Older Carrier 58GP furnace AC wiring problem


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Old 05-26-18, 11:27 AM
J
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Older Carrier 58GP furnace AC wiring problem

Hello;
I just finished installing a new Honeywell Programmable Thermostat(RTH2300) on an older Carrier 58GP furnace with AC. There was only 3 wires, so I switched it out for 18/5. While I got everything working, I'm confused as to why it is working. From what I understand, the AC compressor relay should be connected to C & Y. But the outside compressor doesn't start when its wired that way. Instead, the AC Compressor works when its connected to the C and "GC".
Furnace Terminals
C to AC Relay
Y Yellow Wire to Y on Thermostat
GC Green wire to G on Thermostat & to AC Relay
GH NC
R Red wire to R on Thermostat
W White Wire to W on Thermostat
From what I understand, R=Hot 24VAC, W=Heat control, Y=Cool Control, C=common 24VAC, G(or GC in my case?)=Fan Control. What am I not getting?
 
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Old 05-26-18, 02:43 PM
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I agree with you. The condenser must be connected to Y and C.
You have wired it incorrectly. The most likely reason for your symptom is the 5 minute timer that the thermostat has is holding the condenser out for 5 minutes and you just didn't wait long enough.
 
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Old 05-29-18, 10:01 PM
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Thanks Houston204. I need to try connecting it correctly and give more time to kick in. Too damn impatient I guess.
It all makes sense now as I actually looked at the schematic. The Y Terminal on the furnace is just a dummy terminal. Its basically just a convenient place to splice one side of the Compressor Relay with the Y term on the Therm.
 
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Old 05-30-18, 12:11 AM
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On some furnaces.... a connection to Y causes the blower motor to run faster.
On yours.... Gc = blower/cooling speed and Gh = blower/heating speed.

So the Y is a dummy as you've found out.
 
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Old 05-30-18, 06:03 AM
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Those old Carrier boards were strange. Gh is actually connected to R so most of the time not used.

If Y from the a/c and thermostat are both on the Y terminal and things not working then you may have a break in the wire going back to the thermostat.
 
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