Looking to switch from an old Honeywell CT87A thermostat to a smart thermostat, probably something requiring a common wire. There are 2 thermostats (one on each floor) in a 2 zone forced hot water oil burner system, heat only.
I want to make sure I have the right wires before buying a new thermostat, and was worried with an old system like this I'd have just 2 wires. But looking at the existing wiring, there are 4: red, black green and yellow.
- Red and Black wires are on the thermostat's R terminal,
- Green and Yellow on the W terminal
- nothing on the Y terminal
Looking up thermostat wire color charts, I am now very confused as I have no White wire which is pretty much always shown as being for heat.
Can I assume either my Green or my Yellow is actually for heat, since they are both connected to the W terminal?
Assuming my Red is for power (since that's consistent with 99% of color guides, and it is connected to the R terminal), is my black wire a common?
Can the wire roles be deduced in this situation just from the terminals they're connected to? Is there a way to find out for sure?
On the boiler end, there's only one thermostat-gauged cable coming from the junction box (red arrow):
And that one cable goes from the junction box to the following taco box, and has just black and white wires: