Power booster for any Omnistat2 thermostat or Omnistat RC-80B thermostat (sold separately). This optional module may be needed in retrofit applications where "HVAC common" wire is not available. This device helps you use the power-stealing device in HAI by Leviton thermostats where no common exists. Connects to Common, W, and Y terminals at furnace or air handler. Easy low voltage installation.