Voltage potential between white & red
Hello hvac gurus.
I realize this might now be the place to post this, but I'm stuck...
We'd like to power a small device (tiny tiny current <20ma... connected to standard thermostat wiring) using the typical supply from the furnace. Is there any standard, or typical voltage potential measured between white and red? Yes, I realize that C, common, and red would be a better way to provide a power supply, but it seems not every furnace supplies a C to the thermostat. What are your experience?
Can we count on there being a C?
Can we count on there being a 24vac differential between W and R?
Also note that I realize when R and W are connected, we no longer have a potential to work from. In this case I think I've seen designs that use another transformer in the thermostat to then isolate and capture 24vac on the secondary while R and W are connected and the furnace is going.
Thanks!
B