I think I have now 2 wires at the box. One in, across the switch, then using the other on to the light.
I assume theres a 'return' from the light to complete the circuit.
(as if one white and one black.)
For a standard switch, newer houses have 4 wires (not including ground wires) at either the switch box or the fixture box and 2 wires at the other. In the box with 4 wires, 2 of the wires connect the two boxes together and 2 of the wires.
I don't have much experience with the knob and tube wiring that is found in older houses, but it sounds like they are wired with only 2 wires at both the switch box and the fixture box. My guess is that the hot wire runs from the electrical panel directly to the switch box, a switched wire runs from the switch box to the fixture box and a neutral wire runs from the fixture box directly back to the electrical panel. This makes things very awkward for using a WS13A.
I gather you are suggesting pulling a 3 wire "Romex" (with two "real" wires and an exposed ground wire ,all in a heavy flexible jacket) back to the panel, other end at the switchbox. What happens to the existing 2 wires?
Yes, although they call it 2 wire since they never count the ground wire. A 3 wire cable has has a Red, Black and White wires plus a bare copper Ground wire. This wire would replace the (black?) wire that goes from the panel to the switch box. You would still use the switched wire that travels from the switch box to the fixture box and the neutral (white?) wire that travels from the fixture box back to the panel. If you wanted you could upgrade the wiring and drag a new wire from the switch box to the fixture box and not use any of the old wires. If you do this, use 3 wire so that you have the option of controlling the light and fan separately from the switch box.
Wouldn't using just one of those for the missing neutral make the X10 switch work? I guess the Romex would look safer than just one loose wire.
You answered your own question. The electrical code in most areas don't allow single loose wires anymore (like you currently have).
If I'm doing all this wiring, maybe there is an easier way to control the fan, other than pulling the cords?
I'd probably be happy just to have the light on the X10, which might be easier if I take down the fan, rewire it so the fan has its power separate from the light? Would avoid running a wire back to the panel. There's an attic with access above the fan, and the panel is about 20 feet away. Whaddya think?
Yes you could run wires from the panel to the electrical box to power the fan (replacing the existing neutral wire) and use a regular wall switch to control the light. It wouldn't be my preference though as most better quality "X10" wall switches require a neutral wire for improved reliability, but if it makes your life easier, that is worth something.
Should this be a separate thread?
Good idea. As you can see, I split it into a new thread.