I'm installing a Honewell zone control system (Honeywell HZ311K 1 Heat / 1 Cool TrueZONE Zoning Kit). The kit comes with a low volt transformer, but my existing furnace has an onboard transformer wired into it. Do I need to install the second transformer just to be able to run the control panel and the motorized dampers... OR can I use the existing transfomer that's already built into the system?
Thanks

Generally the zoning systems need their own power supply.

The furnace usually has a tranformer rated for no more than 40 Watts (VA or Volt-Amps). You would have to determine the size of all the components operating at the same time to really decide if you can use it. It's best to keep the power supplies separate anyway.