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Thread: Replacing a Honeywell Thermostat EIM with Nest

  1. #1
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    Replacing a Honeywell Thermostat EIM with Nest

    Hi I'm trying to replace my Honeywell thermostat(THX9421) with Nest, I opened it up hoping it would be fairly straight forward, but I'm stumped.

    To start with my Thermostat has only two wires(as shown in the attached picture) - R & C. On further research found that this Thermostat has a EIM(Electronic Interface Module) that it communicates with wirelessly and that is the one hooked up to my HVAC. So I went into the attic and found a Honeywell Redlink that has more connections(also picture attached). There's also a Honeywell HZ322 in there that I have no idea what it does. I'm attahing the pictures here and hoping someone can shed some light on how this whole thing works and what I need to do to replace that thermostat with Nest.
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  2. #2
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    HZ322 is a zone control panel to work with multiple thermostats.

    Your other pictures are not showing but in general to use a Nest (or any other thermostat) you will need to remove the EIM* and use the additional wires that are in the cable behind the thermostat. Those wires would connect the HZ322 and the Nest.

    *Need to be careful as the EIM might be supporting multiple thermostats so might not be easily removed.

    If you just want to add Internet/wifi you might be better served by keeping the current Honeywell system and just adding the Internet gateway module - https://www.amazon.com/gp/product/B006BD1K4A
    Last edited by Stuff; 05-08-2017 at 10:24 AM.

  3. #3
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    Quote Originally Posted by Stuff View Post
    HZ322 is a zone control panel to work with multiple thermostats.




    Your other pictures are not showing but in general to use a Nest (or any other thermostat) you will need to remove the EIM* and use the additional wires that are in the cable behind the thermostat. Those wires would connect the HZ322 and the Nest.

    *Need to be careful as the EIM might be supporting multiple thermostats so might not be easily removed.

    If you just want to add Internet/wifi you might be better served by keeping the current Honeywell system and just adding the Internet gateway module - https://www.amazon.com/gp/product/B006BD1K4A
    I have a similar question about installing a NEST in a system that currently uses three (of four) wires between a Honeywell thermostat and EIM. It is a two stage gas system with AC and whole house humidifier. I think I can pull a 5-wire cable if necessary. Is it? I'm pretty sure I'll need further advice on connecting everything.

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