Let's say you were going to make a heating system for a driveway. Using a two-circuit system, that is one circuit: Ethylene glycol-water circulating through the pipes under the driveway and also through a heat exchanger. The other circuit: Regular house hot water system. But it also has a loop going through the heat exchanger, and you use, say, twice[1] the normal heating capacity.
What would be a good way to make the water heat exchanger? Is there something commercially available?
I don't have any current plans to make such a system. I'm just contemplating.
[1] Just guessing. I'd have to figure out the size.