> > Probably what you are hearing is the control transformer in
> the furnace.
> > Your thermostat and controls runs on 24V. There will be a small
> > amount of eddy current loss in the transformer, but that's about it.
> Probably 10 watts or so. (If it's old, maybe lots more.)
That seems way high to me. Whatever it dissipates is converted almost
entirely to heat; and 10 watts into something the size of a control
transformer would cause it to get quite warm even in free air. Mine is some
40 years old, and just barely gets warm to the touch. Might be 1 watt, but
I doubt even that.
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