Replacing Honeywell EIM with Nest Learning Thermostat (4 gen)

[Bahnstern]

Good evening!

Joining the long list of folks trying to figure out if the Nest Learning Thermostat (4 gen) is the right swap for my current Honeywell System. Based on what I can see, I am running the following:

Current Equipment:

Thermostat: THX9421R5021WW
EIM: THM5421R
Natural Gas Furnace, Electric Heat Pump

Thermostat upstairs is only using 2 wires (R and C), but the wiring in the wall behind the thermostat and the EIM has several more conductors available that are unused. As is shown in the pictures below, there are several different wires connected to the EIM. From what I can see (validate with the images below), we have the following connections:

(24 VAC) To Thermostat:

R: Red wire
C: Blue wire

Equipment:

W O/B: White wire
W2 Aux 1: Black wire
Y: Yellow wire
G: Green wire

(24 VAC) Power:

RH: Jumper to RC
RC: Jumper to R
R: Red wire
C: Blue wire

Images:

EIM Wiring: https://postimg.cc/v4R8ZvtR][img]https://i.postimg.cc/v4R8ZvtR/EIM-Wiring-Copy.jpg

EIM Model: https://postimg.cc/75M4YrLt][img]https://i.postimg.cc/75M4YrLt/EIM-Model-Copy.jpg

Questions:

1. Is the Nest Learning Thermostat (4 gen) compatible with my current system?
2. Would I lose any functionality by replacing the current Honeywell EIM and Thermostat with the Nest?
3. I have sufficient wiring in the walls to route the existing wiring to the present thermostat location, but what would be the appropriate way to make those connections?

Thanks in advance for your help!

[Derickf]

Hi @Bahnstern,

Thank you for posting in the community. I understand you are looking for a confirmation on whether the Nest Learning Thermostat (4th Gen) is a compatible replacement for your current Honeywell system. I appreciate all the relevant information you shared. I'm happy to provide information.

After checking the wiring listed on your post, your system is compatible with the Nest Learning Thermostat (4th gen). You can double-check using the Compatibility Checker, considering only the equipment wiring and the 24 VAC power, excluding the jumper wires. Since I wasn't able to view the pictures you shared. To fully assess your wiring and confirm you won't lose any functionality after switching thermostats, I would appreciate you resending those images.

Please keep me posted.

Best regards.

Derick.

[Bahnstern]

Looks like my image hosting timed out. See below for an Imgur link.

https://imgur.com/a/9Sxnnon

I do have both a heat pump and conventional furnace, so trying to figure out which set of connection labels to use as well.

[Derickf]

Hi there!

Thanks for your reply. I appreciate you sharing the pictures of your current thermostat wiring. 

After checking the pictures, here is the correct configuration for your Nest Learning Thermostat (4th Gen) terminals:

• O/B: White wire.
• W1: Black wire.
• Y: Yellow wire.
• G: Green wire.
• Rh: red wire from (24 VAC) power.
• C: blue wire from (24 VAC) power.

Please, before you proceed to install your Nest Learning Thermostat (4th gen), look for the installation video in the following article: Get started with your Nest Thermostat and Nest Learning Thermostat (4th gen).

Let me know how it goes.

Best regards,

Derick.

[Bahnstern]

Thanks Derick!

Haven't had a chance to run the install yet as I've been travelling for work. Want to make sure I understand what I'm configuring here. Based on your recommended configuration, we're connecting the white wire (W / O/B on my eim) to Nest's O/B and black wire (W2 / AUX1) to W1 on the Nest. My other reading generally says to treat a white wire in a dual labeled port as a conventional system. Is there another reason you determined that should be O/B?

My understanding of my current system is that my heat pump (maybe the wrong word, the unit outside is an American Standard 4A7L6048A1000AA Air Conditioning Compressor) is used solely for cooling and my gas furnace (American Standard s9v2c100u4psbbc Gas Furnace) should be providing heating.

Thanks again!