upstairs rad on (when heating off)

[Jennie]

Hi everyone,
My customer has an unvented cylinder, with two two-port valves.
At night, she's noticed her bedroom rad (the rad nearest the cylinder) stays on when heating is turned off. The hot water in the cylinder is the correct temperature (i.e., not overheating, so cylinder stat and two-port valve must be OK). Central heating is otherwise OK - all rads get hot.
I'm puzzling over it.
If when the hot water was being heated, the central heating 2-port valve was letting by, surely the pump would distribute the heat to all the rads, not just one or two. So I'm not sure it's the hw valve at fault.
The only reason I can think of is that the boiler is producing too much heat - maybe it isn't turning off properly when both hw and ch demand are off? And the excess heat is going up the return pipe to the rad?
Any ideas anyone?
Jennie.

 

[Ant Parkes]

Let it go cold, call for just hot water and feel the pipe just after the S plan valve to radiators, see if it starts to rise in temperature.

Has it always been a hot radiator or is it a fault only recently? Could it have been a radiator they used as a bypass so will come on with both hot water and central heating demand from boiler?

 

[Chester]

reverse circulation possibly?

 

[Ant Parkes]

reverse circulation possibly?

Another possibility yes. Check the 3T rule is followed.

 

[Jennie]

Thanks everyone. I hadn't thought about the 3Ts. That gives me a bit more to go on.

 

[Chuck]

It's possible that there is an unintended convection loop from the HW cylinder coil to the radiator and back. There should be a valve somewhere in the loop to stop this but it may be missing or letting by. Not uncommon if it was once a vented systems with a bypass radiator that's been modernised incorrectly. It's more usual IME for a bathroom heated towel-rail to have been used as the bypass.