Atwood makes a Water Heater that has a Heat exchanger built in to it. Nobody uses it anymore but they still make it.
Model GCH6A-10E
http://www.americanrvcompany.com/Atwood ... Trailer-RVIt was originally designed to heat water in a water heaters tank from a Motor homes Engine coolant system. As I said, nobody uses this system anymore (That I know of)
In the mid to late 80's a few Manufactures used this product in a different way. Allegro was one.
They would take the "In and Out" on the water heaters exchanger, create a sealed system , filled it with Anti Freeze and connect it to the "IN and out" in a heater core. Some of these Heater cores had Fans, and some were Gravity systems. (simple explanation)
As long as the Water heater was on, you got heat to the Heater Core and it was simply shut off in warmer months.
Now, This was definitely a "Lowering the River instead of raising the Bridge" approach.
But!
They worked! Despite there Logistical Problems
How ever over time were no longer used.
Happy Trails
Len