I bought a honeywell ct8775c1005 thermostat model to replace an old 1970's thermostat. HOwever, there are three wires that connected the old one that controlled heating and air conditioning and the fan. I tried connected the new one and it worked for heating, but NOT AC. I connected the red wire to R, White to W, and Green to G.

The digital temperature works and heat works when I turn on the Heat, but when I switched to COOL on the mainswitch in the basement after the winter, the temperature reading disappeared, and the AC does not turn on.

Should I connect the red to Rc?? How can I get both the heat and AC to work? Thanks

You need 4 wires to control the fan independently. Now I see, the heat/cool mode is selcted downstairs. You need to jump Rc to Rh and connect to red.

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Originally Posted by KeepItSimpleStupid

You need 4 wires to control the fan independently. Now I see, the heat/cool mode is selcted downstairs. You need to jump Rc to Rh and connect to red.

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So does "JUMP" mean I have to split the red wire and connect it to both the Rc and the Rh with that red wire?? ALSO, do I keep the Green wire on G and the White wire on W??

Thanks

Well, it means that there needs to be a wire from Rc to Rh and Red connects either terminal.

Electrons don't care what color the wire is. Yep keep the colors as you have it. Without Rc connected you won't have cool.

So then can I take the red wire and wrap it around Rc and Rh with the same wire like doing anm 8 figure with the one wire??

Yep.

I connected the wires like you said, but it still doesn't work. I connected red wire to Rh and Rc... Green to G... and white wire to W. There's a slot for Y which is supposed to control COOLING, but I only have 3 wires.

The heating seems to work, but when I switch to COOL downstairs, the digital reading on the thermostats disappear. There are two thermostats I installed replacing the old ones. DO you think one of the thermomemters should have the White wire connected to Y to control Cool, while the other one should connect the White wire to W to control heat?

Can the white wire be connected to BOTH the W and Y??