Only UFH in screed needs to be 24/7, not every UFH is in screed though and as such can be timed to be used just like normal CH.
Sorry I was not considering expansion and contraction, which also means the less it is switched the better, but the time between switching on, and the room getting warm.
I know with mothers house it was an hour before the floor felt warm to the touch, and likely 4 hours before the room would get warm with UFH on its own, only real way to warm the wet room was the towel rail.
I know the problem with heat pumps is the temperature differential and the less it is the better they work, and with a maximum floor temperature of around 27ºC depending on floor surface, using UFH with heat pumps is one way to increase efficiency. Also with old peoples homes the lack of any radiator which can burn the residents must be good, and in the main they are heated 24/7.
But the problem with UFH is the reaction time, so the sun shines in a bay window, and although the heating turns off, heat is still going into the room, so it over shoots, so one has to fit special glass to stop heat coming into the room, which seems to get rid of the whole point of having bay windows or conservatories.
So UFH in the main has to be selected at the design stage of the home, it is not really suitable for most homes, mainly due to the work digging out the floor and putting in all the insulation, I know with mothers at least 9" of polystyrene was put down before the floor.
But as
@Swwils says for UFH not to work, some thing is wrong with the design, in mothers case it was the room design, it had an extractor fan which caused the replacement air to be drawn from the hall, which was an area kept cool, and the UFH could not heat the cool air from the hall to the temperature required for a wet room with the amount of air being pumped outside.
Also
@DIYWell makes a good point, fans move air around and any fan even if not a heater will help extract the heat from the floor and put it into the air, and like the UFH fan heaters can deliver heat without using up loads of wall space. The kick space heaters used in kitchens are a good example.
But one needs to look at the design as a whole, how efficient are the heat recovery units for example. And are they working as they should, the type using a heat exchanger built into the pipe through the walls only blow in one direction, they rely on the rooms being sealed to get the air flowing in both directions through the heat exchanger.
Heat rises, so a double height vaulted ceiling will end up being hot at top, but cool at bottom of room, the heat recovery unit if set up correctly can help bring this warm air down, the same applies to ceiling fans, which are often reversible summer and winter.
But what we are looking at is the design, I got my degree in electrical engineering not heating and ventilation, and we call them heating and ventilation "engineers" as it is a complex subject well above the level 3 training given to plumbers, electricians, etc. It is level 5 education to be called an engineer, and this knowledge is not going to be taught on a forum.
I have made errors in my own house, I considered heating, but forgot cooling, and put the thermostat in the wrong place.
With mothers house I did not want to fit the earth mat over the electric heaters, so used cheap resistive heating with built in earth braid, rather than the better quality chemical heating cables which are self regulating, this was because it was a wet room, would not need the earth mat else where, also of course do not need earth mat with a wet system, but we have no idea of the system being used.
I have seen UFH fitted with also radiators, and the heat pump had to lift the temperature high enough for the radiators, which seems to get rid of the whole idea of using UFH to start with, some types of central heating can use the near enough engineering ideas, but not UFH and heat pumps.
