Conventional fully-pumped system, new gas boiler and pump and associated plumbing ten years ago, has been working fine until last month, but has now started cutting out, and the problem seems to be getting worse.
The system is on all day. Most days, in the afternoon or early evening, the house goes cold. The radiators are going cold. The pump, which is upstairs in the airing cupboard, is running (or, at least, vibrating and making a noise) and the pipes in the vicinity are hot. At the boiler, the upper pipe (which I assume to be the output) is hot but the lower pipe is only warm. The boiler occasionally fires for a minute or two.
If I switch off at the timer, wait five minutes, and turn the system back on, everything starts working normally again for the rest of the day. My impression (though it's hard to be sure) is that it's necessary to switch off the hot water as well as the heating, and half a minute isn't long enough.
Gut reaction says it's either an air lock or an electrical cut-out that automatically resets.
Any suggestions where to look?