Heating only works when boiler is heating the water

[leelee]

I have replaced the PCB on my potterton suprema 50 as it wouldn't reset all seemed ok for a few weeks but now its cutting out again. It run ok if the thermostat on the hot water tank is at about 90 degrees but if i turn it down to say 50 degrees so that only the rads are being feed then it won't spark up. I can override this if i switch the motorised valve to manual but then the water temp goes up to about 80 degress. Both thermostats seem to be ok, i have lifted the motor off the valve and all seems free in the valve. Any ideas.

 

[oilman]

Which motorised valve is it?

 

[leelee]

Hi
It's a danfoss 3 port type. it's about 8 years old. I know that it's going to feel hot due to it sitting on top of the hot valve but if you put your hand on top of the plastic cover it is all most to hot to touch. Don't know if this has any relevance?.

 

[ollski]

had one of these today which turned out to be a faulty cylinder thermostat which was preventing the heating microswitch on the valve from making....worth a try before replacing the valve.