Can I install a smart thermostat in a house with no C-wire?

Yes. The ecobee includes a Power Extender Kit that provides C-wire power without running new wire. The Emerson Sensi works without a C-wire entirely for most systems. The Nest Thermostat uses a Nest Power Connector as an alternative.

Will a smart thermostat work with my radiator heat?

Most smart thermostats work with central steam or hot water radiator systems. They control the boiler on/off cycle. However, they cannot control individual radiator valves — for that, you need smart radiator valve controllers like Tado or Eve Thermo.

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