Heh. Good point. That's the right thing to do if it's not sitting on the lowest level (basement in my case) with a floor drain nearby. And codes have required those overflow pans for a long time. If you didn't add a pipe to the temperature-pressure relief valve to carry water down to the pan in the event it opens, you should do that, too - and not just to catch the mess, but also to divert any blast of steam and scalding water away from face-height of anyone nearby. Also a code requirement. Of course some areas don't have any codes, but those are good things to do, regardless of any statutory considerations.