Older homes rarely have the common wire most smart thermostats expect, which makes the usual best-of lists misleading. This guide matches thermostats to the wiring and heating system you actually have. It covers how to check your wires before buying, how ecobee, Nest, Honeywell, Sensi and Mysa each handle a missing C-wire, and what to do with two-wire boilers, heat pumps, millivolt heaters and electric baseboards. It also looks at steam radiator homes, realistic energy savings from ENERGY STAR and independent research, and the fixes for blank screens and dropped Wi-Fi after installation. Claims link to manufacturer sources.