I'm replacing a builder-installed ceiling fan. The wall switch is standard ON/OFF single switch (NOT 3-WAY). I haven't taken the switch plate off, but I'm curious about...

At the ceiling "fan rated junction box" the BLACK is twisted with a RED wire, then the usual neutral (WHITE) and ground (green).

What is the RED wire for? There is no separate switch for a light. Since it's a "tract-built" home there must be a standard purpose for this RED wire, and why would it be twisted with the BLACK?

Need to know all wire in the box, they could have run out or red or black, if it worked like this leave it alone.
You may find that is a constant hot and another wire is the switchleg.

Yes, I agree that I should leave it alone. If it ain't broke...

Those are ALL the wires coming out of the box. I took the switch plate off and the red wire IS switched. When the switch is off, the twisted red/black is NOT hot; meaning neither is constant hot, yes?

I'm still curious, even though I know it will work fine the way it is.

I've seen a number of "track" homes where 3 wire cable was run between the switch and the ceiling box. Indeed, I've seen homes where the switch, wiring and ceiling box were installed by the developer with no fixture installed. Particularly where there is vaulted ceiling and no attic space. No developer gives you anything he doesn't have to, so electrician was probably required to do this so builder could offer an over priced option of an installed ceiling fan/light.

The other end may be capped off in the switch box. If the red and black are in the same cable, they should not be twisted together, You can't Parallel Conductors unless it is under 50 volts, or larger than 1/0. Cap 1 wire.

I looked in the switch box and nothing is capped off. There are a couple of red wires, but no three way switches. This box has just two single-pole switches; one for a wall outlet (for a table lamp, and the other for the ceiling fan).

If you're curious, get a voltmeter. Otherwise, if it isn't broke.. .

What wires go to the fan switch?