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?