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The power is supplied by a transformer, and when the thermostat makes contact between the 24 volt power and one or two of the other wires, a relay back at the heating/cooling unit activates the corresponding heat/fan/cool function of the unit(s).
Where ever a thermostat is required, communication and control wiring and in some cases an addition power circuit either a control voltage such as 24 volts or line voltage of 120 volts must be installed. Adding this power source may require the installation of a control transformer to convert 120 volts to 24 volts.n a VAV application, the VAV boxes (assuming these are not replaced) will need a new control module for the heating coils, dampers, and fans. There will also be the requirement to replace the temperature and air flow sensors for each box.