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Originally Posted by dave.irwin.59@gmail.com

I have replaced both of mine with Honeywell residential units.

You must use a replacement that does NOT require a "C" wire. The right one will typically require batteries (instead of powered by then "C" wire.

The only other advice is to not follow the colors of the wires in the wall. Cut the wires (they are hard wired) near the old thermostate and match them.

The Honeywell thermostat should have terminals for 5 wires: "Y," "G," "Rh," "Rc," and "W." Note the jumper between the "Rh" and "Rc" terminals? Leave that jumper installed.

This will operate your heat and A/C, but you will loose the Hi/Lo fan option. I opted to wire in order to use the fan on high as follows:

Red wire to “Rh” terminal (Rh and Rc utilize a jumper)
White wire to “W” terminal
Yellow wire to “Y” terminal
Green wire to “G” terminal
Gray (not connected)
Blue (not connected)

Hope this helps.

X2 on this. 5 minute job