Rads heat up when HW only selected

[nekomatic]

I've got a fully pumped system with a conventional (non-combi, non-condensing) boiler and indirect hot water cylinder with cylinder thermostat. For some time now the radiators have been heating up when only HW is selected on the programmer; also, I've noticed that the boiler tends to cycle on and off rapidly.

I suspected the motorised valve (Honeywell V4073) but even though the radiators get hot, the CH pipe coming from the valve is cold! (inlet and HW outlet are hot as they should be.) Any suggestions please?

 

[gas4you]

As you say first suspect would be the Honeywell valve. Other than that it could be some strange back flow through the system by pass. You need to check the valve properly, microswitch could be knackered and telling boiler to come on constantly.

 

[nekomatic]

Thanks for the advice. I checked the voltages going to the valve and when HW only is selected there is about 180 V ac on grey, nothing on white and 240 V on orange which is supposed to be an output from the valve! So I'm now thinking that the fault might be in the cylinder thermostat. It's a Sunvic clamp-on type and the wires are earth, brown, blue and black. Brown joins to a wire coming from downstairs (I assume from the programmer), blue to another from the programmer and to grey from the valve, and black from the thermostat goes to orange on the valve (and again to another wire from the programmer). Can anyone tell me how the thermostat should behave, so I can check out whether that's the problem?

That doesn't solve the mystery of how the hot water is getting to the radiators without going through the valve though. Where would I start looking for a bypass?