Determining Your Existing Wiring And Equipment - Johnson Controls T9100 Owner's Manual

Installation Instructions
Before you go any further, determine what your
existing wiring and equipment situation is.
If you have a Heating only system without Air Conditioning, the JCI
A.
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 JCI 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 JCI 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 72.
If there are only 4 wires present that are connected to the existing
thermostat, there are at least 2 options available to connect the JCI
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.
C. If you have a multi-stage HVAC system comprised of a fossil fuel
heater with air conditioning, the JCI 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 74, or use the optional Add-A-Wire accessory.
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