Question About American Standard Condensing Unit Furnace Wiring

[ChilledBeam]

I had another thread on my Ecobee thermostat but I got that figured out, I still have some remaining questions. My system is running fine so its really just curiosity at this point.

See attached wiring diagram. I though "O" was the terminal for the reversing valve (if I had a heat pump, which I don't), any idea why that is jumpered to the "R" terminal per note 2?

I know "Y1" and "Y2" on the stat are for 1st and 2nd stage of cooling and same for "W1" and "W2". I don't know why "Y2" is wired to the "BK" terminal on the furnace. What is the "BK" terminal? I know "G" is fan and and am assuming "R" is power with "B" being common. I also have a "Ylo" terminal on my furnace that is not used, any idea what that is for?

Sorry for all the questions, just trying to understand this wiring.

American Standard 4A7A6048 Condensing Unit (2 Stage)
American Standard AUY100R9V4W5 Gas Furnace (2 Stage)

Andy

[BaldLoonie]

The jumper has to do with how the furnace handles the 2 stage A/C. Ylo is for a XL20i which really has a low stage near 50%. The blower drops in 1/2 for low vs 80% in low for your A/C.

[catmanacman]

the r to o jumper is so comfort r will run the ramping profile if dip switch 5 and 6 are turned on