• (Weathertron is a Brand name of General Electric)
• GE/Trane thermostats use an outdoor anticipator similar to the one described here.
• The new electronic Hunter Stat will not need an out door stat ("T" lead) but some units do have them.
• The only purpose of the common lead on most stats is to operate the lamps unless it needs the 24 volt transformer to power the electronics. See Note below.
• The Hunter 44428 has the advantage of not using the electric heat unless necessary and when it does it will bring the room to the set point and shut the system off instead of running the heat pump forever. It has 2 W terminals that allows the electric heat to be broken into 2 stages.
• There is a reversing valve switch. It should be set to "RO" in most cases, "RB" if Rheem/Ruud.
• See Note below.

     Many manufacturers use the same terminal designation except for a few that change like "E" is emergency heat relay but it is not used on most units.
"B" is energize to heat (Rheem/Ruud) the opposite of "O" but here it is "Common" what would be "C" or "X" on Carrier or Bryant. I checked  a 3.5 ton Trane unit (General Electric is the same) and I found that "X2" is one set of auxiliary heat and "W" is the other. When the thermostat calls for auxiliary heat it energizes "W" when the outdoor unit calls for defrost it activates the "X2" line. If the unit has an outdoor thermostat (smaller units don't have one)  it will connect "W" and "X2" together. I recommend for proper operation of the Hunter thermostat to disconnect the outdoor thermostat (pull one wire loose and tape it) and let the Hunter do it's job. When the thermostat is placed in the Emergency heat mode it also connects the two together. The "W" terminal is connected to the "AUX. HEAT" lamp, the "X2" is not. Smaller units under 3 tons may not have this extra heat circuit. I recommend using "W" as the 1st stage of heat (W1) and "X2" as the second stage (W2).  If you are unsure leave it disconnected and if there are no problems leave it off while you decipher the wiring. It is better to have something not work than to fry the transformer or thermostat. Most GE/Trane units have a wiring diagram the inside cover of the outdoor unit control panel.

Notes: the older Hunter 44428 does not have a common terminal ("C") the newer ones do!

• On GE/American Standard/Trane and York "B" is common, please do not confuse with Rheem/Ruud that use "B" to energize the reversing valve with the system switch set to heat. If you connect "Common" to the reversing valve "B" (or "O") you will blow out the transformer and possibly smoke the thermostat.
• "X" can be common on some systems, or the indicator lamp. On Trane/American Standard/GE "X2" is the second stage of electric heat connect it to "W2".

If you have a Lennox system that uses terminals like: F, V, R , P, L and H ect. triple check the function of each wire before you connect a new thermostat!!!

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    I have amended my wiring page and I will add it here too. I want to urge any one changing a thermostat, especially one this expensive in the strongest terms to check the function of each wire BEFORE you blow up your $80+ thermostat and the control transformer. Obtain a set of clip leads (available from Radio Shack) and a 1 amp fuse and test each function by connecting from "R" to each of the wires that needs to be connected to make the system work. If the fuse blows instantly then you most likely found the common wire or a defective component.
     An assistant is useful for verifying each function on the outdoor unit, the reversing valve and contactor. The electric heat functions should turn on the indoor blower.

I also insist that you never replace a thermostat or any other components as a diagnostic or on a system that is not working correctly. Isolate the problem first.

This part Written by Scott Meenen
 More to come...

Trane electric heat modification that will save you lots of $$$...

If you have a larger Trane heat pump with 2 sets of circuit breakers for the electric heat then consider making this minor modification that could save you lots of money. On the indoor unit there are 2 electric heat channels (inputs) W1 and W2. W1 is on the first circuit breaker with the fan and controls and W2 is on the second one and stands alone. Move the black wires (X2) to W1 and the white wires (W1) to W2 and power down that circuit breaker, remove any jumpers that may connect both inputs. This simple modification will give you emergency heat and defrost heat but allow the heat pump alone to heat the house without the benefit (or cost) of auxiliary heat.

    If the weather gets too cold for the system to keep up then you can close the second circuit breaker. This mod works on other brands of equipment with a similar configuration.