US Patents 7,922,100; 7,737,762
Auxiliary Switch Installation Page 1 AW000745-C
AS20 Auxiliary Switch Installation Guide
Missing Common Wire on Single transformer HVAC System
The Auxiliary Switch facilitates the installation of a thermostat
where a common wire connection from the heating and/or
cooling equipment to the thermostat does not exist.
The single transformer instructions on this page should be
used with a Heat-
only HVAC installation or an integrated
Heat/Cool installation using a single transformer servicing
both Heat and Cool parts of the equipment. For installations
with separate Heating and Cooling equipment, each with its
own transformer, please refer to the instructions on Page 2.
The Auxiliary Switch is installed on or close to the equipment
so that the five wire cable reaches the control terminals inside.
The Auxiliary Switch should not be installed inside the
HVAC equipment enclosure (doing so may void the HVAC UL
certification).
Before Starting the Installation:
Check that the package includes 1 Auxiliary Switch Unit, 1
signal coupler and 5 Splice Wire Caps.
Turn off the power to the furnace, air conditioning and/or
heat pump, either at the shut-
electrical panel.
Thermostat site:
Step 1 - Remove the old thermostat from the backplate. If
there is no backplate then remove the thermostat from the
wall to access the wiring. Next, label the wires according to
which terminal block they connect to (R,Y,W,G). Then
disconnect the wires and remove the existing backplate.
Step 2 – Mount the new backplate and connect the wire
marked R to the R terminal block. Also connect the wires
labelled W and Y to the corresponding terminal blocks.
Step 3 – Install the signal coupler provided with the Auxiliary
Switch between G and C terminal blocks on the backplate.
Twist the signal coupler wire with the wire labelled G to
ensure that it is held securely.
Connect the wire labelled G to the C terminal block. Any
remaining wires should be connected as per their installation
instructions.
Step 4 – Make note of the color and/or labels on the wires
attached to each of the terminal blocks. Attach the thermostat
to the backplate.
Equipment site:
Step 5 – In the equipment area, find a convenient location to
mount the Auxiliary Switch. The location should be outside
the equipment but close enough so that the five wire cable
reaches the thermostat connections inside. Mount the
Auxiliary Switch using 2 screws or adhesive pad provided.
Step 6 – Before disconnecting wires at the equipment, label
them according to which equipment terminal block they are
connected, verifying that these labels match the labels you
attached in step 1.
Step 7 – Pass the five wire cable through a convenient access
hole in the equipment.
Step 8 – The wire labelled G must be spliced to the Auxiliary
Switch cable using a wire cap as follows:
• Wire labelled G to the Auxiliary Switch WHITE wire.
Step 9 – Connect the blue and green wires from the Auxiliary
Switch to the equipment as follows:
• BLUE wire to the equipment C terminal.
• GREEN wire to the equipment G terminal.
Step 10 – Connect the remaining two wires from the Auxiliary
Switch and the wire labelled R from the thermostat as follows:
• Auxiliary Switch YELLOW and RED wires and the wire
labelled R to the equipment R terminal.
Step 11 – Restore power to the equipment and follow the
equipment configuration instructions in the thermostat user
manual for testing.
Step 12 - Record the serial number of the unit, name and
address of the owner, and make and model of the heating
and/or cooling equipment. Install the Connection Diagram
sticker (AW000785) on a visible place on the HVAC cabinet or
on the wall close to the Auxiliary Switch.
Auxiliary
Switch
Single Transformer
Connection Schematic
WHITE
YELLOW
RED
GREEN
BLUE
R
G
W
Y
C
R
G
W
Y
C
Thermostat
HVAC Equipment
Signal Coupler
Legend
R – 24VAC
C – Common
W – Heating
Y – Cooling
G – Fan
Example for single stage
Heat/Cool system.
G
R
W
Y
Labels per steps 1 and 6.
Note: If the old thermostat has no connection to G, but
instead to W, then connect the W wire to C at the thermostat
and the signal coupler between W and C at the thermostat
location. At the equipment location, connect the Auxiliary
Switch green wire to W instead of G at the equipment.
Connect the Auxiliary Switch White wire to the W wire with a
wire cap at the equipment location.
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