Nest Thermostat and Heat Pumps w/ Aux

Posted: February 24, 2017 in Uncategorized
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I recently found myself in a position of having the learn about heat pumps. I house I recently bought had one installed with auxiliary electric heat. What this means is that the house will heat up using the heat pump outside first. If it can’t catch up it will use the auxiliary heat to supplement. My auxiliary heat is provided by electric coils installed above the A-Coil in the air handler. But getting heat from the heat pump is about 300% more efficient than electric coils even though it will be on longer. Therefore we want to use the heat pump over auxiliary heat for cost reasons. This is called a “single-fuel” system. From what I understand, if the air handler has gas heat instead, this would be called a “dual-fuel” system.

While researching different aspects of the heat pump, I noticed that the blower was always on. Searching Google I found most people said this was normal. Though I thought it was odd. Why would it be blowing when the heat pump and coils are off?

When we had a few nice days, I decided to turn the heat off. But yet the fan still ran. This I know is not normal. Once I removed the head unit of the thermostat I found the batteries badly corroded. The electronic board was likely shorted out on the fan circuit.

I’ve been interested in getting a smart thermostat like Nest or Ecobee and have been indecisive. But now that I needed a new thermostat and the fact that I’ve got Nest Protect units installed, I decided to stick with the Nest ecosystem. I find Nest to be like Apple. Quality products but they really keep you inside their proprietary eco-system. But I decided to go with it anyway.

I have an American Standard system with an American Standard LCD thermostat. What got tricky was the B wire, which Nest has an O/B slot and the X2 wire, which Nest does not have.

20170218_140403

From my understanding this is what each wire does:

X2 : Black : Turns on another set of standby electric coils for emergency heat
W1 : White : Turns on electric heat coils
RC : Red : 24V AC Power Hot
O : Orange : When on, reverses the heat pump valve to go from heat to cool in the summer
Y : Yellow : Turns on the heat pump compressor
G : Green : Turns the fan on (in my case, on low)
B : Blue : 24V AC Power Common

Inside the air handler, the following wires a hooked up to the following codes:

Black : W2 & W3
White : W1
Red : Bk
Orange : O
Yellow : Y
Green : G
Blue : Bk

20170223_185327

Needless to say, there really needs to be a better labeling or coloring system. In fact, thermostats should really go towards an IP system and use an Ethernet cable.

After calling Nest, after I figured out that it thought I had dual fuel instead of single-fuel, I figured out that black goes to * and is programmed for emergency heat. (* can be used for emergency heat or humidifier circuit) The blue wire becomes C.

Let me say that Nest support was awesome. I could talk with a clearly spoken woman who worked in Oklahoma and took the time to understand, address, research and resolve my issue promptly. She sent me an email to reply back with photos of the original thermostat wiring and my current wiring on the Nest which helped things along. Their number is 1-855-469-6378.

Here is how I now have my Nest 3rd Generation thermostat wired and working:

Y1 : Yellow : Turns on the heat pump compressor
G : Green : Fan
O/B : Orange : When on, reverses the heat pump valve to go from heat to cool in the summer
Rc : Red : 24V AC Power Hot
W2/AUX : White : Turns on electric heat coils
C : Blue : 24V AC Power Common
* : Black : Turns on another set of standby electric coils for emergency heat

screenshot_20170224-145603

 

After I got the corrected wiring, I went into the pro settings and switched it from dual-fuel to single-fuel. At that point in time the Heat Pump Balance option showed up. I started with “balanced”, but that seemed to aggressive still. So for now I have “Max Savings” on with “Early-On” turned on to 5 hours.

Note: System and wiring vary widely. This is a reference only to my specific system and wiring. Yours likely varies. Professional installation recommended.

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