How to Wire a low voltage thermostat for a 220v baseboard heater?

You will need a relay with a low-voltage coil and 220v contacts rated for the current load of the baseboard heater. If you have a low-voltage transformer, see what its output is rated at (typically 24 volts or less). Then go to an electronics parts store or call some place like Digikey (phone 800-344-4539) and tell them you need (for example) a double-pole single-throw normally-open relay with a 24 volt coil and contacts rated at 30 amps at 240 volts (change these numbers to match your situation). If you do not already have a low-voltage transformer the parts store call sell you one to match the relay coil voltage (be sure to specify if you are connecting the transformer input to 120 volts or 240 volts). Then wire it up as follows: connect the transformer output (24 volts in the example above) in series with the thermostat contacts and the relay coil. Connect the 240 volt supply to the moving arms on the relay, and connect the contacts the arms touch when the relay operates to your heater. Now when the temperature drops below its setting the thermostat contacts will close and connect the relay coil to the 24 volts, the relay will operate, and the 240 volts will be connected to the heater. When the temperature rises above the thermostat's setting, its contacts open, the relay contacts open, and the heater is disconnected. Depending on where you live there may also need to be circuit breakers, protective ground wires, etc.--ask an electrician.