[Richard]

This is my opinion, and I have not laid eyes on your specific tank. First, you definitely want the incoming water to be on the outside loop. Second, I think that a complete encapsulation of the mastic around the original coils and the new coils will work better than the original concept which was to use the mastic only where the coil touched the tank.

I have read the mastic is runny until it sets. I would think about using something to act as a "form" to keep the mastic from dripping away from the coils while it sets. The key will be to get complete contact of the entire coil with the mastic. Something like heavy duty aluminum may work. Fine weave fiberglas cloth would work, but would remain in place when the mastic cures.

I have gotten really excited about the use of a double walled flat plate heat exchanger to augment the existing hot water loop. I know those things are amazing when it comes to heat transfer. The double wall design is made for domestic hot water heating so you have some protection against glycol contamination of the drinking water. I don't know that I would do it exactly the same way that the guy did in the link I posted. I would mount the FPHE on top of the AH cover. I also would rob the electrical circuitry associated with the engine preheat pump to power the circulation pump for the FPHE. I never use the preheat. If the concept works, then I would figure out something to have it come on when showering. The reason I don't want to use the burner on as a trigger for the pump, is that it commits you to use the diesel for showering.