Things You'll Need
A T8112D1005 programmable thermostat, the Tradeline replacement model for the Honeywell T8112D, controls single-stage heating and air-conditioning systems that use a 24-volt control circuit. A T8112D1005 will not operate multistage systems, such as a heat pump. The T8112D1005's LED screen continuously shows the temperature, time and day. Its "5-2" programmable feature allows a user to create separate settings for weekdays and weekends. Normally, technicians install a Honeywell Tradeline T8112D1005 on an interior wall, near the air-conditioning system's return vent.
Turn off the circuit breaker controlling the air-conditioning system's inside unit. Typically, the inside unit's circuit breaker uses a "Furnace" or "Air Handler" label. The circuit breaker with the "Air Conditioner" usually operates the outside unit, called the condenser. If in doubt, turn off both circuit breakers.
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Pull the end of the T8112D1005's low-voltage wire set out of the wall. The wire set contains four color-coded, insulated wire strands.
Remove 1/2 inch of the insulation from the end of each low-voltage wire strand, using wire strippers. Bend the stripped end of each wire into a hook shape, using needle-nose pliers.
Push the low-voltage wires through the large hole in the center of the T8112D1005's mounting plate. Position the mounting plate against the wall. Mark the mounting plate's mounting holes, the two slots found near the sides of the plate, on the wall with a pencil. Remove the mounting plate.
Punch two holes into the wall on the pencil marks, using a Phillips screwdriver to create the holes. Slip one of the T8112D1005's wall anchors into each hole.
Insert the low-voltage wires into the large hole in the T8112D1005's mounting base. Position the mounting base against the wall, aligning the base's mounting holes with the wall anchors. Slip the thermostat's mounting screws through the plate's mounting holes and into the wall anchors. Tighten the mounting screws, using a slotted screwdriver.
Loosen the T8112D1005's wire terminal screws, using a slotted screwdriver. The letter next to each terminal identifies its function.
Position the red-colored, low-voltage wire's hook on the T8112D1005's terminal screw with the "R" label. Tighten the terminal screw. The red wire powers the T8112D1005.
Slip the green-colored, low-voltage wire's hook under the T8112D1005's terminal screw with the "G" label. Tighten the terminal screw. The green wire sends power from the thermostat to the furnace's fan relay.
Slide the hook on the yellow-colored, low-voltage wire around the T8112D1005's "Y" terminal screw. Tighten the terminal screw. When activated, the yellow wire turns on the cooling circuit's relay or contactor.
Position the white-colored, low-voltage wire's hook under the T8112D1005's "W" terminal screw. Tighten the terminal screw. The white wire controls the furnace's heating circuit.
Push the excess low-voltage wires into the wall.
Align the tabs on the bottom of the T8112D1005's face with the tabs on the bottom of the mounting plate. Snap the top of the thermostat face against the top of the mounting plate.
Open the T8112D1005's battery door, located to the left of the LED screen and function switches. Slip a slotted screwdriver or coin into the slot found under the door. Depress the slot's latch, and lift the door off the thermostat.
Install two AA batteries in the battery slot. Close the battery door.