Old Carrier 24v Fan-Coil Unit - No cooling after Nest Learning Thermostat install

[fritz]

Attempted to install my brand-new 3rd gen Nest Learning thermostat yesterday, replacing a Honeywell thermostat. I say attempted because even though everything in the install went well (took 30 minutes), the unit would not cool when set to 'Cooling'. Listened for the condenser kicking on after the fan, but it never did.  Ran Nest's tests on Cooling multiple times. Always came back ok. Seems like the Nest doesn't realize that it's not switching the condenser on when set to cooling. And after I switched back to the Honeywell thermostat, cooling works fine as before.

My hypothesis is this - This Carrier fan-coil has a single 'R' wire terminating on the thermostat wiring block along with a jumper wire to the adjacent 'Rc' terminal. Per Google's instructions, I removed the jumper wire as Google says it isn't needed and may harm the unit. I SUSPECT that the Nest isn't giving the correct voltage over to the 'Rc' terminal for the condenser now because there's no longer a jumper wire. But any ideas on how to correct??

I do have a Common ('C') wire running between the unit and the thermostat as you can see below. It's my understanding that any extra power needed would be drawn from the C wire. Is that not the case?

Note the silver jumper wire to the left of 'R'

[Patrick_Caezza]

It looks like you have a Heat Pump system and not a Conventional system. Move the orange wire to the OB terminal and set up the thermostat again.

[fritz]

If I had a heat pump system, then wouldn't my old Honeywell thermostat already have the orange wire run to the OB terminal on it? 

[Patrick_Caezza · Accepted solution]

The orange wire is run to the O/B terminal. That note pointing to the O/B label references the user instructions and explains that the W label and the O/B label are the same terminals.

Do as I said and it should work. I've been doing this for a very long time.

Or you can look at the terminal block on the control board in the furnace/air handler and see where the orange wire is terminated, which should be to a terminal marked with one of these labels or something similar, O/B, OB, O, or B.

[fritz]

Ahhhh, I didn't realize that W & O/B were the same terminals! That makes sense now...

I moved the orange wire from W to O/B and now it's cooling. Thank you!  (And thanks for the explanation too 🙂)

[MelbaDT]

Hey folks, 

Thanks for taking care of this, Patrick_Caezza. 

fritz, glad to hear that you've got the answer you're looking for and that you now have a working thermostat. Feel free to let us know if you have further questions in mind. 

Cheers, 

Melba

[Ryan_G]

Hey everyone,

I'd like to jump in here and check if you've seen Melba's response. Let us know if you need more help by replying to this thread.

Thank you for your help, Melba.

Regards,
Ryan 
 

[MelbaDT]

Hi, 

We haven't heard from you in a while so we'll be locking this thread if there is no update within the day. If you have any new issues, updates or just a discussion topic, feel free to start a new thread in the Community.

Best, 

Melba