Nest compatibility check for heat pump

[DJMarsh]

We had an hvac technician out recently to service our system and we discussed switching out our current thermostat to a Nest.  The technician seemed confident Nest would work, but when I went to order one the Google Nest compatibility checker said it would not work.  I contacted Google support and the support person said it wouldn't work but he had no idea why.  

We have the following wires hooked up in our current thermostat: Y1, W2, G, RC, C, O, B (there is also one black wire that is not hooked to anything, and there is one jumper from RH to RC).  Printed inside the thermostat cover there is a guide that identifies each wire.  The guide says: RC = 24VAC Power Supply, C = 24VAC Common, Y1 = Compressor Stage 1, W2 = Auxilliary heat, O = Cool mode powered reverse valve, B = Heat mode powered reverse valve, G = Fan.

Any HVAC pros have an idea why Google thinks this won't work for us?

[CoolingWizard · Accepted solution]

@DJMarsh , your heat pump is either an O model or it is a B model. There cannot be both a B and an O for the same heat pump.   Since you have a C terminal you need to further investigate to see what type you have.  Now then, O models are heaters by default, and the thermostat must energize the O to switch it to Cooling mode.  The opposite is true for B models. The B models are Air Conditioner by default, and the thermostat must energize the B wire to switch the heat pump into Heating mode.

How to test what model you have.  DO NOT PERFORM THIS TEST IF THE OUTDOOR AIR TEMPERATURE IS BELOW 70°F.  On your thermostat remove the B wire and remove the O from the terminal connector.  Then turn on the Heating and see if the air blows out ice cold or warm.  If ice cold, you have a B model. If Warm are comes out, you have an O model. 

AC Cooling Wizard