Why is my boiler leaking water?

There are lots of reasons why your boiler could be leaking. Here, we explore some of the possible causes and explain what you need to do next.

If you’ve noticed your boiler’s leaking water, even if it’s just a trickle, chances are you’re going to be a bit worried. 

With this in mind, let’s get right to the point. Here’s why your boiler could be leaking water and what you need to do to fix it.

Look, we’ve even added some links so you can jump straight to the good stuff. In this guide, you’ll learn:

Please note: We don’t recommend trying to fix leaks yourself, as a botched job could mean you end up paying more for repairs than you initially needed to. Get things repaired properly and safely the first time around by booking a professional through JustFix.

Why is my boiler leaking water from underneath? 

Water dripping from underneath your boiler could be a sign of corrosion or broken seals. If your boiler’s quite old, this can start to happen. Corrosion in pipes and connections can cause small holes that allow water to leak out. 

Likewise, if you’ve just had your boiler serviced or installed, water leaking from underneath could be a sign that some joints need tightening. 

What to do if your boiler’s leaking water from underneath

If you’ve found water leaking from underneath your boiler and you feel confident doing so, take off its cover and examine the pipework. Be careful not to touch the boiler, as it can get very hot, and you may burn yourself.

Once you’ve found the source of the leak, contact a Gas Safe registered JustFix heating engineer to come and take a look and tighten any loose connections. 


Why is my boiler leaking when the heating is on?

A common reason boilers leak while the heating is on is high pressure. This can happen when your boiler is under a lot of strain and trying to relieve some of the excess pressure. 

If you find your boiler leaking when the heating is on, check the pressure gauge. If it’s above the sweet spot, between 1 and 1.5 bar, you may need to release some pressure to prevent further leaks. 

What to do if your boiler’s leaking water when the heating is on

The boiler’s manual will have instructions on releasing the pressure, so if you feel comfortable, you could give this a go yourself. 

Turning your boiler off can temporarily stop the leak, but this isn’t a permanent solution. If you don’t want to mess with the pressure gauge, it’s best to turn the boiler off and contact a boiler repair specialist. 

Another thing you can do is bleed your radiators. A build-up of air in your heating system can increase the pressure and stop any leaks. 

Why is water dripping from the bottom of my boiler?

Water dripping from the bottom of your boiler could be linked to the pump seals. If they’re not installed correctly, water can leak out. 

What to do if water’s dripping from the bottom of your boiler

This problem is more closely associated with old boilers, as the seals and pump will wear and corrode over time. Leaks can also be a sign of a larger problem with the boiler’s pump. If you notice water dripping from the bottom of the boiler, get in touch with a Gas Safe registered engineer, and they’ll be able to diagnose the issue and decide whether the seals or pump need to be replaced.

Why is my boiler leaking from the relief valve? 

A leak from your boiler’s relief valve could indicate a high temperature. Boilers are fitted with temperature controls to monitor the water and ensure it doesn’t get too hot. If the temperature in your system is too high, water can leak from the relief valve. 

What to do if your boiler’s leaking from the relief valve

If you’ve found water leaking from the relief valve, check the boiler’s temperature. If it’s too hot, it’s usually because the system has been on too long and overheated or because the temperature’s too high. 

Always consult the manufacturer’s instructions, but if you think the boiler has been on too long, it may benefit from being switched off for a while. 

If the temperature is too high, lower it and turn it off to allow the whole system to cool. 

Persistent leaks from the relief valve may be an indication that there is a more serious problem with the boiler, so if this is happening regularly, you’ll need to contact a fully qualified engineer

Why is my boiler leaking when it’s turned off? 

A boiler leak when the system is turned off can be due to issues with the heat exchanger or corroding components. 

Water will stay in your system when the boiler’s turned off, so if there are any cracks or holes, these can cause leaks. 

What to do if your boiler’s leaking when it’s turned off

Faulty parts or corrosion must always be treated by a qualified professional. If you try to fix these issues yourself, you can make them worse or void your boiler’s warranty. Either way, it could cost you a lot of money to get things sorted. 


Five common causes of boiler leaks

  1. Internal corrosion - Over time, the constant heating and cooling cycles can cause internal components like heat exchangers and pipes to corrode from the inside out. As the metal thins, cracks and holes develop, allowing water to escape.

  2. Pressure problems -  Boilers operate under high pressure to heat water effectively. If the pressure gets too high, leaks can occur at joints, valves and seals that aren't designed to withstand the excessive force.

  3. Frozen pipes - When temperatures plummet in winter, condensation inside your boiler's pipes can freeze and expand, stressing the pipes until they split apart.

  4. Faulty components - Like any appliance with moving parts, boiler components can wear out over years of use. Circulator pumps, relief valves and washers can fail, leading to leaks.

  5. Improper installation - If your boiler isn't installed correctly by a qualified professional, it increases the risk of leaks developing due to poor connections, inadequate support, or other errors.

Why you shouldn't ignore a leaking boiler

A few drips might not seem like a big deal, but ignoring a leaking boiler can have disastrous consequences:

  • Water damage to floors, walls, and surrounding areas
  • Increased risk of deadly carbon monoxide leaks
  • Decreased boiler efficiency and higher energy bills
  • Complete boiler failure requiring an expensive replacement

The smartest approach is to have the leak inspected and repaired by a professional heating engineer as soon as possible before further damage occurs.

The importance of regular servicing

While leaks can sometimes develop suddenly, annual boiler servicing is the best way to catch small issues before they become major problems.

During this comprehensive maintenance, your heating technician will thoroughly inspect and test all components to ensure everything works as it should. A professional will identify areas of potential concern, like corrosion, and replace worn parts before a leak can occur.

Don't skip your annual boiler service - it could save you a fortune in potential water damage and repairs or replacement costs.

Call in the pros at the first sign of a leak

Whether you have a minor drip or a steady stream of water around your boiler, don't try any DIY leak repairs. Boilers are complex pieces of kit that require an expert's attention to fix safely and properly.

At the first sign of a boiler leak, call a Gas Safe-registered professional immediately. They’ll have the tools and training to diagnose the underlying issue quickly and get your boiler back in perfect working condition.

With JustFix, you can find and book a boiler repair online in minutes. Simply tell us a bit about the problem you’re facing and where you live, and we’ll connect you with a local boiler repairer in a jiffy. 

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