I'm hoping to replace my thermostat with an Arduino-powered one of my own. The furnace is a Trane XV90 Upflow 2-stage condensing gas-fired one with upgraded inducer motor using Trane Furnace Kit KIT15017. No cooling.
I'm following this guide to understanding thermostat wiring, which recommends:
Measure the voltage between the terminal C and the rest of the terminals. Set your thermostat on the different modes (AC, HEAT…) and check what terminals are energized on each function.
This will give you a pretty good idea of the terminals that you will have to connect to activate the different modes.
So I have two issues:
- No exposed C wire for measuring with multimeter
- Accessing the terminals requires removing the thermostat. Won't this turn off my heat, and I won't be able to measure any activity anyway?
Sanity check passes that the colors/labels align on this end, but it bothers me it doesn't match the wiring diagram from the manual:
For the silver lining, I spy that this end labels "B" as "B/C." Am I on to something?
Second edit: As for how to measure with a multimeter during operation, I postulate I could probe these furnace-side terminals instead of thermostat-side terminals. The system senses the furnace burner door being opened (required to access terminals) with a simple switch, foolable with an inch of duct tape. Now I can probe while the thermostat is plugged in, giving commands, and outputting system status.
Can I get a sanity check that my logic is correct before I proceed? Once I proceed, I can quickly solve the prior issue (whether 'B' is my common, I guess?) by sensing voltage difference between it and the 'R' wire.