Dave wrote:
I am attempting to replace my current thermostat (mercury type) with a
White-Rodgers 1f82-261. The current stat is wired with 4 wires;
red-'r', green-'g', yellow-'y' and orange-'o'. The new stat
requires 24v power from the system which means I need to get the
common side of the trans back to the stat. The cable at the wall has
7 wires in it. When I put a meter on the red and blue wires I show
24vac. I opened the panel on the heatpump (Heil) and the blue wire
is not connected to anything so I'm left wondering how the circuit is
being completed.

Obviously, it is a different blue wire.

You are over your head and may be better off calling a professional
before you let the magic smoke out of something.