Heating strategy - constant or intermittent?

What I found is that to maintain - say - 18C under occupancy it was almost impossible to switch off before leaving and needed hours before coming home to reach that, AND it always overshot a degree or so anyway..the savings on having it on low, and using something else for faster heating, of specific rooms outweighed the costs of having it try and keep the place the right temp when actually needed.

The more mass inside the insulation, the less a 'heat when needed' solution works. Conversely you don't need massive aircon in summer either.

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Overnight last night when it was particularly cold my thermostat didn't quite trigger the heating during the night time phase but it still dropped a bit over 7 degrees C in the 6 hours before the early morning heating kicked in and it took more than 12 hours to get within spitting distance of the evening set temperature. I have found in the past that when the heating has unexpectedly turned itself off (or down) I begin to feel uncomfortable before the temperature has dropped as much as 2 degrees. It took 45 minutes to drop 2 degrees last night when the thermostat switched to the night time phase.

ISTR you have underfloor heating where the lag is much greater than with a radiator based system.

I have plenty of mass but insulation is seriously lacking and what little there is is immediately under the slates. It takes a prolonged heat spell in summer to make my house uncomfortable and I don't take kindly to excessive heat.