Wanted to share it, though, because it might help somebody else. Pumping-over tanks are quite common, and they can be really very dangerous.Okay, I went up into the loft a couple of weeks ago, and the central heating header tank was warm, verging on hot. It was well insulated, so there was not much heat loss really, but even an idiot like me knew that wasn't right. But what could be causing it?
"Ooooh", said the internet plumbing forums, as they sucked their collective teeth. "Your system's pumping over". Which is to say: "There's a partial blockage in your main CH pipework (the great big 28 mm pipe) that's forcing some of your hot water to go up the vent pipe because there isn't enough room for it all to flow through to the intake side of the pump. And the blockage will be made up of iron deposits that have all arrived at a very specific place, close to the pump inlet, and you'll need to replace a section of pipe."
Yeah, right, I thought, pull the other one. My central heating's working just fine. But dammit, the experts were right.
Sure enough, there was about 3mm of ferrous deposits at the very bottom of the pipe, which doesn't sound like much of an impediment in a 28mm channel, does it? But it was, and replacing the pipe (and the T joint where the deposit happened) completely fixed it. Here's the hot tip, though. There's a really good test for finding out whether you've got iron deposits inside a copper pipe - and where. Use a magnet, dummy. If the magnet sticks, you've got your diagnosis. Durn it, why didn't I think of that?
You can't just scrape an iron-fouled pipe out, cos it's as hard as iron, innit? So a T-shaped piece of pipe does indeed have to be swapped out. Two hours and twenty quid later, job done, everything sorted, and my status as a DIY household god duly restored.
Why is hot water in a header tank dangerous? Because they can split and spill near-boiling water over anyone unlucky enough to be standing under them. (There have been deaths.) More prosaically, any tank that's pumping over will be introducing air to the system, and that'll rot your radiators sooner or later.
Enough. Pass it on.
BJ