Installing ecobee SmartThermostat with Voice Control on older home

Alright -- the trick with the PEK for you is to treat everything wirenutted to your two thermostat cables as the "furnace" from the PEK's standpoint. You'll need some 18AWG pigtails for the connection from the PEK to the "furnace", though, as follows:

• Red/black nut to PEK cover (furnace) R
• White nut to PEK cover (furnace) W
• Green nut to PEK cover (furnace) G
• Yellow/white nut to PEK cover (furnace) Y
• Blue/red/white nut to PEK cover (furnace) C

and then you can connect the base as follows:

• Red Left to PEK base (t-stat) R
• White Left to PEK base (t-stat) W
• Red Right to PEK base (t-stat) G
• White Right to PEK base (t-stat) Y

Just make sure to connect the Red Left (R) wire at the thermostat end to the Ecobee's Rc terminal or else the Ecobee won't turn on (this is a limitation of using the Ecobee/PEK combo).

It is normal for a 24vac control transformer to measure somewhat high under no load (i.e. 27v vs 24v). It is 24vac system.

"B/C" on the furnace should be the common. I am surprised the existing thermostat works connected like it is. The only thing I can think of is that the thermostat is internally configured for a single power supply system and the "Rc" terminal is open instead of being shunted to Rh as is the case with many thermostats. In this case if Rc were shunted to Rh it would pop the overload protection on the furnace board or the transformer.

Should be good to go on connecting the black wire to C on your new thermostat. Do not connect any of the wires to Rc on the new thermostat.

Edit:

Schematic for Goodman AMV9/ACV9_XA furnace: