Hunter 40120 Electronic Thermostat Installation Instruction


Hunter 40120 Electronic Thermostat


For assistance with wiring or operation, our Technical Support Group is available from 8 am to 5 pm CST. they may be reached toll-free at 1-888-830-1326.

Electronic/Mechanical Thermostat

Thank you for choosing a Hunter thermostat. Your new Hunter thermostat will provide years of reliable service and year-round energy savings. Please read this manual before beginning installation and save this booklet for complete operation instructions.

This thermostat is designed to work on the following heating and cooling systems:

  • Gas – Standing Pilot Gas – Electronic Ignition
  • Gas–Fired Boilers Gas–Millivolt Systems
  • Oil – Fired Boilers
  • Oil – Fired Furnace
  • Electric Furnace

This thermostat is not designed for heat pump systems or 110/220 V baseboard heating systems. If you are unsure what kind of heating and cooling system you have, please contact a qualified HVAC Technician for assistance.

This thermostat includes two #8 slotted screws and two wall anchors for mounting. To install your new thermostat, you will need the following supplies:

  • Flat-head screwdriver
  • Small Phillips-head screwdriver
  • Hammer
  • Electric drill and 3/16” bit
  • Two fresh 1.5 Volt (AA) size alkaline batteries

NOTICE! Do not disconnect the wires from the existing thermostat before reading these instructions. The wires must be labeled prior to removal to ensure proper reconnection.


Uninstalling the existing unit

  1. Turn the system power OFF from the existing thermostat. Turn the power to the HVAC system OFF at the main power panel or at the furnace.
  2. R remove the existing thermostat cover to access the wires from the wall. (Some thermostats may have multiple covers, screws or other locking devices that must be removed or disengaged.)
  3. L locate, but do not disconnect the wires. (If wires are not visible, they may be connected to the back of the wall plate. Some models may have doors that open to expose the wires and mounting screws.)

If your existing thermostat is marked  label the wire with this sticker


See page 27 for a complete wiring diagram.

  • Using the provided stickers, label each wire according to the chart.(If the terminals are not labeled, contact a qualified HVAC technician.)
    Note: Wire colors do not always comply with standards, so wire colors should be ignored. Refer to the existing terminal designation for proper identification.Hunter-40120-Electronic-Thermostat-Installation-Instruction-fig-3
  • If a wire marked C is present, C may be a Common wire and should not be used. If you have a wire marked C, do not connect it to any terminal. This wire is used only for non-battery-powered thermostats.
  • Do not let the wires slip back into the wall during disconnection. You may want to secure the wires to the wall as you disconnect them. After all wires are labeled, disconnect each wire and remove the existing wall plate.


  1. Remove the new thermostat from the back plate by pulling the two halves apart.Hunter-40120-Electronic-Thermostat-Installation-Instruction-fig-4Hunter-40120-Electronic-Thermostat-Installation-Instruction-fig-5
  2. Position the new wall plate on the wall and pull the labeled wires through the opening.
  3. If the existing holes do not match those on the Hunter wall plate, or if there are no existing holes, level the wall plate and mark the wall for two holes. remove the wall plate and drill two 3/16” holes where marked.
  4. Tap the plastic anchors into the holes until they are flush with the wall.
  5. R repositioned the wall plate on the wall, pulling the wires through the opening. Insert the mounting screws through the mounting holes and into the anchors. Verify that the thermostat is visually level and securely tighten both screws.Hunter-40120-Electronic-Thermostat-Installation-Instruction-fig-6


  1. loosen, but do not remove, the terminal screws.
  2. Match and connect the wires from the wall to the terminals as shown. Wires should be inserted behind the black terminal shields. Tighten each screw after the connection has been made. (The ends of any extra wires should be wrapped in electrical tape and carefully pushed back into the wall.)
  3. Push any excess wire length back into the wall to prevent interference.Hunter-40120-Electronic-Thermostat-Installation-Instruction-fig-7
  4. Locate the HE/HG switch on the back of the thermostat. Set the switch to HG for a gas furnace or oil burner. Set the switch to HE for electric furnaces.
  5. nsert two fresh AA alkaline batteries.
  6.  Align the thermostat with the wall plate, and snap the two halves together.
  7.  restore power at the electrical panel or furnace.Hunter-40120-Electronic-Thermostat-Installation-Instruction-fig-8Hunter-40120-Electronic-Thermostat-Installation-Instruction-fig-9


  1. Set the Set Temperature slider to the desired temperature.
  2. The System Switch determines the operating mode of the thermostat. Select Off or Heat.
  3. The Fan Switch should normally be located in the Auto position. To run the fan continuously, slide the Fan Switch to the On position.
    Note: In Auto, The fan will be turned on along with the normal operation of your system. In a normal gas or oil furnace, the fan will be turned on by your furnace after its warm-up delay.


  1. The THERMOMETER indicates the room temperature.
  2. The BATTERY INDICATOR displays two stages of WEAK BATTERY WARNING, indicated by the BATTERY INDICATOR flashing once approximately every second. When this warning appears, install two fresh AA alkaline batteries at your earliest convenience.
  3. The second warning is the SHUT-DOWN INDICATOR, indicated by the BATTERY INDICATOR flashing approximately twice every second. When this warning appears, the thermostat is shut down and will turn your system OFF. Your system will remain shut off until you install two fresh AA alkaline batteries.Hunter-40120-Electronic-Thermostat-Installation-Instruction-fig-10

Your thermostat is preset to cycle your system off and on at 1oF above and below the Set Temperature. For maximum system efficiency and a sustained comfortable temperature, your system should cycle 3 – 5 times per hour. If your system is cycling either too often or not often enough, then use a small Phillips-head screwdriver to turn the SPAN ADJUST left or right to change the cycle time.

  1. Remove the thermostat from the wall plate.
  2. To make your system run LONGER, turn the SPAN ADJUST clockwise.
  3. To make your system run SHORTER, turn the SPAN ADJUST counterclockwise.
  4. Reattach the thermostat to the wall plate.

Note: The Span range is 0.6oF – 3oF. The Span setting remains the same in both the Heat and Cool modes and can be changed regardless of what position the System Switch is in.

  1. My heating or cooling will not turn on or off.
    • Check the HE/HG switch to ensure it is set to the correct position.
    • Check the circuit breakers and switches to ensure there is enough power to the system.
    • Replace the batteries with fresh AA alkaline batteries.
    • If applicable, make sure the furnace blower door is closed properly.
  2. My system continues to operate when the thermostat is in the off position.
    • Replace unit.


Is there help on the web?
Yes. Visit for more information.
Can’t I just call someone?
Sure. Our Technical Support Group is available from 8 am to 5 pm CST. They may be reached toll-free at 1-888-830-1326.



© 2007 Hunter Fan Company


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Hunter 40120 Electronic Thermostat Installation Instruction


Hunter 40120 Electronic Thermostat Installation Instruction





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