An ADD-A-WIRE device can be used when upgrading to thermostats that need a "COMMON" wire from one that didn't have one... and there is no way to run the additional wire needed. This device, ranging from $15 to $40 can come in very handy especially in the case of zone control where one or more thermostat does not have enough wires to accomodate a modern thermostat such as a WiFi thermostat.
Be sure to follow the wiring diagram for the specific model of "ADD-A-WIRE" as well as the system type. Show here is the LUX Products LPB-WH Power Bridge wired to a GAS/ELECTRIC thermostat and SmartZone-4X controller, however, the basic principal is the same for any application.
All of the different ADD-A-WIRE models come with instructions, read them carefully and follow exactly and you won't have any problems.
For more information about the method of ADD-A-WIRE when there is no way to run an extra wire from your zone panel to your equipment, check out this post:
https://zoningsupply.com/blog/upgrading-thermostats-solving-a-common-problem
From an engineering perspective this is an elegant and simple solution. Without giving away trade secrets, the ones i have tested use the a half wave bridge using diodes at the thermostat and some simple relay logic plus the another half wave at the equipment (no micro necessary) to use only 1 wire for both Y and G signals.... brilliant.
For more information, or with any questions, please contact us.