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

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

Boiler leaking water from the bottom

Apart from the fuel system, the principles of boilers’ 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 a leaking boiler instead of forking out for a replacement.

The cheapest company you can hire to fix a gas boiler is typically one of your local Gas Safe boiler engineers.

If you’re sick of sinking cash into your rotten old boiler, a new boiler replacement is an option. HEATABLE are an online boiler installation company that are able to offer installation prices lower than most. You can use their boiler calculator to get a fixed price online in minutes.

Viessmann Vitodens 100-W from Heatable

How to Fix a Boiler Leaking Water?

There IS a DIY fix and repair 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 one of the best DIY repairs for a leaking boiler on the market. A lot of boiler leaks are tiny, and F4 acts as a boiler leak sealant and 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

If your boiler is leaking water from the casing at the bottom, these are the internal parts that can harbour the root cause. If you notice your boiler leaking, and suspect that a defective internal component is the culprit, our best advice is to get a Gas Safe registered engineer to inspect your appliance.

#1 – Pressure Release Valve (PRV)

The pressure release valve is designed to discharge water if the boiler operates above it’s working pressure range. Most boilers work at an optimum pressure of 1.3 bar, although anything in between 1-2 bar is acceptable.

If you’ve overfilled your boiler using the filling loop, the PRV will likely release this excess water to stop the boiler’s internal components from self-destructing! In this case, you’ll need a heating engineer to investigate the cause behind this excessive pressure. You can read more about high boiler pressure in our handy guide.

#2 – Boiler 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 within boilers to leak.

The most common reason a heating pump will leak is due to blown seals, which deteriorate as the pump ages. However, excessively high pressure could also cause the seals to fail.

#3 – Boiler 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

If your system is new, there’s a chance the boiler flow and return pipes have not been properly set up. It’s an unlikely scenario, but not an impossible one.

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, a corroded heat exchanger is one of the few faults where boilers leak water from the bottom that may require a boiler replacement.

A boiler heat exchanger is incredibly expensive. Over time, it can corrode, and if it splits, it can lead to the boiler leaking water from the bottom of the casing.

You’ll need to have a Gas engineer check this out, and make a decision: should you just replace a boiler that is leaking water, or pay for boiler repair?

Unless your boiler is under warranty, it’s usually much more cost-effective in the long-term to get a new boiler than to repair an old one with a broken heat exchanger. You can learn how much boiler installation costs here; or, you can get a fixed price for a replacement boiler from Heatable and compare it with quotes from local installers.

#6 – Corroded Pipes & Soldered Joints

Check the pipes and soldered joints around the boiler. Over time, pipe fittings will corrode, leading to leaks under the boiler.

What to Do With a Water Leaking Boiler?

Hopefully, by now, you’ve determined why your boiler is leaking water.

What kind of fuel does your boiler use?

Fixed price online with next day installation

Worcester boilers from as little as £1,965.

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

A Gas Safe engineer will not only be able to fix the problem, but they’ll be able to assess important and expensive internal components and quote you a repair estimate.

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

However, if your boiler is old, and you choose to forgo repairs in favour of a new boiler, we recommend you check out Heatable. The online boiler installation company usually offers great deals, starting at £1,675. All you have to do is fill out a quick, anonymous survey about your property and you’ll get a range of fixed price new boiler quotes on your screen right away.

FAQ For Boilers That Are Leaking Water

Are leaking boilers dangerous?

If you have a boiler leaking water from the bottom of the casing, there’s a good chance the water is dripping over electrical parts. And although most electrical units are sealed, it’s always wise to switch the boiler off and call an engineer.

Apart from potential danger, remember that a boiler leaking water could be damaging to its electrical components. For example, if the water leaks and penetrates the printed circuit board (PCB), you’d have to fork over around £500+, including parts and labour, for a replacement PCB.

Does a boiler leaking water lose pressure?

Any boiler that’s leaking water will lose pressure. In fact, any leak in the central heating system will contribute to boiler pressure loss. This includes leaks from towel rails, radiators, piping and more. And this is why you should get a Gas Safe engineer to investigate any boiler leaks as soon as they become apparent.

What’s Next?

Bottom Line — a boiler leakage can be dangerous and also expensive to repair or replace. Take the time to assess the leaking boiler professionally before taking action.

Hopefully, our 5-minute guide has helped you understand why your boiler is leaking water from the bottom. If not, head over to our boiler problems guide. Alternatively, you can leave us a comment below and we’ll try our best to respond quickly.

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    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

  1. 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.

  2. 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.

  3. 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?

  4. 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 ?

  5. 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

  6. 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

  7. 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.

  8. 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.

  9. 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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