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tomo7 09-27-2013 12:14 PM

Adding to AC Thermostat
3 Attachment(s)
Had central air added this summer and want to connect my heat to the new thermostat that located on the second floor. Pic 1 is the new thermo with AC attached. Pic 2 is the old thermo for heat. Seems straight forward but can someone to confirm the wiring to the new thermo? From the old thermo: white wire goes to the W connector and red wire goes R on the new thermo?

Also, when running new wire to the new thermo for the heater, what wire should I use? 18/5 or 18/7?


gregzoll 09-27-2013 02:47 PM

You would be correct. Just move the W & R to W on the other thermostat, remove the jumper on the new thermostat between RC & RH and connect R from the old one to RH on the new one.

As for wire, standard 18-20ga Doorbell wire, which you can find in rolls from 10-100 feet in the same aisle for thermostats, ductwork, heaters in your local big box store, or in the electrical aisle.

As for that last pic, looks like that is your doorbell transformer. Really would have nothing to do with your hvac gear, unless you know that transformer runs to the furnace or boiler the heating thermostat is connected to.

tomo7 09-30-2013 08:02 AM

Adding to AC Thermostat
Thanks for the reply.

The reason I ask about the 18/5 or 18/7 wire is that the system is old and will need to be replaced in the next few years. Do the new systems require 5 or 7 wires or will 2 wire suffice? I want to run the correct wire the first time and not have to rerun it again when we install a updated furnace. Thanks

gregzoll 09-30-2013 11:32 AM

It does not matter how old the system. What matters is how many connections. Just purchase a 4 pair roll (8 wires inside the jacket), and you will be fine. If you plan on updating to say a heat pump, or 2 or 3 variable speed furnace, and 2 speed a/c or heat pump, then yes account for that number of connections, and purchase enough wire for that.

Usually you would have to pull two runs on anything over twelve connections, due to the wire will get thicker with more wires in the jacket @ 18 ga.

I just used Cat-5e for connecting our thermostat to the furnace. It is 20ga, and let me have enough pairs for it. It was what I had laying around when I got the 3m-50 Thermostat I bought at that time.

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