Fitting a frost thermostat

[Gilray]

My boiler is housed in an unheated outbuilding so i want to fit a frost thermostat. I guess the correct place to connect it is probably to the white wire on the mid position valve but I dont want to rip up all the carpets and floorboards in a bedroom and a landing to get the wire back to this point.

I was thinking of connecting the Frost stat output to the yellow demand line to the boiler but this will connect AC to the mid position valve motor and bypass the contacts that control the position of the valve potentially causing it to operate incorrectly.

I thought a solution would be to fit a relay with a N/O contact in series with the output of the mid position valve with the coil operated by the white wire. Hence the valve o/p will be isolated when normal central heating controls mean C/H is off, and if the Frost thermostat turns on it wil call the boiler without affecting the valve. If the C/H is on and Frost thermostat operates the valve will still operate as normal.

Has anyone got any thought on this idea or done this themselves?

 

[SimonG]

Don't forget your pipe stat on the return pipe to shut it all down once warm or it will keep going util the frost stat has warmed up.