[Phantom]

I'm partially in the same boat as bschloop. I think that one of the best ways to get the heat from the exhaust would be to use a larger pipe.

If you go from a 3" exhaust pipe to a 3.5" and run tube inside with a size of 0.25" and the tube covers the complete stretch it should give you an effective 3" to 3" to 3" pipe.

I think that it would work best running the tube as a spiral with the inlet at the bottom of the tube shaped as a "J" and have the outlet on the top side of the pipe. Air bubbles getting trapped in the pipe should not be a problem as long as there is enough water in the system so that the inlet side water level never drops below the highest point in the loops. If the water on the inlet side is cooler than further down the pipe it will push/pull from the temp difference.

If you are really worried about trapped air seal the system and place a vacuum on the outlet side to promote the movement to the lower pressure side and place this pipe on the exhaust down-pipe so the inlet side will be much lower than the outlet side. The only issue I see there is that the hotter part of the pipe will be getting the heat first but that could help as water takes on heat 8x faster than air.