Hello Aptosnest,
It is difficult to see which cable is connected to which contact.
Can you tell us which contacts the two white cables are connected to?
Presumably, your original thermostat is a White Rodgers. (or Emerson)
Basically, you can connect the Nest to any low current system.
What functions are currently available to you ? Cool and Heat? Just cool ? Just Heat ?
Please locate the correct contacts at the source!
The source is actually the control board on your furnace.
If you dare, take a look inside and, if possible, trace the wire leading from your thermostat to the control board.
Are the wires from your thermostat to the furnace connected directly or are they perhaps spliced somewhere along the way? This is very important to know!
In your photo, the two white wires look like they are some old speaker wires. This does not meet any code and should be fixed. You might say "cable is cable, it doesn't matter" but you should make sure you are using the right cables. (18/5 is recommended)
Please note that ALL work on a system that uses electricity is dangerous to life and can result in death. Always turn off the breaker to the system you are working on.
If you are unsure or in doubt, always consult a professional!!!
