A thermostat should run the system. Most are run by an app.

A heating system runs best when it is on for longer at lower flow temperatures rather than short and hot. A good thermostat keeps it there by reading the building and the outside conditions and asking the boiler or heat pump to modulate accordingly. That is the unglamorous job most thermostats are bought to do, and most do badly.

• Modulate the heat source rather than cycle it on and off.
• Compensate for outside temperature in real time.
• Run per-room schedules where the architecture allows it.
• Hand control back to the OEM controller when the OEM controller does the job better.

On a modern heat pump the manufacturer's own controller, Mitsubishi MELCloud, Vaillant SensoHome, Daikin Madoka and equivalents, generally outperforms third-party thermostats on weather compensation and modulation. Use the OEM controller as the brain of the system and layer per-room TRVs underneath it where individual rooms need their own schedule. Replacing the OEM controller with a third-party thermostat is usually a downgrade dressed as an upgrade.

Tado, Drayton Wiser, Honeywell evohome and Netatmo all do per-room zoning with TRVs at broadly similar quality. Pick on ecosystem fit and how the schedule editor feels to live with rather than on the marketing. Hive and Nest are well-marketed but limited on proper per-room zoning, so avoid them when zoning is the goal.