Boiler Leaking Water From The Bottom: How to Fix a Leaking Boiler

Is your boiler leaking water from the bottom? This 5-minute guide to leaking boilers includes combi boilers, as well as system and oil boilers, and covers all brands, including Baxi, Vaillant, Worcester, Glow-Worm, Ideal and more.

Apart from the fuel system, the principles of their operation are all similar.

Although there are some faults that could turn out to be terminal, overall, there’s a cost-effective way to repair the boiler, rather than having to fork out for a new replacement.

The cheapest companies fixing are local Gas Safe boiler engineers.

If you’re sick of sinking cash into your rotten old boiler, a replacement is an option. BOXT are a company partly owned by Worcester Bosch. So, they’re going to be able to offer installation prices lower than most. You can get an online fixed price using their boiler calculator.

How to Fix a Boiler Leak?

There IS a DIY fix and alternative to paying a boiler engineer 100s of pounds to fix your boiler, in the form of Fernox F4.

Fernox produce a range of filters and chemicals that protect, fix and prevent boilers from breaking down. At my time working in a plumbing merchant, it was definitely one of the most popular brands in this space.

And, the Fernox F4 is the best DIY fix for leaking boilers on the market.

A lot of boiler leaks are tiny, and a sealant like the F4 helps to plug those gaps. It works quickly, so within 1-24 hours you’ll know if the leak is fixed.

Here’s What Causes A Boiler To Leak From The Bottom

Below are a few internal parts that can cause a boiler to leak water from the bottom of the casing.

#1 – Pressure Release Valve (PRV)

The pressure release valve is designed to release water if the boiler operates above it’s working range.

Most boilers work at an optimum pressure of 1.3 bar, although anything in between 1-2 bar will see them work without issues.

If you’ve overfilled your boiler using the filling loop, it’s likely the PRV will release this excess water to stop any of the internal components of the boiler self-destructing!

#2 – Heating Pump Leaks

We’ve created an entire article dedicated to heating pump faults. As you can see here, it’s common for heating pumps to leak.

The most common reason a heating pump will leak is due to blown seals. This is usually down to the old-age of the pump and the seals deteriorating.

However, if there has been extra pressure put on the seals recently, this could’ve contributed to them blowing.

#3 –  Auto-Air Vent

The auto-air vent is another fail-safe in a boiler in case the operating pressure is too high. When the boiler is operating at a pressure that’s too high, the pressure is transferred to the auto-air vent.

Like most vents, they act on a pressurised valve. When the valve notices a pressure that’s too high for the boiler to cope with, it releases pressure. The problem is, these valves can stick, and they’ll not just let air out, but water too.

You’ll find the boiler leaking water from the top as it drips through the casing and down the pipes beneath the boiler.

#4 – Boiler Connections

It’s unlikely, but not impossible. If your system is new, there’s a chance the boiler connections for the flow and return are not done up correctly.

Dab the connections with tissue so they are completely dry. Once they are, check back in a few minutes to see if any leaks have appeared. Usually, a simple ¼ turn to tighten them up will do the trick.

#5 – Heat Exchanger Corrosion

Unfortunately, this is one of the few faults where boilers leak water from the bottom that may mean a replacement is necessary.

The heat exchanger in a boiler is incredibly expensive. Over time, it can corrode, and if it splits, it can result in water leaking from the bottom of the boiler.

You’ll need to have a Gas engineer check this out. Unless your boiler is under warranty, it’s usually much more cost-effective in the long-term to have a boiler replacement rather than repair.

You can read more about how much a boiler installation costs, here.

#6 – Corroded Pipes & Soldered Joints

Check the pipes and soldered joints around the boiler. Over time, fittings and copper pipe will corrode.

Excessive corrosion can lead to leaks below the boiler.

What To Do With A Boiler That’s Leaking?

Hopefully, by now, you’ve determined what’s causing your boiler to leak.

What kind of fuel does your boiler use


Grab your fixed price online by 3PM, and get next-day boiler installation.

Worcester boilers installed from as little as £1,795.

Some fixes are simple. But, the cost of boilers and boiler parts means we’d always advise contacting a Gas Safe engineer. There’s a tonne of qualified Gas engineers ready to give you a repair quote here.

They’ll not only be able to fix the problem, but they’ll be able to assess important and expensive internal components.

If any of those are close to failing (or need maintenance), they’ll be able to fix them while they repair your boiler leak; saving you cash.

Leaking Boiler FAQ

Is A Leaking Boiler Dangerous?

If your boiler is leaking water from the bottom of the casing, there’s a good chance the water is dripping over electrical parts.

Although most electrical units are sealed, it’s wise to switch the boiler off and call an engineer.

Asides from potential danger, remember a boiler leaking water everywhere could be causing more internal damage.

For example, if the water was to leak and penetrate the printed circuit board, it could lead to needing a replacement.

And, you don’t want this. A replacement PCB could cost £500+ including parts and labour.

Will A Boiler Leaking Water Mean It Loses Pressure?

Any boiler that is leaking water will cause the boiler to lose pressure. In fact, any leak in the central heating system will contribute to a loss of pressure.

This includes leaks from towel rails, radiators, piping and more.

What’s Next?

Hopefully our 5-minute guide to why your boiler is leaking water from the bottom of the casing has helped. If not, head over to our boiler problems guide.

If it hasn’t – we’re happy to offer help and advice by answering questions.

Just leave a comment below.