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

You should have 24 volts AC between the red and white. You could wire in a high resistance, low current, load between them, but it would supply current to the heat circuit in the furnace. It might or might not be enough to fire up the furnace. I am not sure having a constant low voltage to the W input at the furnace would be good for the circuit board. Some thermostats may do that to power themselves, but I am not sure how low is low. It might be simpler in the long run to pull a new cable if you have to, or find another source of power.

Go to the furnace and open up the panel with the filters. Usually the control wires are right there. Look and see if the thermostat cable has a blue wire. Even if it isn't connected to the com, you could connect it and then use it at the thermostat. And there is nothing magic about a blue wire. Any unused wire would do.

labman,

Many thanks for your response. I really appreciate it. I've been experimenting with ways to power our device in a way that doesn't require batteries, and still works with lots of different installations of existing furnace systems. Many modern thermostats seem to be just battery powered, others use the 24vac supplied via Common and Red. Some also seem to do it with the Red and White and just use a low resistance transformer with R and W connected on the primary side (allowing current to flow... thus turning "on" the furnace) AND also having a high resistance load between R and W to get power when signaling the furnace to turn "off." Does this align with your experieces?

I agree than just connecting the blue wire to C at the furnace controller (and using R and C, rectifying, etc.) is the easiest way to do it, but its not a very nice use case for the average homeowner. This is what I had to do at my home. In your experience, how many installations out there actually have the (typically blue) wire connected to Common?

Do you know how I could get more information about what an acceptable current level from R to W might be -- without going over some threshold and turning "on" the furnace?

Many thanks again,
Brian
Brian "-at-" ecosynse "~dot~" com

I think the standard thermostat cable has red, white, yellow, green and blue, although most thermostats don't use the blue. Judging by the questions here, I am surprised at how many old 2 wire set ups there are out there. Heat pumps and thermostats controlling 2 stage heating and cooling take more wires.

You might do a search on "voltage stealing" either here or on the net. Even if you find thermostats that use the white as a common, likely they won't give technical details about the voltage used.

Northern Heat might be a much better source as far as what you typically find. He is in the business. I am just a homeowner that occasionally shares some of what I have figured out the hard way in my own house.