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Old 12-02-2013, 02:30 PM   #1
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Wiring for Honeywell Equip Interface Module THM5421C to Thermostat RTH8500


So on the interface module and old thermostat everything is wired to 1, 2, and 3. Not exactly sure of what this is equivalent to on new thermostat.

So on the Equipment Interface Module from furnace I have coming in:

R Red
C Blue
W1 White
W2 Yellow
G Green

Wires from Thermo to Equip Interface module
1 red
2 white
3 green

The three wires are the same for the module and the old thermostat. My question is what do these relate to on new Thermostat Model RTH8500? Or do I have to redo and get a whole new Interface module? I have a dual stage fan which I believe is controlled by the white and yellow wires on module.

I do have on extra wire coming into house but have always just used three wires at the thermostat. My initial plan was to use Red to R, White to W, and Green to C.

Thanks in advance for any help, Bill

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Old 12-02-2013, 06:29 PM   #2
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Wiring for Honeywell Equip Interface Module THM5421C to Thermostat RTH8500


The RTH8500 is not a communicating thermostat, so can't use it will an EIM.

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Old 12-02-2013, 07:06 PM   #3
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Wiring for Honeywell Equip Interface Module THM5421C to Thermostat RTH8500


Just to say again, you can't do that combination of EIM and thermostat.
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Old 12-02-2013, 07:19 PM   #4
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Wiring for Honeywell Equip Interface Module THM5421C to Thermostat RTH8500


OK so in looking further I had 5 wires into house where thermostat was and they seemed to be all that was used on furnace it is High Efficiency Nordyne KG71L 080D.

Used on furnace....

R Red
C Blue
w1 White
w2 Yellow
g Green

Thermostat

R Red
c Blue
g Green

w o/b White
w2 e/aux Yellow

Then I thought correct settings in thermostat mods would be
170 4 (Heat only fan)
240 3 (more then 90% eff)

Constantly cycling on/off and the fast setting on blower/fan not kicking in.

Reason to replace LCD messed up on initial thermostat. Would like wifi in replacement not critical and definitely like the blower kicking in on high when returning to cold house. Not as important but nice. So did I totally blow it getting this thermostat?

Appreciate the comments and help. Seems pretty simple but typical home owner not liking results at this point!!
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Old 12-02-2013, 07:39 PM   #5
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Wiring for Honeywell Equip Interface Module THM5421C to Thermostat RTH8500


fast setting on the blower? What is that exactly?

Even though you don't have cooling, the proper setup is:

170=8
180=0
240=3
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Old 12-02-2013, 07:51 PM   #6
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Wiring for Honeywell Equip Interface Module THM5421C to Thermostat RTH8500


Appreciate the input...

Fast setting on blower....my version of two speed fan/blower. When the blower/fan initially kicks in if really cold in house it will kick in super high you can hear much louder, than as it gets warmer the fan/blower kicks down. Not sure how else to word it. Guess it is called variable speed blower..

Seems to be the solution!! thanks to all who provided help...
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Old 12-02-2013, 07:53 PM   #7
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Wiring for Honeywell Equip Interface Module THM5421C to Thermostat RTH8500


With the previous settings it would not go into 2nd stage heat. It should with the new settings

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