What temperature are you needing?
This sounds a somewhat dangerous and very inefficient heat regulating solution.
A water bath with say an AC water heating element switched on and off via a relay may be more what you want?
The water in the bath gives a high stability.
As long as the wiring is OK and you have properly accounted for any ratings in the components of the relay, then it would seem a lot safer than having a 240degreeC thing left on in your house.