trader_4 wrote, on Mon, 22 Dec 2014 08:21:13 -0800:

No, you can reverse the hot and neutral on AC. But as I said before,
it's virtually impossible for that to have happened and I doubt anything
to do with the power to the furnace is the problem.

I agree that hot and neutral *can* be reversed, but, from a practical
standpoint, other than the safety of two-wire grounded items, it
makes no difference.

So, when they're talking polarity reversal in the continuous-blink
scenario, I wonder if they're talking about the DC voltage, where
polarity can make a huge difference.

I doubt either one was reversed, simply because nothing was touched,
but, methinks they may be talking DC and not AC anyway.

Nonetheless, I'll pop that box off in the middle of the furnace
and measure the voltages (yes, I know what all the colors mean,
including white, black, red, if it's there, and green or bare).

I didn't check if it's 220V, but that just means there will be
two hots and a neutral, instead of a hot and a neutral.