@Wayne-TP 

I actually use this (well as close as I can get anyway) I have a temperature sensor on the house boiler outflow and a temperature sensor on the hot water tank. If the temperature on the outflow is higher than the hot water tank then I turn on the valve to heat the hot water, if it is lower then I turn it off. (Currently I use a 40degc threshold)

I also have a temperature sensor in 2 different rooms. One room has a woodburner in it. If the temperature is higher in one room than the other then I turn on an extractor fan to blow warm air from one room to the other. (Currently I have a 21deg threshold)

I actually run my whole central heating system off a combination of temperature sensors and light switches, with rules to combine them... I'm just waiting for the ke100 to be integrated and the e200 to be released to complete the picture!