When saying things like this, it is better to contextualize by giving some information about where you live. Some people live in areas where temperatures go as low as -20~30 C and not leaving the heating on can damage the house infrastructure. In other areas, where the temperatures only go down to -2~3C, you can get away with never turning the heating on.

Who doesn't? Near zero temperatures outside mean that it gets cold awfully quickly and with sub-zero temperatures there is lot of risks with pipes and so on.

Sure but you just set the heating to 12C or something. There's no need to leave it fully on at 20C.

If you have floor heating + heat pump, then the manufacturer recommendation is to keep it running 24/7 from October to April. That's for the climate where I live, elsewhere this period may be shorter.

Why though? Is it to not have disappointed customers because it takes 4 hours to heat up and it's harder to explain to people how to set a timer to turn on at 3am, or because keeping a higher temperature delta between indoors and outdoors is actually more efficient with this system?

I don't know all the details and reasons, because I don't have enough knowledge about heat pumps. But the explanation given was that takes many hours for the whole system to 'warm up' and start working within optimal range. Turning it on and off frequently would actually make it perform worse, wasting more energy than if it's running all the time.

I'd wager it's to prevent damage if the stuff inside the pipes freezes/expands.

Floor heating won't go from heating temperatures (30°C? I don't know exactly) to freezing across 8 hours of sleep, even if it's -25°C outside.

The advice also doesn't have to be "turn it off", just "turn it down to 5°C" which would be safe indefinitely. There has to be another reason why the advice is to keep heating 24/7.

The controls available aren't in the sense of thermostats - for example, you can't ask it to keep temperature at 21 degrees during the day and 18 degrees during the night. Instead, it has something called 'heating profiles'. There are summer (passive cooling, essentially) and winter profile (heating). And for the winter profile you can set the level of heating: from -5 to +5 degrees compared to what it considers optimal. I have mine always set to -5, because even at that setting it's warm enough. Note that 'degrees' in this instance is not the air temperature in the rooms, but I believe the water temperature in the pipes.

In other words, it will automatically adjust depending on the time of day, temperature outside, etc. It is possible to influence how it works, via 'administrator' settings meant for the repair guys, but I haven't touched those.

Well that just sounds like a badly designed heating system then.

I have radiant floor heating, and setting the thermostat to my target temperature 3 hours before wake-up is fine.

But note that the OP has not just radiant floors, but also a heat pump (which isn’t using gas, so we’re far afield from the overall question, but setting that aside).

Heat pump systems are generally sized so that compressor’s sweet spot is pretty close to your expected normal/max output.

To be honest, this is more about the lifespan of the compressor than it is energy efficiency, but there is also a small loss of efficiency if you get the system into a state where it’s turning on/off frequently.

So it seems plausible and sensible that a radiant-heat pump system would recommend circulating 24/7 in the winter rather than cycling on/off a lot.

I don't know. It's not what I was used to before in my previous homes (thermostats in different rooms, specific temperature settings, etc.), but you know what? It keeps me warm and is cheapest heating I've ever had to pay for, by far.

My house has district heating, feeding into floor.. turning heat up/down takes effect many hours later.

House is also well insulated, with heat recycling ventilation. Objective is to keep a consistent temperature.

Every places I lived at in Germany the heating system is turned off at night for the entire building. Same in Switzerland. I always assumed that was the norm?

Not off. Throttled down.

Thanks for correcting, I just checked and you seem to be right

Many buildings hear use electricity for heating, electricity is cheaper at night. We have heaters that heat some kind of ceramic material and keeps releasing the heat during all the day.

Who doesn't? I keep my house at 23C. The cost is negligible. I don't want to wake up to pee and be shivering in the bathroom.

I do (suburbs of Seattle), otherwise my house would not stay above 17C overnight, especially with the last few nights we had.

Is there some reason why that's an issue if you're in bed? I live in the UK and well we don't put the heating on at night ever. Doesn't matter if it's a snowstorm or below freezing outside. Heating comes on first thing in the morning, but overnight you'd be pretty much fine in bed.

Yeah, you don't want to set the thermostat too low to protect the outdoor faucets (not all of them are freeze-proof, especially in the places where temps below freezing are uncommon). I also don't want to wait for 2hrs in the morning for the house to reach comfortable temperature, and would like to be able to go to the bathroom in the night w/o it being too cold.

Edit: is it more common to use forced-air heating where you live? Are buildings brick/concrete? Heating vía radiators w/ warm water and concrete buildings tend to have more inertia in my experience, the temp will not drop down that much overnight, but that's not the case in my current home.

Yeah, and that's the reason Irland and the UK is full of moldy shitholes.

Everytime I watched the housing offerings in Dublin I wondered how any sane being would spend upwards 1M£ for a small, moldy goat pen while parking their Porsche infront.

Isn't mold more to do with not airing out the house to let moisture out?