I'm installing Hunter thermostats (replacing my old Honeywell thermostats). There is an orange wire in the old thermostat, but not in the new one. Is the orange wire for power (which I wouldn't need for the new battery-powered thermostats)?

Normally the orange wire is for "O" in the thermostat, this orange wire energises the reversing valve on a heat pump system. If it were used for something else then you would have to go to the air handler or furnace and see what it is connected to. Hope this is helpful, Mike

Does your new thermostat have a "O" and a "B" terminal ? Is your system a heat pump?

If you are not sure if you have a heat pump, it is covered in the sticky at the top of forum. If you have a heat pump, you must use a thermostat made for a heat pump and select heat pump mode.

I'm installing Hunter thermostats (replacing my old Honeywell thermostats). There is an orange wire in the old thermostat, but not in the new one. Is the orange wire for power (which I wouldn't need for the new battery-powered thermostats)?
Acetec: I do not have a heat pump. The new thermostat does not have an "O' slot. There is only G, Rc, Rh, Y and W.

If you go to the thermostat and follow the thermostat wires to the point of connection you should see what it connects to. If it connects to a terminal board you will see a letter on the board, if it is "C" then that means common, if it connects to the transformer and the red connects to the transformer opposite terminal then it is the common, cap this wire off you will not need a common on the new thermostat.