Dave wrote:

Hello all,
I'm hoping someone here can help.

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.

Do the control wires typically connect to something else (transformer?)
between the heatpump and thermostat? I had assumed the transformer is
located within the heatpump and the 24vac is supplied via the red wire to
the stat which then determines which path it returns on. Is that correct?
If so, I was thinking I could just connect the common side of the
transformer to the blue wire and violla. Since I'm already showing a
circuit with the blue and red, and the blue is not connected at the
heatpump side, I'm stumped.

Any help is appreciated!

The transformer is located in the air handler. The open end of the
unused blue is probably just touching some sheet metal in the control
section, thus grounding it. It shouldn't be tied in at the t-stat until
it has been confirmed that it's connected to the transformer common. If
the house burns because of your negligence your HO insurance may not
cover it.

hvacrmedic