Dan S. MacAbre wrote:
My mum and Dad's boiler wants to stay on all the time. I thought the
best thing to do, besides just switching it off for now, would be to try
and work out how it's all wired together, and then work out possible
causes. I've only so far had time for a quick look, but something has
struck me as odd. They have two motorised zone valves, but there is no
room thermostat anywhere. I'd always thought that each zone valve
responded to a thermostat. Is there any valid configuration that has
two zone valves, but no room thermostat?

Well, I've had the floorboards up, and made a few drawings, and I think
I've got my head round it now. There is no room thermostat - the timer
output for the CH goes straight to activate the motorised valve, so it's
basically acting as a relay. I can understand this if the timer output
is not capable of driving the pump and the boiler; however, it's a
mechanical timer, so I don't think it's /that/ delicate :-)

I see from other posts that not having a room thermostat is valid, if
you rely on the boiler to respond appropriately, however, mum and dad
are likely to turn it high in Winter, which seems to be a bad idea in
this configuration. It would be easy to add a room stat into this
wiring, I think.

I was a bit confused because mains sockets and plugs have been used to
make some of the connections. So what I thought was the main power feed
to the whole thing (because it is right next to the boiler) is, in fact,
just the connector coming down with the power from the motorised valves.

Anyway, it's obvious now that a knackered CH motorised valve will keep
it all going regardless of what the timer etc. does, since they have an
uninterrupted feed from the mains. So this is what I need to check. I
suppose having it open all the time the heating is on doesn't help. The
actual valve underneath it feels stiff, so it may just need freeing up.

But the whole thing is a bit ropey. The heart of it is a terminal block
dangling in the void space. I need to at least put it in a box, or
something.