Drayton programmable thermostat major problem

[freddyuk]

I have this unit controlling my underfloor heating. It has recently been taking the temperature to in excess of 23F when it is set for 19F.
The thermostat is not calling for heat (no flame showing) but the zone valve is opening.
I have not altered the wiring recently and it has been working fine for over two years. It only happens on certain days.
I have checked every day for each time period and all is as it should be.

I have replaced this unit before as the last one just packed up and was replaced by Drayton.

Is this possible ? There is no other control to affect the valve except the system programmer which is "on" all the time so the room stat does the work.

 

[Elkato531]

Is there a frost stat wired into the system somewhere which could be activating the zone valve?

 

[freddyuk]

No frost stat.

 

[Agile]

If it has a relay in the unit then it could be sticking.

They are often magnetic latching types which are just pulsed to change over.

Assuming its battery operated has it got recent ALKALINE batteries fitted?

Is it directly driving higher powered devices like a pump? If so that may be outside its ratings and causing relay contacts to stick together.

Tony