I'm trying to update my thermostat to one that has a scheduling feature. My plan is to just copy how it was hooked up in an old 129N Thermostat from Armstrong however, I can't figure out why the "W" and "E" terminals are connected and how I should mimic that on my new thermostat. Additionally, after hooking everything up and turning the heat on, it doesn't seem to be heating the air. Could anyone give some insight into why it would have been connected like this. Below is a picture of how it was connected.

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  • 1
    How did you wire the new thermostat? – Tester101 Jun 1 '15 at 23:13
  • Actually I'm still trying to figure it out. My new thermostat uses the "W" port for the conventional type system and the "E" port for the heat pump system. I connected the white wire to "E" but the heat wont turn on until I connect the orange wire to "B" on the new system but when I do that the cooling no long works until I connect the orange wire to the "O" port. Any suggestions? – TrapLevel Jun 2 '15 at 23:36
  • Did you set the thermostat up for a heat pump? – Tester101 Jun 3 '15 at 0:00

Your photo shows a heat pump thermostat with control for 1 stage of auxiliary heat which is also used as emergency heat if the heat pump completely fails and emergency mode is selected on the thermostat (that's why W and E are wired together). From your description you have a heat pump which may not have any working auxiliary heat and your orange wire should be connected to terminal "O" on the new thermostat. If your new thermostat is not correctly configured for the type of single stage heat pump operation you have, then "O" will not change states from HI to LO when the thermostat call switches the heat pump from cooling to heating.

The standard wiring for thermostat terminals used with a single stage heat pump are as follows:

G: Blower. (ON when any heating or cooling is called for)

Y: Compressor. (ON when either A/C or Heating is called for from the heat pump)

O: Reversing valve. (Typical wiring, ON = cooling mode, OFF = heating mode)

B: Reversing valve. (Alternate wiring, ON = heating mode, OFF = cooling mode)

W: Auxiliary heat, stage 1

E: Emergency heat (In emergency mode "Y" is not energized and E is energized to indicate the need for emergency heat. This mode is used if the heat pump fails during cold winter temperatures and the compressor must remain off.)

C: Common

R: +24vac

Note that either "O" or "B" is used depending on the system, but not both.


E is usually emergency heat. In systems that don't actually have emergency heat but a user might select that mode on the thermostat, connecting E to W will cause the normal heat to come on instead of doing nothing.

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