Help Wiring Nest Thermostat - getting warm air on cool

[Cvan360]

Just moved to a new house, trying to wire up my nest thermostat.  I believe I have a heat pump.  It's a Trane XR.  After wiring the Nest, I was able to get warm air when set to heat, but I also got warm air when I set it to cool.  Here's pics of the existing wiring before installing the nest, and how I wired it.  Note that I did not use S1 or S2 as my understanding is those are for outdoor temperature sensors.  Did you see anything I did wrong?

[Cvan360]

Didn't realize I can't post pictures.  Here's the wiring:

With original Honeywell:

Blue Wire to C
Green Wire to G
Yellow Wire to Y
Orange Wire to W
Red Wire to R (with jumper to RC)
Black Wire to S2
Brown Wire to S1
White Wire to W2 (with jumper to unlabeld slot next to it)

How I wired it on the Nest Gen 3:

Blue Wire to C
Green Wire to G
Yellow Wire to Y1
Orange Wire to W1
White Wire to W2/Aux
Red Wire to Rc

[aperezdeleon]

Hello Cvan360,

Thanks for reaching out to our community.

Installing a thermostat for the first time can be confusing, but don't worry! Let's figure out what's going on and get your thermostat set up correctly.

From the wires you provided, it seems like you might have wired the thermostat as if you have a conventional system. However, if you have a heat pump system, it's important to use an O/B wire on your thermostat.

Simply put, the O/B wire allows a heat pump system to switch between heating and cooling. Without it in the correct terminal, your thermostat will stay in one mode, in this case, heating.

If you've confirmed that you have a heat pump system, make sure to use the correct wiring labels on your thermostat. Some thermostats have two sets of labels on the baseplate. If you still have a photo of the original wiring on your thermostat that would be a great place to double check how can it be set up.

Let me know how it goes.

Regards,
Angel.

[Cvan360]

Thank you.  Which wire is the one that should go to O/B?

[pereztzum]

Hi there!

That’s a good question In this case, the O/B wire is essential for controlling both heating and cooling. It's usually a combined wire from the old thermostat.

So, to answer your question O/B wires are typically orange, but you should not rely on orange color alone to determine the function of a wire. 

Regards,

Brandon.

[Cvan360]

Forgive me if I'm off here.  While I understand taht colors alone are not reliable to determine function, doesn't the fact that I stated that the current configuation with the Honeywell thermostat have Orange wired to W, tell you what the function of "my" orange wire is?

when I wired the Nest, i had Orange goign to W1.  So I should move that to O/B and leave W1 empty?  with all other wiring remaining the same?

[aperezdeleon]

Hello Cvan360,

Thanks for your reply.

The main reason we suggested changing your thermostat wiring configuration is that a heat pump requires an O/B wire.

So, if you're certain that your system is a heat pump, I suggest searching for the make and model of your HVAC system to find the user manual. These manuals usually describe what the wires in your system do, so we can check them out and wire the Nest thermostat accordingly. If you can't find the manual, checking the control board where the wires directly come out can also provide a lot of information. Just make sure to turn off your breakers first.

If the thermostat was wired correctly and you have a conventional system instead, we might need to try a different approach to make it trigger heating and cooling as expected.

Let me know how it goes.

Regards,
Angel.