If each radiator calls for hot water when it needs it, won’t this be often and at different times, and therefore be a roughly similar energy cost to doing it less frequently but all at once?
In theory, the smart valve allows for better control of temperature in each room, i.e. 20c rather than position 3.
You would still have the heating on a timer, e.g. between 6am and 8am, and then each radiator will call for heat during that time the boiler will fire.
It will vary from system to system, I have a Tado system (without smart TRVs) but you still set a schedule or times for the desired temperatures.
With Smart TRVs you could only heat the rooms needed in the morning and heat different rooms in the evening.
It allows for better control which should be more efficient rather than heating the whole house or running around turning rads on and off throughout the day
Imagine that you have two rooms, one of which loses heat quickly and another that holds it well.
A normal radiator system would mean that if the hot water is circulating, it’s adding heat to both rooms at the same rate. But one room doesn’t need heat, so that’s wasted. The higher the temperature difference to outside, the faster that will lose heat, so that warmer room is losing heat faster.
Now, imagine that you have a generally well insulated house, but you have one room with big windows. It loses heat fastest. Your system now heats the ENTIRE house to get that one room to temperature, which is a huge waste. You only want that one room heated.
Keeping the entire house warm all the time takes more energy than keeping certain rooms warm when you need them.
Less heat in the house means less heat can escape, meaning you need to burn less fuel to keep the rooms warm.
Smart valves let you heat less of the house at a time
For instance on a weekday we heat the bedrooms to comfort level in the morning and evening, but have them fairly low in the middle of the day.
In short it’s efficient because the other radiators can stay ‘off’.
That said it’s not as much of a saving as you might think – a lot of boilers are not that efficient when constantly running far below their max output and you might be better off splitting your house into larger zones rather than per-room
My understanding, and I’m not an engineer, is that it’s not to save energy but rather to keep a consistent on-demand flow of hot water. It’s “smart” in a sense that you won’t run out of hot water halfway through your shower