Those panels can accumulate scale - which may affect the performance - but not overflow.

The drain tube may be clogged with scale or slime - is it easy to get at? replace?

If the solenoid was stuck open the water would run all the time.

Thanks, Doug. The two times the drain has overflowed has been when it was very cold out and the furnace was working almost non-stop. The drain tube wasn't clogged, but it has about a 10' horizontal run before it hits the drain.

It's been working fine since I posted a couple of days ago, so all I can think of is that the drain tube gets backed up under continuous use. Odd...

Does it have enough slope over that 10' run? Could it be partially blocked with crud?

You said it, there is not enough slope as the tube runs across the floor crossing through a doorway and walkway. It doesn't seem to be blocked as I can blow through it. I've been thinking of trying to reroute it, but so far no easy or close solutions.

Thanks!