What a room thermostat, TRV, programmer and heating zone each control, and how they cut wasted heat.
Room thermostat
A room thermostat senses the air temperature in a typical part of its heating zone. An ordinary on/off thermostat stops calling for heat when the room is warm enough and calls again as it cools. Other zones or the hot water may still need heat, and the boiler may keep its pump running for a while (pump overrun). More advanced controls vary the demand instead of simply switching on and off.
Turning a room thermostat up does not make the room warm any faster. How quickly a room heats depends on the system, such as the size of the boiler and radiators; a higher setting only holds the room at a higher temperature and uses more energy. Honeywell's guidance for its T6360 suggests finding the lowest comfortable setting and leaving it alone. Its suggested starting point is about 18 °C, increased by one degree each day until comfortable, then left at that setting. The thermostat needs a free flow of room air, so it should not be covered by curtains or blocked by furniture, and a nearby electric fire, television or lamp can upset it.
Thermostatic radiator valve (TRV)
An ordinary mechanical TRV senses the temperature near its head and cuts the flow through its radiator as the room warms. It cannot tell the boiler to fire. Linked smart radiator controls can send a heat request through their controller, so identify which type is fitted.
In the room with the room thermostat, do not let an ordinary TRV shut the radiator off before the thermostat is satisfied. Set the two up together, as the control instructions say. Honeywell suggests that where TRVs set individual room temperatures, a slightly higher room-thermostat setting can let the coldest room reach comfort, while the TRVs limit overheating in the warmer rooms. A thermostat that keeps calling for heat does not make the burner fire non-stop; the boiler’s own controls still apply.
Programmer or timer
The timer sets when heating and hot water are allowed to run. Within those times, the temperature controls decide whether heat is needed. One controller may do both jobs, and frost protection or a deliberate override can run outside the normal times.
Zoning
Separate zones can have their own times and temperatures. That cuts heating where a lower temperature will do, while keeping background heat and frost protection. It does not make every room the same temperature, and the radiators still need balancing. Check the boiler still gets the flow and the load it needs.
Siting a weather-compensation sensor
Weather compensation lowers the flow temperature as the weather gets milder, so its outdoor sensor must read the true outside air temperature. Baxi's sensor instructions, for example, ask for an outside wall of the property facing north to west, never a south-facing wall in direct sun. The sensor goes at about half the height of the living space and at least 2.5 m above ground. Keep it away from anything that warms or cools it, such as a flue terminal, and not above doors or windows, beside vents or close to the eaves.