When I got home, I could smell it outside on the porch. Sure enough, this wasn't just a fried housefly. Maybe it was coming from elsewhere? Coming into the house disabused me of that notion, and the smell was pretty strong. We poked around for a little bit, but the only big sources of heat are the furnace and water heater, so we went into the basement. Following our noses, the water heater seemed like the source of the aroma. Note: we have a tankless water heater that heats the water as needed with a huge powerful gas burner. So I pulled off the cover and had her fetch a flashlight. Sure enough, when I peered inside, I could see the scorched remains of a small snake on the burner. And it STANK! I started poking around to see if I could extract the remains, causing her to wonder aloud if the snake might bite me. No chance of that, it was well and truly cooked. I eventually determined that I would have to remove the heat exchanger in order to get the snake out.
The first steps were pretty obvious, remove the retaining bar from the vent hood and uncouple the water feed pipes. Having done this, I realized the water feed pipes actually extended up into the exchanger and would have to be removed entirely. So I took them off and tugged at the heat exchanger. The water couplings extend through support brackets, so it would have to be tilted out of the hood retaining hooks and lifted out of the brackets. This involved separating the hood from the rest of the unit. That done, it still didn't want to come out. I finally had to brace myself over the unit, get a good grip on the preheating pipe that ran around the outside of the heat exchanger, and just power the thing out.
Once out, I inverted a plastic bag on my hand, and using it as a glove, grabbed the crumbly ashes of the cremated snake, and reversed the bag around it. Then I scrubbed the residual debris off the burner with an old toothbrush (I keep old toothbrushes for just such purposes). Now to reassemble the beast.
I put the heat exchanger back into place (it went in much more smoothly than it came out), reattached the vent hood and retaining bar, and went to bolt the feed pipes back into place. I did the front fittings first, as the rear ones are aligned by their pipes extending up into the heat exchanger. Then I tried to get the nut on the heat exchanger. Wouldn't go, felt cross threaded. As the heat exchanger (and its threads) are copper, I wasn't going to force anything. I tried and tried, but no matter how I attacked it, it wouldn't go on. I disconnected it from the front to give myself more freedom, still no dice. I had fizzygeek try it, but it wouldn't go for her either. I tried it from the back (reaching through the sharp sheet metal) for another angle, but no luck there either. Finally, I decided I would have to remove the feed pipe entirely to see if getting a good look at things would enable me to figure out what was going wrong. So I pulled off the wires going to a sensor in the pipe and had a look. Sure enough, the washer had deformed over the years of pressure and heat, and was keeping the nut from getting to the threads properly. As it wasn't an ordinary washer, I didn't have any around. So I got out my Leatherman and trimmed off the excess. After that, both pipes went on without too much trouble. One joint wanted to leak, but some extra torque convinced it to hold water.
I think I'll order a washer set and valve rebuild kit just in case.