Is your boiler leaking water? Find out why, and how to fix it

27 August 2021 | Celia Topping

It’s getting to that time of year again – when we wave goodbye to BBQs and trips to the seaside, and say hello to radiators and boilers. 

As we all know, it’s a good idea to make sure your boiler is in perfect condition before the arrival of winter. So, if you notice water leaking from your boiler, you should act quickly, as it could be a sign that there’s a serious problem.

In this article, we’ll help you work out what could be causing your leak, and how you might be able to fix it. Or if it’s time to call in the professionals. 

How do I know if my boiler is leaking water?

Modern boilers shouldn’t leak water at all. They work within a pressurised system that’s sealed, so the water can’t escape. But things can, and do, go wrong. It’s not always serious, but no matter how minor the leak, it should be repaired quickly.

It’s possible that you may not even see the leak at first, but you may notice a loss in pressure that causes your boiler to cut out. If you top it up and it loses pressure again quickly, it’s likely you have a leak. 

Is your boiler losing pressure but there’s no leak?

First of all, check out our step-by-step guide on what to do if your boiler pressure is too low, to see if it’s an easy fix. If not, then read on. 

Why is my boiler leaking water? 5 top causes of a leaking boiler, and what to do

When you notice a water leak, the most important thing is not to panic! 

Many leaks can be caused by minor issues, and are easily repaired. Read on to find out how to diagnose what’s wrong and what to do in every situation. 

1. Badly installed or corroded joints

One of the main causes of leaking boilers is an issue with the pipework underneath. You can check for this by having a feel for any wetness around the pipes, as small leaks might not always be obvious. 

If there’s a leak, you need to ask yourself: is your boiler relatively new, or old?

If your boiler is new

This is usually nothing to worry about. Small pipe leaks are surprisingly common in new boilers, and can be a little tricky to spot when first installed. 

If you’re worried, any reputable boiler installation company will happily come back to check the pipes and joints, and fix any issues. Happy days! 

If your boiler is old

Most water leaks come from pipe joints. So even if your boiler is old, they can easily be repaired by having a replacement joint fitted by a heating professional. 

If your boiler is over 8 years old, it’s possible that your pipes are leaking because they’ve corroded. You could get a short term repair, but if it’s out of warranty (usually 8 to 10 years) an old boiler is best replaced sooner rather than later. Otherwise, it will only break down again, costing you more money. 

We know replacing a boiler is a big expense, so we’ve written a helpful guide that explains the different types of boiler and how to choose the best one for your home. The most common boilers fitted in the UK are combi boilers, so we’ve written you a guide on those too.  

2. Too much pressure

It’s easy to check if your boiler is building up too much pressure. On the front of your boiler is a pressure gauge. For most boilers, the needle should remain between 1 and 2 bar. 

If the needle is way over 2, then the boiler pressure is too high. This means there’s too much water in the boiler and the pressure release valve (PRV) will let some of the excess water out – which you’ll see as a leak. 

pressure gauge diagram

How to fix high boiler pressure

It’s a simple fix, done by bleeding the radiators. But first, check if the filling loop tap (the silver flexi-pipe under your boiler, with 1 or 2 taps) is firmly closed. 

Then follow our guide on how to fix high boiler pressure.

 Or watch OVO heating engineer, Rich Laniyan, talk you through how to bleed your radiators.

3. Corrosion and general wear and tear

As mentioned above, boilers don’t last forever. The combination of water and metallic debris within the system eventually takes its toll on the pipes and cylinders, so they become brittle and break. 

Some components can be replaced relatively cheaply, but after time, this wear and tear means your boiler will have to be replaced

4. Leaky, or faulty heat exchanger

The heat exchanger is, unfortunately, one of the most important, and most expensive parts of your boiler. It’s quite common for them to fail in old boilers, and even in new low-quality boilers. 

You’d need a qualified heating engineer to come and check your boiler in this instance. We’re sorry to say that fixing it would probably be more expensive in the long run – it’s probably time for a new boiler

5. Leaks from seals on internal parts

With water running continuously through your boiler, sometimes the seals on the joints inside can decay. High pressure can also cause joints to leak. 

Even if you can see where the problem is coming from, it’s time to call in a professional heating engineer. Don’t try to fix anything inside your boiler yourself. 

What to do if you have a leaking boiler

In most cases, it’s best to call in the experts if you have a problem with your boiler. It can be extremely dangerous to mess around with internal parts if you don’t know what you’re doing. 

But, if you’re a confident DIY-er, you could attempt to repair a very basic boiler leak. If the leak is dripping from one of the pipe joins on one of the pipes visible underneath your boiler, dry the area and double-check that it’s the source of the leak. If it is, tighten the fitting and see if it helps. 

Can I use a boiler leak sealant or sealer?

For a minor leak, sealants could work, although they’re tricky to apply. Plus, sealants are not a long term solution – they’re not much better than using sellotape to fix a wall. So it’s probably best to bite the bullet and call a Gas Safe -registered expert to come and fix it properly. 

How to avoid or prevent a boiler leak

As we all know, prevention is better than a cure. Regular servicing is the best way to prevent your boiler leaking. Make sure you get it done by a professional, as boilers are potentially dangerous. Any tampering with them could cause a gas or carbon monoxide leak, or worse. 

With OVO Energy boiler and home emergency cover, you can relax in the knowledge that any boiler breakdowns, leaks or emergencies are covered, from as little as £15 a month. We’ll keep your boiler and central heating system working efficiently, so you don’t need to worry.

FAQs: Why is my boiler leaking water from underneath?

Is a leaking boiler dangerous?

Without knowing the cause of the leak, it’s not easy to know if it’s dangerous. But as boilers contain electrical parts, gas and hot water, it’s not a good idea to go poking about inside! The best thing to do is switch it off and call a qualified heating engineer to come and take a look. 

Is a leaking boiler an emergency?

Boiler leaks can be minor, or they can be a sign of something more serious. When you don’t know the cause, it’s best to treat all leaks as serious, as any leak means your boiler needs immediate attention. Some leaks could be causing irreparable damage, so switch it off and call a professional engineer as soon as you can.

Can I fix a leaking boiler myself?

If you can clearly see where the leak is coming from and you’re competent at DIY, you could tighten a joint, or add some sealant in one of the external pipe joints underneath the boiler. But the likelihood is, your boiler will need an expert to take a look at some point, so it might be best to give them a call from the outset. 

If you're worried about the cost, our home emergency cover takes the hassle out of any boiler or central heating issues, by providing full cover for as little as £15 a month.

Does a boiler leaking cause low pressure?

A leaking boiler can be a cause of low pressure, because there’s not enough water in the system. If you try to repressurise your boiler but it continues to lose pressure immediately, it’s time to check for leaks, and call in the experts. 

What to do if I have a boiler leaking oil

This is only relevant if you use heating oil in your boiler. If the smell of oil is stronger than usual, or if you spot oil stains or pooling, you could have a leak.

Immediately close the tap or valve on the oil tank and open the windows to release any fumes. Put a bucket under the leak, but don’t wash it away. Call out a heating engineer straight away. If anyone feels sick or dizzy, get out of the house. 

Quick recap

So, there are a few reasons your boiler could be leaking:

  • Badly installed or corroded pipework
  • High pressure
  • Interior corrosion and general wear and tear
  • Leaky or faulty heat exchanger
  • Leaky seals on internal parts

The best thing to do if you spot a leak is to check the pressure gauge. If it’s a high boiler pressure issue, you can read our guide on how to fix high boiler pressure

For any of the other problems, it’s best to call out a heating engineer. They’ll be able to tell you whether it’s an easy fix, or whether you need to think about replacing your boiler.