The home I moved into has a Honeywell thermostat controlling an Amana heat pump. I am trying to replace this with a Nest thermostat instead. Here is a photo of the wiring for the existing thermostat before making any changes:

enter image description here

Nest wants me to identify both of the "Y" wires. How can I tell which one controls heat and which one controls cooling?

I'm also unsure how I can determine whether the heat pump should be set up for "O" or "B", though I've read that most are "O". Will I cause damage to the equipment if I set this in the wrong mode?

Which wire is used for emergency heat via the heating coils? Is that the W2 wire in the image above?

  • Go check the furnace connections as well to make sure that end is setup like you expect. The codes and colors are not 100% firm between manufacturers and you may need to refer to the instruction/installation book for your furnace to be sure.
    – jwh20
    Mar 30, 2020 at 12:47

2 Answers 2


First, the labels you need to look at are the ones on the outside - not the ones under the wires. The labels under the wires are for a conventional system. Looking at the labels on the outside, it's clear that the two yellow wires are Y and Y1, and the white wire on the left is AUX, which is normally Auxiliary or emergency heat.

Y and Y2 are normally used for a modern HVAC system that has dual stage cooling, so neither should be used for heat specifically. On a conventional system, the W wire is used to call for heat, but on a heat pump, a combination of the O wire and Y wire(s) call for heat.

Choosing O for the switch over valve wire is a safe bet. If you choose wrong, the heat pump will start in heating mode when you call for cooling or will not go into heat mode when heat is called for. It shouldn't damage anything.

Also, some of those wires might not do anything for your system. For instance, I can't tell you what L, S1 and S2 do. Maybe nothing, but you might need to look at the old thermostat instructions or the other end of the wiring on your heat pump to figure that out.

  • Thanks for the reply! Can you explain why there are 2 "Y" wires? Nest wants to know what each will be used for.
    – Yuck
    Mar 30, 2020 at 15:39
  • @Yuck Sorry I missed that part - I updated the question. Keep in mind that what I have said are "typical" but its possible your system is a little different. Things are pretty standardized, but there are some oddball setups.
    – JPhi1618
    Mar 30, 2020 at 15:45

Normally y is for cooling y is for cooling & y2 is for 2 stage cooling not all systems have this Rh is usually the heating. Looking at the connections on the control board are the only way to know for sure but it looks like it was wired to common wire colors.

  • Where do I find the "control board" on a heat pump? Is that inside the house or outside on the unit?
    – Yuck
    Mar 30, 2020 at 15:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.