I moved into this house in September 2025, and on the very day we moved in, we noticed massive water stains across the upstairs ceilings. Not exactly the move-in we imagined.
We called out an emergency plumber, who told us that the water tank for the central heating had overflowed and was now seeping through the ceiling. After investigating further, they found the cause: the hot water cylinder had split. More specifically, the coil itself had split, which meant scalding hot water was being pumped back up into the tank in the attic.
That caused the water in the tank to rise, expand, and eventually leak.
So, on the third day of living in the house, we had to pay for a brand-new cylinder. Including installation, it cost about £1,300, which was a pretty brutal thing to deal with immediately after moving in. But the issue itself was fixed, aside from a small pinhole leak on one of the pipes, which the plumber kindly came back and repaired for free.
That resolved the issue itself, but one problem remained.
The Constant Overflow Drip
From the back garden, I could see an overflow pipe dripping constantly. I put a bucket underneath it to catch the water, and it would fill up with around 15 litres every week or so.
For a while, I just emptied the bucket into the drain and ignored it. It was annoying, but not urgent.
But now that our child is walking, she needs somewhere to run around outside, and the back garden is a pretty perfect space for that. The only problem was this big mouldy bucket sitting there collecting water from the overflow pipe.
So I finally decided to go up into the attic and take a proper look. Having done quite a lot of other work on the house by this point, I’m much more confident with DIY than I used to be. I wanted to see whether this was something I could fix myself rather than calling out a plumber and spending a lot of money.
The First Tank
The first thing I found was in the main water tank.
Even when the ball had floated all the way up and should have closed the valve, the valve was still dripping slightly. This didn’t actually seem to be causing the overflow, because the tank is so large that it would probably take months, if not years, to reach an overflow state from such a slow drip.
But either way, it still needed fixing.
Naively, I thought this was the cause of the problem at first. So I removed the entire ball valve and replaced it.
I watched a few YouTube videos beforehand, and in most of them people split the valve into two pieces and reused one part of it. I decided to just replace the whole thing instead. The replacement part was about £8 on Amazon and arrived the next day.
The process was fairly straightforward: I turned off the water to the house, lowered the water level in the tank by running a tap, removed the old ball valve, and fitted the new one. The process only required two spanners, albeit quite large ones.
That fixed the leak from the valve, But it did not stop the drip from the overflow pipe.
Finding the Second Tank
At that point, I was confused, so I went back up into the attic and had another look around, which is when I found a second water tank.
I then looked into how gravity-based water supply actually works, and it turns out that in my 2004 house it is pretty normal to have two water tanks: one for the main water supply into the house, and a smaller one for the central heating system.
The smaller tank was hidden completely behind the larger one. I didn’t even know it was there until I traced some pipes to it.
When I opened it up, the water level was right at the top- so full that just nudging the tank slightly would probably have caused water to spill out. So obviously, that wasn’t right.
I then realised that I couldn’t drain this tank in the same way as the main water tank. The water doesn’t really leave it in the same way; it just cycles through the central heating system. At least, that’s my understanding of it. I could be wrong.
So I got a bucket and a plastic water bottle and scooped out the excess water by hand until the level was well below the valve.
At first, I thought that was the repair done. I couldn’t see any dripping or anything obviously wrong. When I went back outside into the garden, the overflow pipe had stopped dripping, so I was very happy.
The Drip Comes Back
Then we had a surprisingly cold night, so I turned the central heating on. The next morning, the overflow pipe was dripping again.
I went back up into the attic and found that the water level in the smaller tank was high again. It wasn’t quite as high as it had been the first time, but it was still much too high.
So it looked like while the central heating was on, something had caused water to flow into that tank.
That’s when I realised that the float valve wasn’t working properly at all. The ball had to become completely submerged before the water flow would actually stop. By that point, the water was almost at the brim of the tank. The first time I had drained the tank, I didn’t have the water on, and didn’t check to make sure it wasn’t running after the water was on. Upon a second look, I noticed that the valve didn’t actually close until the tank was nearly full, and well over the overflow pipe.
Replacing the Second Ball Valve
I drained the smaller tank again using the bottle and bucket method.
Then I completely replaced the ball valve, just like I had done on the main water tank. I unscrewed the main pipe, undid the plastic nuts, removed the old ball valve, which was very mouldy and disgusting, and fitted the new one.
Since then, I haven’t had any further problems with the overflow pipe or the constant dripping.
What I Learned
Through this process, I learned a lot about how water tanks and gravity-based systems work.
And honestly, I’m quite impressed by how analogue the whole setup is. There’s no electricity driving anything in the tanks themselves. It’s just gravity, water levels, float valves, and pipework. As a tech guy, I find that surprisingly satisfying. I now feel much more confident dealing with basic water tank issues myself, although ideally I won’t have to touch any of it again for another 20 years or so.