Changing the clocks this morning reminded me of the issue I had with an underfloor heating controller when I first moved to this house during the very cold two winters ago.
The combined kitchen/conservatory has underfloor electric heating with a Devireg 550 controller/thermostat. The rest of the house is gas heated. Quoting the Devireg manual : "[it] works dynamically .. to make heating your home as economical and environmentally friendly as possible .... Once it has been installed for a few days it will fully optimise .. [blah blah] .. constantly improving its performance [etc etc]". (Why does the phrase "envonmentally friendly" ring warning bells in my head these days? ).
I had set the controller to come on at 6-8 am and a couple of other two hour period during the day.
After a few weeks, my first electricty bill showed huge usage. I could not understand where it was going - far more than I thought even this underfloor could consume, even accepting that a combined kitchen/conservatory is a very bad idea (latter is a heat sink). It would have needed to be running nearly the whole time.
Then one night I had occasion to go down to the kitchen at about 2am. Outside was 8-10" of snow frozen hard at about -10C (I live in the hills). Yet the kitchen was warm, hours before I had asked it to come on! I realised that the "smart" controller was trying to reach 18C at 6am, the requested settings, and had calculated that it would never do it, but was nevertheless struggling up a logarithmic curve towards the target by running flat out for hours beforehand. Stupid.
I soon fitted a programmable but dumb timer in series before the "smart" Devireg, which I reprogrammed to be just a thermostat. It was like neutralising Preston in "A Close Shave".
The wonder is that the previous owners must have been going on like this for years (the neighbour said they had complained about their bills), pumping something like 5 kW into the sky from an unoccupied room for most of every winter night. Yet people worry about their mobile phones being "green"!