I only have 4 wires for my Wiser Air thermostat - how do I connect it?
Customer has existing thermostat that does not include a common wire.
Wiring practices for low voltage cable are not standardized
The Wiser Air thermostat is delivered with a device labelled as a "Wire Extender". This device will generate the necessary common return path to properly power the Wiser Air thermostat.
A wire extender should be installed in the equipment providing power to the thermostat. Professional assistance may be required.
The attached file has detailed instructions for installing this device on pages 11-15.