Venstar T7900 - Manual (Page 80)

72
Before you go any further, determine
what your existing wiring and equipment
situation is.
A. If you have a Heating only system without Air Conditioning, the
Venstar thermostat will require 3 wires: R (24Vac), C (24Vac) and
W (Heat). Most systems that only have Heating use very simple
thermostats that require 2 wires: the R (24Vac) and W (Heat). The
Venstar thermostat requires 3 wires to the thermostat. In this case an
Add-a-Wire accessory will not work and it will be necessary to install
another wire for the C (24Vac) connection.
B. If you have a single stage fossil fuel heater with air conditioning,
the Venstar model will require 5 wires for independent fan control.
They are R (24Vac), C (24Vac), W (Heat), Y (Cooling), and G (Fan). You
may connect only 4 wires, as instructed in the “Making 4 Wires Work
When 5 Wires Are Required” section on page 74.
If there are only 4 wires present that are connected to the existing
thermostat, there are at least 3 options available to connect the
Venstar thermostat:
1. Use the 4 wires as instructed in the“Making 4 Wires Work
When 5 Wires Are Required” section on page 74, and note that
the fan will only operate with a Heating or Cooling demand.
2. Pull new thermostat wire from the HVAC equipment to the
thermostat so that there are at least 5 wires available.
3. Purchase and install a Venstar Add-A-Wire accessory.
C. If you have a multi-stage HVAC system comprised of a fossil fuel
heater with air conditioning, the Venstar thermostat will require the 5
wires mentioned above (R, C, W, Y, G) plus an additional wire for each
additional stage of Heating or Cooling. You may reduce the 5 wire
requirement to 4 if you give up independent fan control following the
instruction in the “Making 4 Wires Work When 5 Wires Are Required”
section on page 53, or use the optional Add-A-Wire accessory.
Installation Instructions