[thebluehawk]

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.

[frandroid]

The closer you get to the equator, the more you're likely to see these. When I was visiting Turkey, I didn't notice them in Istanbul because most rooftops are not visible from the street, but then I visited Safranbolu, a small tourist town, and there almost every century home had this recently-made metallic hot water tank on top which completely destroyed the bucolic architectural character of the town.

One of my uncles here in Canada built a passive system by just putting a giant 1" PVC tube coil on the backside of his roottop to heat up water for the swimming pool, since the swimmable season is pretty short anyway.

[OkGoDoIt]

Not sure if it’s exactly what you’re talking about, but something similar is pretty common in China as well. I’ve heard people call the system “solar power“, but what they’re referring to is effectively a tank of water on the roof that is heated by the sun which is used as the hot water source for showers, etc.

[xyzzyz]

It is very much solar power, it's just not fancy high-tech photovoltaics.

[csours]

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

I think it might be, if it was built into the home from the beginning, and the engineering cost was spread out over a large number of units. In other words, I don't expect it to make sense for one homeowner to retrofit their house.

It would make more sense in a condo/apartment/multi-unit/commercial situation.

[ip26]

Solar water heating has been on the decline recently because photovoltaics & heat pumps surpassed them thanks to ridiculous scale & modularity.