Typically water heaters weren’t as efficient as boilers, but a lot cheaper. Just by design they’re simpler, but I’ve heard of people doing it, and for larger applications they would be used in tandem. The company Hydrotherm did some stuff with that theory, and they make sequencing controls for commercial boilers. They made a gas unit called the Hydropulse, it was a condensing uit rated @ 90K btu, (I think), if you needed more, just add another, the units looked like DHW heaters. Of course copper is copper and you can always use it for potable water, it’s the pipe thickness on hydronic heating components that is the problem. Since hydronic systems max @ 30psi you don’t need a heavier wall tubing, by add some pressure like 75psi+ and the thin walled tubing will split under domestic water pressures. As long as DHW mixes with cold, you’re drinking it. Do you ever temper your tap water by mixing cold and hot?