OK I'm no expert but I will try to describe this problem as best I can:
The central heating system in our house is a fully-pumped, open-vented Y-plan system. Now it should be possible to have either the central heating or the hot water on separately, shouldn't it? The problem we are experiencing is that the radiators start to warm up when only the hot water is on.
I should point out that we only moved in a few weeks ago, and that the previous owner of the house when contacted said it had always been like that.
After reading around, it seemed to me that it was obviously the three- port valve that was faulty. Even though I'm a novice, I thought that to change the valve was a pretty easy job, so I had a go myself. So I drained the system (I now know that was a bad idea) and successfully put a replacement valve in. However, when I went back into the loft to turn the cold water feed to the header tank back on, it turned out that the tank was already full.
At that point I was stumped, so called out a professional; at first he thought it was simply an air lock but later found that one of the pipes to the air separator had become blocked with sludge. He cut out and replaced that bit of pipework, which allowed the system to be filled back up again. (He also advised me that I shouldn't have drained the system - the three port valve can be changed just by inserting a couple of rubber bungs into the pipes to and from the header tank).
However, when the system was tested out the radiators were still warming up with the hot water. The heating engineer could not work out why, at first he said I'd put the new 3-port on backwards, but after checking, it was correct. The wiring also seems to be correct, and the valve seems to be switching over correctly.
So he finally concluded that something was causing the heat to flow from the boiler return backwards to the radiators, and that I needed a one- way valve putting in somewhere. He left me with instructions to pull up the floorboards and investigate where the central heating pipes run.
Before I start pulling up floorboards, are there any clever people here who might be able to help explain what's going on? Could it be that the system just needs re-balancing? I notice that at some time someone's added thermostatic radiator valves to the bedroom radiators, could this have anything to do with it?