The Hot water and Central heating are a mystery! Both operate well but one or both at odd times cause the header tank to overflow. Switching off either system will cure the overflow. The C/H is an old one pipe system with three returns to the boiler. It is activated by a motorised valve from the room stat. The hot water is a feed and return with a mortorised valve form the tank stat.
Here goes, i'm not a craftsman, but i've been reading a lot, trying to cure my own system's problems. A lot of problems like yours seem to be caused by gas in the system. This can come in thro' leaky couplings, eg at pump, diverter valves, radiator valve glands (this has happened to me), and my present troubles have been identified by 2 plumbers as air ingress thro' 11-year old plastic plumbing, which lets air in thro' the molecular structure even tho' it is water-tight. Another quoted source of gas is hydrogen, as the by-product of water rusting the inside surfaces of radiators. If radiators are left valved-off too long, the gas builds up, and can shift around the system when that radiator gets used next, and can cause air-lock problems. I fitted 2 auto-bleed valves last year (one above the inlet to the hot tank, and even higher than the tank outlet;- the other got fitted in place of the manual bleed point of the tallest upstairs radiator.) However, i have not bothered running the upstairs radiators, or checked for bleeding/sucking couplings and valve glands. Now that i have checked, brown dried Fernox at valves tell a tale, so my 3 cold downstairs may be partly down to air/gas. The diverter valve checked out OK, but pumps do get clogged/clapped. ( Â£65+ labour=Â£200-250). The radiators chilled-out after de-selecting the radiators for just a day away.
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