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I have the Honeywell 9421C1004 thermostat with the EIM in my…

Customer Question
I have the Honeywell...

I have the Honeywell 9421C1004 thermostat with the EIM in my house on two separate units. One of the thermostats display was out, so I went and bought a Nest without knowing exactly what I was in for and what my system entailed. The AC repair man did not recommend a thermostat, but said that it would be easy to replace. I have since learned that the Nest will need to bypass the EIM, which I am not sure if I am losing any functionality or efficiency by doing this. I looked on the Honeywell website thinking I would just by the same thermostat only to find that it has been discontinued. The replacement recommended is the TH8321R1001 which looks like it will work my current EIM. The question I have is given that I will most likely have to get a professional to install the thermostat either way, is the Nest an upgrade or a downgrade to the Honeywell? I haven't found the Honeywell in stores only online, so do I purchase it and have it installed or call the AC company and have them supply a thermostat. The thermostat is on an upstairs unit that I don't necessarily need in the winter, but I need to get it fixed.

Contractor's Assistant: Just to clarify, do you think this is a larger HVAC problem, or something specific to the thermostat?

The thermostat display doesn't work. I can take my downstairs thermostat up and snap it on the mount and the unit works fine. The display is the only thing that keeps you from controlling the unit.

Contractor's Assistant: Do you plan on doing the work yourself?

At this point I am not sure. If I stick with the Nest option, then no. If the Honeywell is a simple attach the bracket and connect the three communication wires back up, then yes.

Contractor's Assistant: Anything else we should know to help you best?

That is it. I am mainly seeking an opinion on am I falling for the Nest marketing and giving up on a good thing with my existing Honeywell EIM thermostat system.

Submitted: 6 months ago.Category: HVAC
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Answered in 1 hour by:
1/9/2018
HVAC Technician: Regan Land, HVAC Technician replied 6 months ago
Regan Land
Regan Land, HVAC Technician
Category: HVAC
Satisfied Customers: 768
Experience: Over 20 years of experience, NATE Certified, Licensed HVAC Contractor, Certified IAQ Specialist
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Hello and welcome to Just Answer. My name is ***** ***** I'll be happy to assist you today. The EIM is used so that the thermostat can be connected and control the system wireless (other than the wires connected to R and C). Quick question: If you remove the face of the thermostat to reveal the wiring, are there other wires that come in with the wires that are connected to R and C that aren't connected to any of the other terminals on the thermostat? If so, how many other wires are available?

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Customer reply replied 6 months ago
I have the 5 wires running from my EIM that is mounted on the unit in the attic to the thermostat. There are only three wires hooked up to the thermostat and those a labeled 1 2 3. If I go the route of the nest I have to remove the EIM. My concern is there are more than 5 wires out of the EIM which I believe are communicating with the humidifiers. I am not sure the Nest will do everything the EIM is doing. If it does, then I just hire an AC company to install the Nest. The issue I have is I don't want to pay the several hundred dollars to put the Nest in and make my system not run property.
HVAC Technician: Regan Land, HVAC Technician replied 6 months ago

I will help determine that, but have a few more questions. Do you know what type of heat you have (heat pump, gas furnace, etc)? Also, is there a single humidifier on each system? Do you know if there's anything else connected? If needed, you can take a photo of the wiring on the EIM.

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