Had a mechanic come down today with a pressure guage and he found the leak in (of course) the only place I didn't check - the hot water heater. I installed this new Seaward unit 3 months ago and it has the heat exchanger connections on the back. They're fine, but the unit is sitting in a puddle of antifreeze. He's sure that it has a defective heat exchanger inside that's causing the leak. So it looks like the mystery is solved, so I'm off to call Defender's customer service and get a replacement. Thanks to all who emailed and posted their advise! Jim @ Home Tradewinds 43 currently waiting for the winds to die in Carolina Beach ;)