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Replacing a honeywell IAQ thermostat (EIM) with Ecobee3

cappuccinomcappuccinom New Jersey, USAMember
edited November 2016 in ecobee

I currently have a Honeywell TH9421C1004 thermostat installed with three wire 1,2,3. These wires are connected to a honeywell EIM which is connected to furnace unit. Ecobee support tells me that ecobee thermostat does not work with EIM setup and if i want to install Ecobee I must replace the EIM with Honeywell HZ322 zone panel.

I first of all do not understand why i have EIM in my current setup? And is it the right thing to replace the EIM with HZ322 ? i will try to explain my current setup:

Two furnace units with two outside condensers. Electric cooling and gas heating. One unit is installed in basement and the other in attic. Each unit supplies heat and cold to the respective areas of the house. I.e attic one for upper level and the basement one for lower level. Both the units are setup as single zone. 1 stage cooling and dual stage heating. The lower level furnace also has apriliare humidifier and an outdoor temprature sensor To meassure outaide temprature. There is no dehumidifier or ventilation enabled on either of the floors. Some times i notice that the fan is running without the heating or cooling So i am not sure what causes that. There is no connection between attic and basement furnaces as both of these operate independently by their own thermostat and EIM.

For above setup, do I really need an EIM? And if so what benefit is EIM adding in my current setup? Apologies to sound so ignorant but I bought the house recently and the previous owners did not explain the furnace setup to me. I am also not an HVAC expert.

I want to replace both the IAQ thermostats with ecobee3 so i can control the temp settings remotely, what is the best way to replace my current setup with ecobee3 without paying 100s of dollar to a HVAC professional?



  • kevinmcckevinmcc Macomb, ILMember
    edited November 2016 Answer ✓

    Sound like you just need wire pulled and two Ecobee3 thermostats. An Ecobee3 lite upstairs and a normal Ecobee3 downstairs to work with the apriliare humidifier. The zone controller they suggested does have a wireless link option for EIM to work, but maybe more expensive than running wires. You can't just use one thermostat you have two systems with their own zones.

    Are both units EIM? If so you would have to buy two zone controllers ($150 each) to get the wireless link to thermostat and furnace. Point of the EIM is the wireless links so wires don't need to be ran all the way from the furnace to the thermostat. I'd ask a HVAC pro how much to run wire, get estimate with at least 7 conductor if not 9 conductor wire.

  • cappuccinomcappuccinom New Jersey, USAMember

    Thanks kevinmcc, yes both systems use EIM currently. I only have 5 wires running between EIM and thermostat. I will check with a hvac contractor on the cost of running wires.

  • Justin93Justin93 IndianaMember

    Sorry for bumping this thread but I have the same exact setup. Although the builder at the time ran 6 wires to the thermostat. So I have 3 wires hooked to the thermostat and 3 left in the wall. While I haven't purchased the Ecobee yet, I plan to in the next coming weeks. My question is that I believe I need to uninstall the EIM and rewire the furnace. Will this effect my 2 stage heating and 1 stage cooling? Do I just match the color of wire with the letter on the furnace / stat?

    Overall I am just confused of the purpose of the EIM. Is it related to the stat or the furnace?

    Thank you!


  • cappuccinomcappuccinom New Jersey, USAMember

    I have finally resolved this. The EIM was controlling the humidifier, condenser, outdoor temprature sensor with only three wire and was not necessary when i ran 7 wires. You can simply connect the ecobee directly to the furnace and bypass the eim completely. If you want more information i can send you detailed info on how i did it.

  • Justin93Justin93 IndianaMember

    If and when you have time. Trying to avoid roadblocks :)

  • stazestaze OregonMember

    I would love to get this information as well. I have a IAQ and am replacing with a Ecobee3.

    Happy new year!

  • Justin93Justin93 IndianaMember

    So I found the right diagram for the thermostat to work with a heat pump. The challenge I am finding is that the board on the furnace doesn't match the wiring diagram that ecobee supplied. Codes on furnace board is C/G/R/W/Y/TWIN. While ecobee is requesting https://www.ecobee.com/wp-content/uploads/2014/09/ecobee_Wiring_HeatPump1.pdf.


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