I looked into waste water heat recovery a few months back and it is a thing you can buy. You buy a special section of pipe that has a straight pipe with another

I looked into waste water heat recovery a few months back and it is a thing you can buy. You buy a special section of pipe that has a straight pipe with another pipe coiled around it. Some of the heat from the waste water gets transferred to the water coming in as they flow past each other in the pipes. They cost a few hundred bucks. I have no idea if the long term savings justify the cost (if you're looking purely at money for benefits).