Yay. I get to learn MUCH more about my water heater than I wanted to. Seems to be leaking from somewhere deep inside. Here's what it looks like when you open it up:
Luckily, the company - Takagi - has a very strong tech support line, and they've emailed me a batch of instructions on how to take apart, inspect, and ultimately replace the part with one they're sending me. Still, a big project that is pretty vital to the house. Luckily, we have another water heater and shower down at the yurt.
Here's the problem area, up inside the thing:
that was hard. There were about a zillion pieces, all connected in strange ways. Somehow I managed to get it all the way out, and all the way back together. and it works!
One thing I'm glad I did was to tape batches of screws together that went for each area, and labeled them. Working over the laundry machine on an outside deck with cracks in it, I was very afraid I'd drop a part and never find it again.