Smart WIFI Home Thermostats and the Notorious C-Wire

This is a short note for anyone struggling to install a new “Smart” WIFI thermostat in their home.

We installed a new Carrier heat pump model 38YRA in our home in 2001. I soon replaced the simple thermostat that was installed with the Lux 1500 with a 5-2 day programmable thermostat so we could take advantage of SRP’s (our local power monopoly) time of usage savings plan.

Numerous vendors started offering Black Friday specials on Honeywell Smart Thermostat 9000 series with color touchscreen for $99 (list price $199). Then I noticed that many local power providers, including SRP were offering a $50 incentive to install “Smart” thermostats so I ordered a Honeywell unit. Included in the box was a C-Wire Addendum. It seems that to power the new touchscreen WIFI thermostat you need full time 24V power which is delivered on a wire labeled C. Except my wiring at the thermostat only had 4 wires connected to G Y B-O and RC-RH on the old Lux 1500. There were 3 additional unused wires at the thermostat but none provided 24V power. The 3 wires were not connected at the Carrier unit installed in our attic during installation.

So I climbed into the attic (difficult) and removed a large cover panel from the Carrier unit and discovered the transformer that converts 120V to 24V and a connecting terminal board with labeled wires R C O W2 and Y (5 wires). Terminal C was never connected to a thermostat destined wire. So I arbitrarily picked the unused brown wire and connected it to the C terminal. Like magic I now had continuous 24V power and the new “Smart” thermostat powered up.

Why didn’t the original installers in 2001 attach one of the unused 3 wires to the C terminal and save me and millions of other installers a lot of trouble installing “Smart” thermostats? Clearly the C wire existed in 2001.