Need C-wire for new Honeywell RTH6500WF Thermostat

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Frank8487

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I am upgrading my old non-wifi programmable thermostats with Honeywell wifi models. These need a C-wire and my wiring only has 4 conductors and no C-wire (see image). I contacted Honeywell support and they suggest using a C-wire adapter, but it appears my system's air handler units for heating/cooling do not have a control board with a terminal block to connect the adapter to, I opened the side panel. They are First Co units that were installed when the house was built 30 years ago. There's a unit in my basement, and one in the attic. All the wiring connections are mounted on the outside of the units. Only 3 wires go into the unit beside a separate AC power line. Can I still use a C-Wire adapter with this configuration? What are my options to provide a C-Wire to my new thermostats?
 

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Frank8487

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In the wiring diagram, the brown wire appears to be the common, so wondering if I should just make a new run with 18/2 to route the brown wire to use as the C-wire on the new thermostat? But that would be much more effort than installing the c-wire adapter. :(
 

Gagecalman

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I was trying to understand how the adapter functions. It must make one of the 4 wires a common and change the function of one of the others.
The schematic doesn't look like it matches the wiring.
 

WorthFlorida

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I never tried one of these. Resideo is Honeywell's thermostat source. The installation instructions are quite good but takes a while to absorb it all.


 

Fitter30

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I was trying to understand how the adapter functions. It must make one of the 4 wires a common and change the function of one of the others.
The schematic doesn't look like it matches the wiring.
They are confusing with using 5 wires on the furnace side and 4 wires thermostat then use terminal k for y. In your case because they didn't use traditional thermostat wiring and colors just throws another level.
 
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Frank8487

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Thanks everyone. Tomorrow, I am going to wire back the brown common to the thermostat. I would rather not use the C-Wire adapter because you lose control of you fan control.
 

Frank8487

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Could use the adaptor or run 18/2 or 5-6 wire t stat wiring.
Guess you have hot water he

I was trying to understand how the adapter functions. It must make one of the 4 wires a common and change the function of one of the others.
The schematic doesn't look like it matches the wiring.
It combines the G and Y wires into a single K wire that controls the fan and AC.
 
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