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gotoman for ac, HVAC Technician
Category: HVAC
Satisfied Customers: 5203
Experience:  51 years of experience install design and repair of all makes and models of AC
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I replaced an old mercury switch thermostat with a new Honeywell

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I replaced an old mercury switch thermostat with a new Honeywell RTH6580WF and connected the wires according to the 'heat pump' directions in the Honeywell manual. I have a Carrier Model 40AQ03000BB (Inside) 38QN030331SM (outside)

There are eight wires and they were connected to the following areas on the mercury thermostat:

Orange to O
White to W2
Black to C
Tan to L
Yellow to Y
Red to R
Green to G
Blue to E

Based on the directions in the manual, I connected the wires this way:

Old Connection/Color/New Connection
E/Blue/W2 (Aux/E)

When I turn on the cool function (Inside temp reads 80 and thermostat set for 75) the heat kicks in and hot air comes out of the vents - even though the thermostat display reads 'cool on.' I tried the opposite - set it to heat and set the temperature to 85 but nothing happened. I am able to turn the fan on manually from the thermostat but can not get the thermostat to turn on the A/C.

Other than not being able to control the temperature, the thermostat works fine. Registered the unit and connected to the WiFi with no issues.

After reading about others with older units and this thermostat, I disconnected the blue wire (E) completely from the thermostat, but there was no change in the function - trying to cool still blows out hot air and trying to heat does nothing.

I tried calling the Honeywell help desk but after listening to the 'all of our operators are busy' message for 30 minutes I figured they are closed for the holiday.

Any thoughts or ideas on how to wire this thermostat so it will work with my system will be greatly appreciated.
Russ here.. Trane Service Manager. Go to installer setup 1 option 5. Touch fan and up arrow for 4 seconds ,hit next till you get to 170 use arrows to set option 5. hit done when finished. Disconnect tan wire and tape off. Russ
Customer: replied 4 years ago.

Hi Russ,


When I go into the installer setup, the numbers cycle through 1, 3, 5, 12, 13, 14, 16, 36, 38, 39, 42, 85, & 90 then start back over at 1. There is no 170.


Should I re-connect the blue wire to the E connection when I disconnect the tan from Y2(L)?







sorry dealing 4 Honeywell tstats at once setup 1 option 5
Customer: replied 4 years ago.

OK, blue wire (E) is also disconnected - should I reconnect that to E when I disconnect the tan wire?

Just leave E disconnected tape off wire and place a small jumper wire between W2/aux and E. Set option and you are ready to cool. Russ
Customer: replied 4 years ago.

On the RTH6580WF, the W2 and AUX/E connector are the same connector. It's just labeled W2 at the top and AUX/E at the bottom. So no jumper needed.


So now the blue wire and tan wire are both disconnected.


Setup 1 is set to 5


The 'cool on' tag blinked for about 5 minutes before the A/C finally kicked in - THANK YOU! Is it normal for there to be that much of a delay? With the mercury switch it turned on right away.


And just to confirm - the blue and tan wires aren't needed for anything - not even heat related functions in the winter?

No not needed for eat in winter. . 5 minute delay on all thermostats. Good job. Russ
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