Most homes heat every room to the same temperature whether anyone uses it or not, which is a quiet waste in spare bedrooms, hallways and the rooms you barely set foot in. Thermostatic radiator valves let you turn the heat down room by room, and used well they trim the bill without any loss of comfort where it counts.
What a thermostatic valve does
A thermostatic radiator valve, the numbered dial on the end of a radiator, senses the temperature of the room and throttles the flow of hot water to that radiator once the room reaches the set level. Unlike the old on-or-off valves, it holds a room at roughly the temperature you choose, so a setting of two or three keeps a room cooler than a setting of five. It is local control on top of the whole-house control of the main thermostat.
Why zoning saves
There is no value in heating a guest room, a utility or a rarely used dining room to full living-room warmth. Turning their valves down to a low frost-protection setting means those radiators barely come on, so the boiler has less work to do and less heat is wasted on empty space. You concentrate the warmth, and the gas, on the rooms where you actually sit.
Setting them sensibly
Set the valves higher in the rooms you live in and lower in those you do not, and leave the room with the main wall thermostat on a high or fully open valve so the two controls do not fight each other. Bedrooms can usually sit a notch cooler than living areas for better sleep. A low setting rather than fully off keeps unused rooms above the point where damp and frozen pipes become a risk.
The two controls together
The wall thermostat decides when the whole system runs and to what temperature the main room reaches; the valves decide how much of that heat each other room gets. Use them as a pair: time the system to your day, set the main thermostat to the lowest comfortable level, and let the valves tail off the rooms you are not in. That combination is the cheapest zoning most homes can manage, and it needs no smart kit at all.