I have scoured this board to see if I can work out what's wrong with my
system, and while there are some posts that almost describe the problem
I thought i'd try my own query before calling in a heating engineer.

The basic problem is that some of the upstairs rads heat up when we are
running the CH on "water only" - from what I know of CH we have a pump
driven system, tanks in attic etc etc. Had new boiler and cylinder
installed about 18 months ago. I'm pretty sure we don't have a
non-return valve anywhere in the system, we have standard three port
valve near boiler and this is working fine.

I can understand the logic about an incorrectly plumbed tee from the
hot water return causing hot water to find its way into the rads....
but what worries me is that I would have expected to find the return
pipe from the rads was the one that heated up...instead it's the feed
pipe. This is a feature of the upstairs rads - particularly the two
bathrooms and the main bedroom.... and I don't think the bathroom rads
are plumbed into the hot water circuit if you see what I mean. They get
really hot....not just "trickle" of heat

Just can't work out what this is happening. If I thought it was only a
bit of poor plumbing I'd live with it by turning off the appropriate
rads, but it worries me that the heat is coming into the rads via the
return rather than the main how feed. I'm concerned there may be a
blockage somewhere and the radiator loop is acting as the escape route,
but that said, surely the hw thermostat would switch off the main
system?

Any ideas...??

J