R
24 VAC Heating
transformer
R
24 VAC Heating
transformer
Rc
24 VAC Cooling
transformer
Rc
24 VAC Cooling
transformer
The THP9045A1098 C-wire adaptor is used on heat/cool systems when you only have four wires at the
thermostat and you need a fifth wire for a common wire. Use the K terminal in place of the Y and G terminals on
conventional or heat pump systems to provide control of the fan and the compressor through a single wire—the
unused wire then becomes your common wire. See THP9045 instructions for more information.
Ventilation is not available on all models. When the U slider is in the down position (2 wires), the U contacts are
a dry set of contacts. If your ventilation system requires 24 volts, move the U slider to the up position (1 wire).
Lower U terminal is internally jumped to the Rc terminal. In this application, you would hook up one wire from
your damper to the upper U terminal and the other to the common side of the transformer.
Setting Slider Tabs
Set R Slider Tab, see Figure 9.
Use built-in jumper (R Slider Tab) to differentiate between one or two transformer systems.
If there is only one R wire, and it is connected to the R, Rc, or RH terminal on the old thermostat, set the slider
to the up position (1 wire).
If there is one wire connected to the R terminal and one wire connected to the Rc terminal, set the slider to the
down position (2 wires).
Set U Slider Tab, see Figure 10.
Use built-in jumper (U Slider Tab) of relay to wire ventilation. Please note that ventilation is not supported on all
models.
When the U Slider Tab is in the down position (2 wires) the U contacts are a dry set of contacts.
If the ventilator is powered by the cooling transformer, move the jumper switch to the up position (1 wire). With
this switch set to 1 wire, the lower U terminal is internally jumped to the Rc terminal. In this application, hook up
one wire from the vent damper to the U terminal and the other to the common side of the cooling system
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