A simple heat exchanger from the drain to the cold water running to the shower. A thermostat on the shower reduces the amount of hot water you use as the cold water gets warmer.
Recovering water from the drain to go into the shower again, probably via reverse osmosis.
Second option requires maintenance and a bunch of engineering. Unless there’s a massive lack of water where you live and you have lots of money.
First option… Well, it requires more piping and that the drain pipe is a heat exchanging pipe - a pipe within a pipe where the cold water is in the outermost pipe, running in the opposite direction of the drain water. I’m not sure how much heat you would recover. The floor of the shower might need to be a bit higher than otherwise. And if you have hard water then I wouldn’t do it. When you heat cold, hard water the calcium carbonate precipitates and you get limescale. To avoid reducing heat exchange efficiency and avoid clogs you would have to descale regularly and it’s just not very accessible. With normal descaling you can remove a lot mechanically but here it would have to be all chemical. And how would you even get the descaler into those pipes?
Tl;dr don’t do option 1, only do option 2 if you have very soft water.
I think the usual way these are installed is they go in the basement with the drain portion going between the sewers and the house’s various drains and the fresh water portion going before the water heater to pre-heat the ground-temperature water before it enters the tank (although the DOE’s diagram indicates it pre-heats all fresh water entering the house, thereby also warming the cold water, which is probably great for showering, but not when getting a glass of cold water while the dishwasher or clothes washer are running)
I think there’s two ways to do that.
Second option requires maintenance and a bunch of engineering. Unless there’s a massive lack of water where you live and you have lots of money.
First option… Well, it requires more piping and that the drain pipe is a heat exchanging pipe - a pipe within a pipe where the cold water is in the outermost pipe, running in the opposite direction of the drain water. I’m not sure how much heat you would recover. The floor of the shower might need to be a bit higher than otherwise. And if you have hard water then I wouldn’t do it. When you heat cold, hard water the calcium carbonate precipitates and you get limescale. To avoid reducing heat exchange efficiency and avoid clogs you would have to descale regularly and it’s just not very accessible. With normal descaling you can remove a lot mechanically but here it would have to be all chemical. And how would you even get the descaler into those pipes?
Tl;dr don’t do option 1, only do option 2 if you have very soft water.
This is very much a thing! Here’s a DOE explainer page and here’s a Home Depot listing
I think the usual way these are installed is they go in the basement with the drain portion going between the sewers and the house’s various drains and the fresh water portion going before the water heater to pre-heat the ground-temperature water before it enters the tank (although the DOE’s diagram indicates it pre-heats all fresh water entering the house, thereby also warming the cold water, which is probably great for showering, but not when getting a glass of cold water while the dishwasher or clothes washer are running)
A recovering water system. I can finally have a real golden shower! /s