A smart thermostat lets you control your heating from your phone, learns your routine, and can hold back the heat when nobody is home. It is genuinely useful, but it is worth being clear-eyed about where the savings come from, because the device does not save energy by itself.
What it actually does
At heart a smart thermostat does what a good programmable thermostat has always done, matching heat to when you are in, but it makes that easy and even automatic. It can sense when the house is empty, learn how long your home takes to warm up so it fires the boiler at the right moment, and let you switch the heating off from the bus when you realise you left it on. The convenience is real and removes the friction that stops people bothering with timers.
Where the saving really comes from
The honest truth is that the saving comes from heating your home less and at better times, which you can do for nothing with the existing programmer and a little discipline. What the smart device adds is making that discipline effortless and automatic, so households that would never reliably fiddle with a timer end up heating an empty house far less. If you already run a tight, well-timed schedule by hand, a smart model will save you little.
Features worth having
If you do buy one, the features that matter are presence detection or geofencing, so it holds back heat when everyone is out, and individual room control through smart radiator valves, so you heat the rooms in use rather than the whole house. Learning algorithms and weather compensation are nice but secondary. Flashy app graphics are marketing; zoning and occupancy sensing are where the money is saved.
Who benefits most
The best candidates are households with irregular routines, people out at unpredictable hours, and homes that are often half empty, because there is a lot of wasted heating for the device to catch. A retired couple at home all day on a fixed routine has far less to gain. Weigh the cost against your own pattern, and remember that the free habit changes on the thermostat settings page deliver much of the same benefit with no kit at all.