What is the difference between a programmer, a room thermostat and a TRV?

Almost every heating complaint that turns out not to be a fault comes from misunderstanding this chain. Once the order is clear, most odd behaviour explains itself.

Step by step

1. Identify your programmer or timer.Usually a box near the boiler or in a hallway, with an on-off schedule and often separate heating and hot water channels. Outside its on periods, nothing else can bring the heating on.
2. Identify your room thermostat.A dial or display on a wall, typically in a hall or living room. During a programmed on period, it compares that room's temperature with its setting and tells the boiler to fire or not.
3. Identify your TRVs.Numbered heads on the radiators. Each one throttles its own radiator when its room reaches the set temperature. They do not talk to the boiler at all in a conventional system.
4. Read the chain in order.Programmer, then room thermostat, then TRV. Heat only reaches a radiator if all three allow it. That is the entire logic.
5. Work out what a cold room means.If every room is cold, look at the programmer and the room thermostat. If one room is cold and the rest are warm, look at that room's TRV and the balancing.
6. Understand why turning a TRV up does nothing sometimes.Outside programmed hours, or when the room thermostat is satisfied, the boiler is off. A wide-open TRV on a system with no hot water in it changes nothing.
7. Set each one to do its own job.Programmer for the hours you are in. Room thermostat for the temperature you want in the reference room, typically 18 to 21 degrees. TRVs to trim individual rooms up or down from that.
8. Keep the reference room's radiator unrestricted.The radiator in the room with the thermostat should have no TRV, or one left fully open, or the two controls fight.
9. Recognise combined and smart controls.A smart thermostat often combines programmer and room thermostat in one device. Some smart systems also let TRVs signal demand back to the boiler, which changes the chain — follow that system's own guidance.
10. Check the hot water channel separately.On a system with a cylinder, hot water has its own schedule and its own cylinder thermostat. Heating being off does not mean hot water is.

Tips

• Cold house, all rooms: programmer or room thermostat. Cold house, one room: TRV or balancing. That one distinction solves most complaints.
• Set the room thermostat to the temperature you want and leave it. Turning it to 30 does not heat the house faster.
• Label your programmer's channels if it is not obvious which is heating and which is hot water.

Common mistakes

• Turning the room thermostat right up to warm the house faster — The boiler is either on or off; it does not modulate to the thermostat setting in that way. It heats at the same rate and then overshoots, wasting fuel and overheating the reference room.
• Using TRVs as an on-off switch for the heating — TRVs cannot bring heat on. They can only reduce what a radiator takes while the boiler is already running, which is why adjusting them outside programmed hours seems to do nothing.

If it doesn't work

Questions people ask