Wiring Compatibility O and A wires for heat pump thermostat?

[bnanna922]

Worried about the A and O wires:

R – 24 VAC hot
C – 24 VAC common
W1 – 1st stage heat
Y1 – 1st stage cool
W2 – 2nd stage heat
Y2 – 2nd stage cool
G – Fan
A – Alarm input
L – Status input
O – Override/reset

[bnanna922]

I initially used the basic nest, but the transformer went out.  I put O to O, but maybe it's not the wire for switching between heat and cool.  Or maybe my wires touched each other.  Not sure.  All I know is it worked for a couple hours then the transformer went out and I didn't have 24V in anymore.  I fixed that by swapping out the bad transformer, and I bought the learning thermostat with more wire inputs, but I want to make sure it's good before I try something dumb.

[Patrick_Caezza]

@bnanna922 

What is the manufacturer and model number of your current thermostat?

With the O terminal being for Override/reset, the orange wire would need to be taped off as it isn't used in any Nest thermostats. The wires going to the A and L terminals must be taped off as they are also unsupported.

[bnanna922]

Would plugging in the O wire to the O terminal when it's override and not for the heat pump to switch be the reason why the system would short cycle when powered and then cause the 240V to 24V out transformer to fail.  It says it's an internal fuse, so I think I have to buy a new one.  Google support sent me a wiring diagram when I gave them the same info, so I figured I'd give it a try, but it failed after a few minutes of short cycling.

[bnanna922]

The answer is yes haha.  Luckily, when I removed the glued on plastic cover for the $100 transformer, I found a cheap 3.5A fuse, soldered on a new one, and bob's your uncle.

[Jake]

Hey bnanna922,

Thanks for the help Patrick_Caezza.

I am glad to hear you were able to get everything working. I wanted to check in with you, and see if you had any further questions from here. Please let me know, as I will be locking the thread in 24 hours with the issue being resolved.

Best regards,

Jake