Alan, you are correct ... our codes have required furnaces to have their own dedicated circuit for decades. That is, the circuit needs to supply nothing but the furnace and associated equipment (for example, a hunidifier or air cleaner). Or, with proper switching, an air conditioner can use the same source of power.
This house, though ....
The panel was on the outside of the house, and the circuit ran up the wall to the attic. Where else it went on it's journey through the house, back to the basement, is anyone's guess. All I can say for certain is that the light, and several other things, went out when the heating guy shorted the wires to ground.
A reminder ... he didn't do that deliberately ... rather, he opened the disconnect, adn shorted the wires while attempting to remove the stack controller. They were still hot, as the disconnect was on the "neutral" side of the circuit.
You often find the neutral and hot reversed in knob and tube installations, as the insulation is often all the same color: black. This house was the exception; there were cloth-covered wires in black, red, green, and white.
Lest one become confused ... remember, Knob & tube wiring pre-dates then use of green wire for grounding. In such a house, the green is very likely to be hot! When I had sorted out the wiring, everthing was hot ... EXCEPT the black, which was the neutral!
I 'solved' this one by running a new, dedicated, circuit to the disconnect, and re-wiring the furnace. Now, the furnace just may be the only code compliant circuit in the house.
