Usually two wires, often AC mains voltage Live and Load but some newer systems use low voltage. My own house has a thermostat that wirelessly communicates with

Usually two wires, often AC mains voltage Live and Load but some newer systems use low voltage. My own house has a thermostat that wirelessly communicates with a receiver near the gas combination boiler (water heating for hot water and radiators). The receiver simply closes an electrical contact to signal heating demand.