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New House, Heat Pump. Will a Nest work?

22892 Views 14 Replies 7 Participants Last post by  Joeboo25
I just bought a new house with a Trane heat pump. It's a XR12 2TWR2036A1000AB, with a variable speed air handler, TWE037E13FB1. It has electric auxiliary heat, but beyond that I don't know much about it.

I'd like to install a Nest but I'm having trouble identifying my wires for compatibility.

Can somebody help me sort these wires? I believe most of them are fairly standard, and I think X2 is for the emergency heat, but I don't know about F. Maybe it's the fan control? Is the variable speed fan controlled by the thermostat or by something in the heat pump? Can the Nest handle a system like this, or would I be better off using another type of thermostat? Also, where is the control panel located so I can see if the wires are labeled differently on the unit?

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The problem with the nest is that they are not fully compatible with certain setups. As for hooking it up, have you looked at the nest forums on their site or contacted them? they have been helpful for others when you call them.

How is/was the previous thermostat hooked up?

R-24 volts
O-Reversing valve (heat pump)
W-first stage heat (heat pump)
X2-Auxilary I think, electric secondary heat
F-have no clue.
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Thanks. I spoke to Nest and they couldn't identify the F wire either. Where would I look on my unit to try to identify the wire?
Thanks. I spoke to Nest and they couldn't identify the F wire either. Where would I look on my unit to try to identify the wire?
There should be a wiring diagram on the service doors, most likely on the inside of the electric air handler, the inside unit.
I'll swing by the new house in the morning and see what I can find out.
ive seen f as a system monitor, Is the f jumpered to w in your pic?
Ok, here's an update.

I opened up my unit and here's what I found.

R = R -jumpered to BK
G = G
B = B
O = O
W = W1
Y = Y

F is unattached

And I can't find X2 (the black wire)

Maybe I need to go back and look a little closer. If it's secondary heat, maybe it feeds in differently. The wiring diagram shows a black/white wire hitting BK, but I'm not sure if that's the same thing.

Also, how is the variable speed fan controlled? Does the air handler know when to vary the speed, or is it possible that my F wire was supposed to control this but it's been disabled? Part of the reason I wanted the nest is the improved effciency (like all programmables), and I want to make sure I'm using my system properly.

I also have more detailed pictures of the diagram, as well as the DIP switches if any of that would be useful.

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"F" wire is most likely a defrost wire. When the outdoor compressor goes into defrost, it will signal the Air handler to turn on the auxilary heater while defrost is taking place. The nest probably will not need this line, but you need to find out how this is wired up. Some Trane thermostats have a blue light that comes on when the system is defrosting, and all this line does is illuminate that light *BUT* you need to figure out how it is jumpered to the air handler, from the outdoor unit. The second stage heat line switches on your heat strips (x2) and the defrost line needs to be connected to it. This is sometimes done at the AHU, but sometimes one internally by the thermostat...depends on the mood of your installer! If not, your system will blow col air when it defrosts (it is actually running in AC moe when it does that) Defrost is controlled by the outdooor unit, not the thermostat.
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The F wire is not attached to anything on the air handler (see second photo in previous post). I still need to find the black wire (X2), and it may be the difference between which Nest Generation is compatable.
If it is not attached at the AHU, aux heat on defrost was probably being handled at thermostat. IIRC Honeywell thermosats like the one I have instruct a jumper to be installed between x2 and F...or you can just put F on X2 at AHU. (I would confirm this is in fact a defrost line)
Hmm... I took another look last night and I don't think X2 is connected either. Of the wires coming into the AHU, there is a brown one that isn't connected. It looks black at the thermostat (in the picture), but maybe it's actually brown. Unfortunately, the floors are being refinished so I can't get to the thermostat for a couple days. Does that mean I don't have a secondary heat source?
W1 of the air handler and thermostat have a wire on them, so you have aux heat.
Maybe you can pull up the install manual of your existing thermostat and see what the wires are labelled as, then try to correlate that to the nest documentation.
The current thermostat is a Taystat 570. I have the users manual, but I haven't found an installation manual yet which shows the wiring.

I think I may have the Nest figured out. If my assumptions are correct, here's how I'm going to wire it (on a 1st Gen Nest).

Taystat === Nest
R (Red) ==== Rh
W(White)=== W1
Y (Yellow) == Y
G (Green) === G
B (Blue) ==== C
O (Orange) = O/B
F (Pink) == Not connected
X2 (Black or maybe Brown) == Not Connected

I'll take one more look to make sure X2 isn't connected. If it is, I'll need to swap this one out for the 2nd Generation.
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