That's a compelling idea. I've seen systems that use essentially solar panels on the roof that heats up a liquid in a loop then goes to a heat exchanger which heats up your water before it goes to the water heater, thus making it so the water heater doesn't have to work as hard.

Could we use the exhaust from your fridge, A/C, furnace, etc. to also heat coolant in a similar system that circulates to your pre-water heater, thus boosting efficiency. Especially if you can dump the exhaust of your fridge outside rather than paying to heat up that air, then cool in down again with your A/C.

The question is if the cost savings would be worth the cost of the system.