I think I know the answer to this, but I wanted to get some conformation before I cut into my wall.
I installed a direct vent gas system a few years back. Over all, the system's been working great. However, I've always had a problem with the wall switch (which is a millivolt switch) not working. It seems that it will work for a bit, then static will build up in the line and then it will stop working until I pull the switch out and discharge the static. Then it works fine again. This is true even if I pull the switch out and just connect the wires directly.
I think I know why, but I wanted some more opinions on this. The run is probably less than 5 feet from the fireplace, and I was careful to keep it separate from the 120V lines. However, I ran the line into a double gang, because I wanted the fireplace fan switch next to it (stupid, I know). However, the fan switch is a 120V line and I'm guessing I'm getting some induced current from that line in my millivolt system, causing static build-up. I'm guessing moving the millivolt line a little ways away will probably fix the problem. Don't know why I didn't think of this when I was installing it . Anyway, does this seem like the probable cause, or could there be some other factors at play?