Darned that control system

[RP61973]

I have a problem that has just sprung to light.

I have a traditional wet central heating system with a pumped rad circuit and a gravity hot water circuit. I am running a Honeywell ST699B controller and a thermostat in the lounge. The radiators have started operating even if the heating control on the ST699B is turned to 'off'. The only way to prevent them running is to turn the thermostat down. I think it is a 3 way valve problem? Any ideas?

 

[twgas]

I've never seen a gravity system with a 3-port valve, I think you mean a tank fed (hence the gravity bit) or a vented system. I think your 3-port may be letting-by

 

[RP61973]

Mick, many thanks. Yes you are right, tank fed. Sorry for the pigeon English. I suppose it has not done badly given it is believed to be around 30 years old now (all fed by a Mexico Stelrad). I just need to haul my butt into the loft and scribble down the valve part no......