necessarily had a cowboy installation undertaken, small water leaks are remarkably common in new boiler installations and can be quite difficult for installers to spot any and all potential leaks at the time the boiler was installed.

Any conscientious boiler installer will return to check and correct the work for you and fix those leaks.

How to check for a water leak underneath your boiler:

It sounds kind of obvious, but, look directly underneath your boiler for any signs of leaking. If it’s a small leak, you might need to feel around the joints of the pipework for wetness. 

How to fix a water leak underneath your boiler:
Gas Safe Boilers

If you’re certain that the pipework underneath your boiler is leaking, you’re going to need a gas engineer to fix it. It’s important you don’t undertake any boiler work yourself.

You can, however, ensure the leak is contained – placing a bowl or bucket underneath the boiler or even carefully wrap the leaking pipes in some cloth.

It may also be a good idea to start researching how much a new boiler may cost

Is your boiler over 10-years old?

Corrosion of the pipework under the boiler is the first sign your boiler is moving on to a better place.


Need help? Here are our quick help guides:

#2 Pressure issue (too much!)

Boiler Leaking Water Underneath

You’ve probably heard the boiler and pressure mentioned a lot and never quite understood what it means.

Boiler Pressure Leak

Boilers under too much pressure (too much water) break down and cry. If the boiler's pressure is too high, the PRV (pressure release valve) will discharge the excess water, or, parts will fail internally causing the leak.

How to check for pressure issues:

Any self-respecting boiler will have a pressure gauge on the front – the one that looks like a low-end speedometer. 

For combi boilers and system boilers, the pressure gauge on your boiler should be around 1 bar and within the green markings. Unsurprisingly, anything above that green bar would indicate an over pressurised boiler.

How to fix pressure issues:

Reducing the pressure of your boiler can be done at home by ‘bleeding’ the system. This isn’t as sinister as it sounds and requires no human or animal sacrifice. 

But first – and particularly if you have recently topped up your boiler pressure – you should check that the filling loop tap underneath your boiler is firmly closed. 

Boiler Leak Filling Loop

As pictured above, this will be a silver flexi-pipe with either one or two black taps.

Ensure these taps are firmly closed – closed should be them pushed all the way left or clockwise. But if you’re unsure – quickly and slightly pull the other way to briefly notice the pressure increase before returning and firmly closing.

Bleeding radiators should be your next step...

Bleed Water From RadiatorThis is somewhat more of a pain and will require some basic tools and a relatively low level of confidence, approximately the amount conjured up by Gareth Southgate during his Euro ’96 semi-final penalty miss against Germany.

You'll need to use a bleed key and loosen the nut on the end of the radiator. As you turn it, water from your system will come out.

Boiler still leaking? We can fit you a brand new boiler as quick as tomorrow - Get a quote online, today.

#3 Corrosion to your system and general system wear

Boiler Leak Corrosion

It’s true that in most cases boiler leaks are caused by corroded pipes and hot water cylinders, which become brittle and gradually degenerate. If only a single component is corroded, then a Gas Safe Engineer may be able to replace it relatively cheaply, but extensive corrosion may indicate it’s time for a new boiler

Time heals all wounds. Except for boilers. Time kills boilers (and causes leaks).

Extensive corrosion over a long time is a huge clue that you’re due a new boiler, although do get it checked by an engineer first in case they can still save it – just weigh up the cost of repairs and future repairs against the cost of a new boiler.

Lots of corrosion = lots of leaks.

How to check for corrosion:

Visual checks are about as far as the uninitiated in Gas Safety can go here. It’s really just a case of visually observing how monumentally knackered and rotted to the core your boiler is.

How to fix corrosion:

Again, there isn’t much you can do but prepare your bank balance here – whether it be for a costly repair or probably more suitably, a new boiler installation. Get calling a heating engineer and check out the best new boiler deals online

Boiler still busted? We can fit you a brand new one as quick tomorrow. Get a quote online, today.

#4 Leaky, or faulty heat exchanger

Leaking Heat Exchanger

Unfortunately, the heat exchanger of your boiler is the most expensive component. It’s not uncommon for them to fail or crack in old boilers and particularly cheap low-quality boilers – irrespective of age. 

If you bought your boiler at the farmers market, in exchange for a basket of cabbage then get ready for a broken heat exchanger.

How to check for a leaking heat exchanger:

This isn’t something you can identify yourself and you’ll have to get a qualified gas engineer to diagnose the problem. 

Unfortunately, once diagnosed as a broken heat exchanger, your boiler is likely terminal. Or going to cost you several hundred pounds to keep in the limbo between life and death.

How to fix a leaking heat exchanger:

Heat exchangers can be replaced by boiler engineers but likely at a cost that would be considered a false economy.

Your best option here is to get yourself a new boiler, whatever type of boiler you need. Ideally an outrageously well priced range topper from Viessmann with a stainless steel heat exchanger that Thor’s hammer couldn’t crack.  Make sure to check our Viessmann 050 review for the full rundown. 

Educate yourself silly with our article on the best combi boilers for 2020.

Boiler still busted? We can fit you a brand new Which? Best Buy boiler as quick as tomorrow. Get a quote online, today.

Curious to know which boiler brand performed worst? Find out in our report of the worst boilers

#5 Leaks from seals on internal parts

Leak from Boiler Seals

Boilers are like humans, there are fluids running through their whole system but only in the right channels. Internal bleeding (and leaking) is bad and needs to be stopped.

Sometimes the seals on parts and joints inside your boiler can decay over time or if your boiler is over pressured, once the seals fail, water leaks out of them at a sometimes surprisingly rapid rate.

How to check for leaks on seals:

You can check for leaks in seals at home if you’re comfortable removing the cover of the boiler and having a look around. If you aren’t a gas safe engineer (hopefully not, if you’re reading this) then you should treat your boiler like salmon on a fresh fish counter and look – but don’t touch.

Greek mythology has it that when Pandora opened up her boiler to check the internal seals, all of the evils flew out over the Earth. In the end, only hope remained. Hope of a new boiler.

How to fix leaks on seals:

Outside of attaining a gas safe qualification and becoming a fully fledged heating engineer, you should leave this one to the professionals.

Small sealant issues can be solved relatively cheaply however more substantial seal issued caused by corrosion over time might well leave you looking for a new boiler.

Boiler repair cost looking too steep? Get a new boiler installed by us, tomorrow. Get a quote online, today.


Want an exact price on a new boiler? Use our boiler quote calculator (your fixed price will appear on screen).

What should I do if my boiler is leaking water?

  • Complete aforementioned visual checks
  • Minimizing leaking/collect water drops in a bowl, bucket or your mouth if thirsty
  • Correct pressure if possible
  • Appoint a heating engineer
  • Prepare for bad news and research prices for a new boiler

Is a leaking boiler dangerous?

Possibly, but highly unlikely.

It’s largely an inconvenience and your boilers way of letting you know that its time in this world has come to an end. A leak is no laughing matter.

An old boiler does, however, pose a large and relentless danger to your bank account. Do the maths on the repair work and likelihood and probable cost of future breakdowns vs. a new boiler, covered under a 10-year warranty - a new boiler is probably going to be cheaper in the long run. This is especially true if your boiler is particularly old. 

It may also be a good idea to start researching how much a new boiler may cost you.

How to fix a boiler leak

It's usually best to leave leak repair to the professionals, especially if you suspect an issue with an internal component. However, in some cases and if you feel confident, you can do a quick DIY fix. 

If you see the water leaking from a pipe joint, dry it off using a clothe and attempt to tighten it. Sometimes, a leak may be the result of a loose joint, so if you suspect this is the case, it's worth a try. 

Should you use a boiler leak sealant? 

If the leak is a pinhole leak, then you may be able to seal it up by using a special additive you add to your central heating system that will automatically seal the holes as it circulates through the system. However, these are not always straightforward to add to your system and in the majority of cases will not offer a solution. 

How to stop, or help to prevent future boiler leaks

Getting your boiler serviced annually is an essential part of boiler ownership. Not only will it ensure minor issues are caught and fixed before they develop into major repairs, but it's almost always a requirement of your boiler manufacturer's warranty and without it, your warranty will be invalid. 

It's also worth looking into system flushes, or chemical flushes to cleanse your heating system.

Heatable include a chemical flush & magnetic filter as standard with every installation – included in the price! It borders on financial insanity.

Make sure any new boiler installation you purchase is fitted with a magnetic boiler filter. And don't forget to check out our list of the best boiler brands to make sure you avoid a dud. We also offer the option of adding a powerflush - essentially a deep clean of your boiler and heating system. 

These are essentially magnets more attractive than Margot Robbie – collecting any and all metallic system debris from your heating system. This will prolong the life of your boiler, prevent blockages and minimize the risk of the boiler leaking.

Learn all about combi boilers in our what is a combi boiler guide and if you're considering having a new boiler installed in a new location, our cost of moving a boiler guide will be helpful. 

Common leaking boilers (with error codes)

Worcester Bosch Boiler Leaking
Worcester Bosch boiler leaking water:

Likely error codes: A1, E9, CE207, HO7

*error codes applicable to Worcester CDi Class, Worcester CDi Highflow, Worcester CDi Compact, Worcester Greenstar 25i, Worcester Greenstar 30i & all Worcester regular & Worcester system boilers.

Vaillant Boiler Leaking
Vaillant boiler leaking water:

Likely error codes: F.22, F.24, F.13, F.73, S.41, S.53

*error codes applicable to Vaillant EcoMax Pro, Vaillant EcoTEC Pro, Vaillant EcoTEC Plus

Ideal Boiler Leaking
Ideal boiler leaking water:

Likely error codes: F1, L1, FD

*error codes applicable to Ideal Logic, Ideal Logic Plus, Ideal Mini, Ideal Vogue, Ideal Independent, Ideal Isar

BAXI Boiler Leaking
Baxi boiler leaking water

Likely error codes: 117, 118, 125, E78, H.02 – 06

Is your boiler making a lot of noise? Find out why in our noisy boiler guide. 

Conclusion - a boiler leak is no laughing matter

There is no need to sound the alarms and light the beacons of distress to alert the elders of the boiler world for a leak.

But it does need investigating.

If you happen to be growing an array of herbs and small vegetables directly under your boiler, a small and manageable leak may serve you well

If it’s a small leak caused by a loose seal then it can likely be repaired easily and most importantly, cheaply.

If you are curious to know how much a new boiler installation may cost, use this clickable form to get a fixed price on the screen instantly. 

Larger leaks in older boilers, particularly boilers weathered by the sands of time are more of a handful and could lure you into an endless pit of wasted money. And if you have an immersion heater, their poor running costs make upgrading to a combi well worth it in the long run. 

In this instance, check out our selection of combi boilers with solid, reliable internal parts and a decade long parts & labour warranty – that covers boiler leaks 😉 

Wondering how much a boiler replacement may cost? Check our new boiler costs guide and see our guide to boilers on finance if you want to spread the costs and pay monthly. 


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