Do smart thermostats actually save money?
Sometimes, and considerably less than the marketing claims — the savings depend almost entirely on what you were doing before, and a household already heating efficiently has little left to gain.
Where genuine savings come from:
Not heating an empty house, through scheduling or presence detection. This is the largest single effect and is available from any programmable thermostat.
Zoning, heating only occupied rooms, which requires smart radiator valves as well and is where multi-room systems earn their cost.
Weather compensation and learning, adjusting flow temperature and start times to conditions rather than running to a fixed schedule.
Visibility. Seeing consumption changes behaviour — modestly, and with the effect fading over time, which is the consistent finding across energy feedback research.
Remote control, which mainly prevents the specific waste of heating an empty house when plans change.
Where the claims overstate it. Manufacturer figures are typically derived from households upgrading from no controls at all or from very poor habits. Independent trials have generally found savings well below headline claims, and some find little difference against a well-used conventional programmer. A thermostat cannot save energy that a competent schedule was already saving.
What produces far larger savings, in rough order: insulation, particularly loft and cavity walls; draught-proofing, which is cheap and underrated; lowering the boiler flow temperature on a condensing boiler so it actually condenses, which is free and frequently saves several per cent; hydraulic balancing of radiators; and reducing the room thermostat by a degree.
Where smart controls matter more than they used to: with a heat pump, where run strategy materially affects efficiency, and with time-of-use tariffs, where shifting heating to cheap periods and using the building's thermal mass can save real money. This is the case that is strengthening.
Practical advice: fix the schedule and the flow temperature first, which costs nothing; then consider smart controls if you have irregular occupancy, a heat pump, or a variable tariff.