Thermostat Wire Colors

An email came by my inbox today asking about wiring a Tstat10 in to an existing system which is using the R, G, B, W1, W2, Y1, Y2 naming convention. Its a good question so am putting some effort into a thorough reply.

The short answer is that since the Tstat10 is a programmable thermostat any of the outputs can be wired to any of the relays on the stat. The program will then associate the relays to the various functions of the existing equipment.

My convention is to use output1 for the fan, next comes the heating stage1, stage2 and reversing valve if present. Then finally the cool stage1 and sometimes stage2.

Here’s a more complete list of the wire names you may come across in the field.

And here’s a random thermostat backplate taken from the internet to show the example in more detail. Starting with the red wire at the top right and moving clockwise, R is the 24VAC from the mechanical equimpment. This is used to power the Tstat10. Next comes an orange wire which is labelled as both W and O/B, so after a bit of reading I see that orange wires and the O/B label are normally used by the reversing valve. The wire is also labelled as W which is ‘primary heating’ in the chart above. We would need to consult the equipment data sheets to confirm the full details but since the outputs are controlled by our program its easy to change once we have it wired up to the Tstat10.

Next is the yellow wire and labelled with a Y which is the cooling. Then comes the green wire with a G label for the fan. C is blue and is the GND of the 24vac from the equipment, it is also used for the 24vac supply to the Tstat10.

Moving around to the other side is the white wire labelled W2 which is heating. Finally there’s the black wire on the E terminal which is the emergency heat, from my reading this is only turned on manually by the user in extreme cold weather. If it is turned on automatically by the thermostat it would be laelled with an X for auxilliary heat which is not present here.

And finally, here’s the wiring diagram for the Tstat10 to the existing cabling. You’d need to take off the existing backplate and install the Tstat10 backplate. The wiring is all low voltage so just tape off the 24VAC hot while you are working. Better yet power down the equipment at the main power switch of the unit.

Notice the jumper from terminal 16 to 17. The thermostat relays are isolated from the thermostat power by default. Most of the time you will just jumper the 24VAC over to the relay common on terminal 17 which in turn is fed back to the equipment through relays K1 to K5.

All that remains to be done now is create a program for the application. There’s an example program for two heat/ two cool on this forum which can be easily tweaked for single stage cool, two stage heat. I see that its not 100% complete so will work on that later this week.

All the inputs on the Tstat10 are spares so if you can get a wire in from the stat to the equipment you can monitor all sorts of parameters on the heat pump like a dirty filter alarm, air flow switch, current sensor showing the power draw, refrigerant temperatures and outside air temperature to name a few.