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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 Viessmann, 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. HEATABLE are an online boiler installation company. They have lower costs and have streamlined sales, so they’re able to offer installation prices lower than most. You can get an online fixed price using their boiler calculator.

Viessmann Vitodens 050-W from Heatable

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.

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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 £1,770.

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.

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  1. The cold water pipe going to my boiler is leaking can this affect the pressure

    1. The boiler is dripping water from the bottom corner not a constant drip but dripping,I’ve turned it off any help would be appreciated thank you

    2. Any leak will result in pressure loss.

      There’s a few posts on pressure/leaks here: https://heatingforce.co.uk/blog/boiler-fault-finding/

  2. Hi my boiler has developed a small ‘drip’ overnight. Heating (using Hive) was actually off. Turned it all back on, but one small drip from the underneath of the unit and it’s wet in the front left bottom corner of the boiler. Using a bucket. Have a homecare agreement but with the nightmare weather, can’t get an appointment for another week+ – my question is, it is safe to have the heating on. All seems to be working, but just worried about the water drips. Only serviced last Saturday with parts replaced. Thanks.

    1. I can’t advise on safety over the internet for obvious reasons.

      What parts were replaced? I think I’d stress the fact that to the engineer & get them out as soon as possible.

      To stop the leak you can open the bleed valve on radiators which will release water. You’ll need towels to protect floor/carpet & something to catch the water. By releasing some water, you’ll drop the pressure, and that should be enough to stop the leak.

      Alternatively, you could drain the whole system if you know where the drain valve is.

  3. Boiler is leaking after releasing some water into it to increase pressure last night. Is it possible that we just released too much water and the leak will stop once it has balanced out?

    1. It could be that something has damaged inside. It might be that the leak will stop, but it’s something I’d keep an eye on.

  4. My automatic air valve is leaking water on a Glow Worm CX30. Can I replace the valve myself and if so approx cost of replacement please?

    1. I guess it depends how good of a DIYer you are 🙂

  5. Hi ,I have a valiant boiler and it’s only leaking water when the heating is switched on from the left hand bottom ,any ideas what this is ,also the pressure is fine ?

  6. My Vokera combi boiler stopped working. Repair done but is still,leaking from bottom. Boiler bone dry inside , bolder working perfect , but floor around is still quite wet when boiler first comes on in the morning. Help

    1. Call the people who installed it to check.

  7. Why would there be water on the floor around my boiler only sometimes? I’ll check other times and it has dried up.

    1. Because the network may have ups and downs and not constant water pressure.

  8. I lost pressure on our combi boiler so re pressurised it. Whilst doing this I notice the valve behind the pressure gauge was crusted up. After turning it a bit I’ve noticed a drip every 30 seconds or so. Pressure and system are working fine and this is the first time I’ve had to do this,is this a problem and shall I get someone out or is there anything I can do myself? Thanks

  9. Hello,

    A very good article, actually gives a solution, why FERNOX F4 surely, any water resistant, leaking sealant should work, i know Fernox f4 is the best and excellent, it is only, about 5 pounds to buy, a good price.

    This might be a solution however you still have to find the leak, which would be difficult for an amateur to find, and even if he does, there might be other leaks, that he has not seen. So it can be tricky.

    My boiler was leaking from the bottom Valliant, i find three leaks, small ones coming from the heat exchanger, so i put some water proof sealant on it, two layers, it was Evo Stick Sticks, like Sht. It did not stop the leak, however did reduce it, i was lucky, i managed to find the leak area, if it was, elsewhere, and hard to get too, no chance for me. As legally you are not allowed to work on boilers, unless you are qualified, which makes sense.

    A leak fix, is a small, risk, so i had a go. It has helped a lot.

    An excellent article.

  10. I noticed my boiler was dripping water from the bottom, I called to have it repaired but the only appointment they have is for next Thursday. I don’t know anything about boilers, I’ve put a bucket underneath it but I don’t know if I can use my boiler in the meantime.

  11. My Grant combi boiler was losing water and pressure to the extent I needed to top it up 3 times in one day. I called out a plumber but by the time he arrived the leak had stopped and the garage floor was dry. That was 3 days ago and everything is still okay.

    Any ideas as to what is happening here please? Should I just keep an eye on the situation or is there something else to do at this stage?

    1. Please don’t think the boiler fairy is going to protect you for the winter. It may be that you will face the same problem in the coming weeks, or months, or days. Have a knowledgeable engineer look into it and tell you what may be the cause of it.

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