A few days ago my cold water tank started overflowing at a steady trickle. So I emptied the loft cupboard of all the old tennis racquets, puzzles and bits of wood and carpet that I have bunged in there in case they might one day be useful, and I climbed inside on my hands and knees to have a look at the tank. I checked the ballcock and adjusted it a bit but couldn’t find any problem. Thinking that as I couldn’t find any problem there mustn’t actually be one, and that somehow the mere act of climbing into a cramped cupboard and fiddling about with a ballcock might suffice to make the non-problem go away, I gave up and did what all good DIY-ers do: just hoped it wouldn’t happen again. Much to my surprise the overflow trickle seemed to have stopped so I must have done something right and I felt suitably smug.
Half an hour later and the trickle resumed. Bollocks.
I turned off the rising main, turned on a hot tap for a bit, went back inside the cupboard and fiddled again, this time adjusting the ball to make the water level lower. I was surprised how pleasantly lukewarm the water felt as I sploshed around in it with my adjustable spanner. Surely that would do it.
Again the trickle returned.
Huh. I turned down the boiler in case the cylinder was “kettling” – where it get so hot it ejects steam up through a vent into the water tank. That didn’t work. I had been on my own for a few days, not using much water. Could there just be too much hot water? Is that even a plumbing phenomenon? (Of course it isn’t. There’s a thermostat which stops that but I was beginning to lose my mind, not being able to figure out what was going on.)
I went away for 36 hours and when I got back there was no hot water and the trickle was a constant stream. The boiler was working but the cylinder was full of cold water. Was it the thermostat? Once the water was hot again I could feel the pipe that normally feeds the cylinder with cold water from the tank getting warmer and warmer. I ran a hot tap and the feed pipe got cold again, and the overflow trickle briefly stopped.
Aha! I figured out that somehow water was flowing the wrong way – from the cylinder up into the cold water tank. Madness. What could be causing it? Was there something wrong with the cylinder? The limescale is terrible around here and we’ve already had to replace the cylinder once because the inside of it looked like Wookey Hole.
I was on the brink of calling the plumber so that he could systematically empty my wallet when I thought I’d give Google one last go. And bless the patron saint of search engines but she came up trumps. I found someone who’d had exactly the same problem.
The culprit for the whole mis-functioning of the hot water system was… (drum roll)… the kitchen mixer tap. About the last place I would look and as far away from the leaking overflow as it is possible to be. A bit like saying the root of the Middle East crisis is a pub in Truro.
The kitchen mixer tap (pissed Cornish anti-Zionist bastard that it is) is fed by the hot water from the cylinder (natch) and by the rising main – the high pressure source of all water in the house. Something in the mixer had died of old age and while the tap was off, inside it the cold rising main water was forcing its way (it is much butcher after all) into the hot water’s territory, pushing the hot water back to from whence it had come, all the way through the cylinder and into the water tank in the loft, making the tank overflow. Who’d have thought it eh?
So if you ever have a overflowing and baffling tank here’s what to do to check for the same problem I had: turn on your mixer tap to hot only, until the feed pipe under the sink is warm. Turn off the tap and keep feeling the pipe. It it starts to cool rapidly then you know that your mixer is knackered, the rising main is pushing the hot water backwards and the tap needs replacing. Mine cost £49. I have a hunch if I’d called the plumber I’d already have a new cylinder, a new stopcock, a new thermostat, no new tap and a leaking overflow.