Replace Honeywell THM5421C with a new WiFi thermostat


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Old 05-17-21, 08:20 PM
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Replace Honeywell THM5421C with a new WiFi thermostat

Hello Gurus!

I'm in the middle of switching out my current THM5421C thermostat with a new one and was hoping to get some direction on how to properly connect it to my current unit setup.

THM5421C is a 3-wire thermostat that is connected to the unit via the EIM. Based on what i understand, i need to run a new 6-wire cable from my unit to the new thermostat.

My question is: How do i bypass the EIM and connect it directly to the Control Circuit Board?

I'm including some images below in hopes that someone can point me in the right direction.

Current setup:
Old Thermostat:


The EIM: top 3 wires on the left are from the thermostat, the other 8 wires are going to PCBBF106 Control Circuit Board and connected to EIM as follows:
Left side:
1 (to thermostat)
2 (to thermostat)
3 (to thermostat)
C - Brown and Blue
R - Red
HUM 1 - Connects to DHM1

DHM 1 - Connects to HUM1
DHM 2 - Orange

Right Side:
W - White
W2 - Black
Y - Yellow
G - Green


Out of 8 wires 6 are connected to PCBBF106 Control Circuit Board:
W2 - Black
W1 - White
R - Red
G - Green
B/C - Blue (also has Whie wire leading to outside AC unit)
YLO - Yellow (also has Red wire leading to outside AC unit)

The other two wires (orange and brown) are connected to the 9100Y233Q00 -Tyco relay (bottom right), which is also connected to the Broane HEPA Filtration system



Thank you all very much in advance!
 
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Old 05-17-21, 09:49 PM
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Welcome to the forums.

I assume the new stat you are getting will have direct humidifier control. That means you need seven wires between the stat and your furnace. You can make splices directly at the EIM or go directly to the furnace with your new cable. I would recommend running an 8 conductor thermostat cable.

W - White - heat stage 1
W2 - Black - heat stage 2
Y - Yellow - A/C
G - Green - blower
C - Blue - common
R - Red - 24v
? - humidifier control wire

Noticed something strange.... I see red, white and blue connected to the stat but red, white and green connected to the EIM. What's up with that ?
 
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Old 05-18-21, 09:37 PM
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Oh my! Thank you, Mr. PJmax!! I attached the wrong picture of the thermostat!! Itís red white and green on both ends! Good catch

there is a separate stat I have for the humidifier and I was not going to mess with that one and leave it as is.

if I chose to go directly to furnace, do I completely disconnect the EIM? What happens with the orange and brown connecting to the relay?

thank you very much, Sir!

 
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Old 05-18-21, 10:26 PM
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Yes... you can completely disconnect the EIM and go directly to the furnace.

The humidifier relay is handled based on what you are going to use to control the humidifier. Since there is a relay there that means either you have a powered humidifier or you are using the transformer that came with it.

You could keep the relay there and connect it to W1 and common but that means the humidifier will only run in first stage heating. If you wanted the humidifier to run in W1 and W2 stages.... you'd need to replace that relay with a 120v coil and connect it to HUM-hot and HUM-neutral.
 
 

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