Heating Controls and Thermostats Explained

What a thermostat actually does

A room thermostat is a switch, not a dial that controls how hard the boiler works. It reads the air temperature around it and tells the boiler to fire when that reading drops below your set point, then tells it to stop once it's reached it. Turning the thermostat up to 30°C to "heat the room faster" does nothing except make the room overshoot and then have to cool back down — the boiler runs at the same output either way, for longer than it needed to.

How to set it properly

Find the lowest temperature you're genuinely comfortable at, rather than defaulting to a round number. Most people land somewhere between 18°C and 21°C. If you're not sure, start around 18°C and nudge it up by half a degree every day or two until it feels right — each degree lower typically saves a meaningful amount on your heating bill over a winter, so it's worth taking the time to find your actual comfortable minimum rather than guessing high.

Where to put a thermostat

Position affects accuracy more than almost anything else about a heating system, and it's the thing most commonly done wrong in a house that's had had furniture rearranged or an extension added since the thermostat was first fitted.

• Away from radiators and other heat sources — a thermostat that's warm from a nearby radiator thinks the whole house is warm and stops the boiler early
• Out of direct sunlight, which has the same false-warm effect
• Clear of draughts from doors, windows or letterboxes, which cause the opposite problem
• Not blocked by curtains, furniture or anything else restricting airflow around it
• In a room you actually use — a thermostat in a rarely-used hallway will happily let the rest of the house run cold or hot without knowing it

Thermostat vs programmer vs TRV — the difference

These three get confused constantly, and understanding the difference is most of what you need to run an efficient system.

• Room thermostat — controls the temperature. One per system (or per zone, on a zoned system), switching the boiler on and off based on air temperature.
• Programmer or timer — controls when the heating is allowed to run at all, regardless of temperature. Set it for the hours you're actually home and awake.
• Thermostatic radiator valve (TRV) — controls an individual radiator's output, letting you run bedrooms cooler than the living room without touching the main thermostat.

The one rule worth remembering: never fit a TRV in the same room as the main room thermostat. They'll fight each other — the TRV closing the radiator while the thermostat is still calling for heat because the room hasn't reached temperature, or vice versa — and the room's temperature becomes unpredictable.

Smart thermostats — worth it?

A smart thermostat does the same core job as a standard one, with scheduling, geofencing and app control layered on top. The genuine saving comes from heating the house less when nobody's home or the schedule doesn't need it running — not from any special technology in the thermostat itself. Paired with TRVs so you're not heating empty rooms, it's a reasonable upgrade, particularly in a house with an irregular routine that a fixed weekly timer never quite matches.

Written by the engineering team at Synergy Gas & Plumbing, Gas Safe registered 594385. Last reviewed August 2026.