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Even more effective: drain water heat recovery, preheating the incoming water with the outgoing waste heat.

Quick overview from one manufacturer here: https://renewability.com

That's just a heat exchanger. I wonder if it's worth it tho.

Perhaps only if you fill a large tub or a pool every day.

These devices actually require a similar amount of water to be moving through the drain pipe/heat exchanger and the cold water line that's refilling the water heater. So they're really only effective during showers.

DoE says they breakeven is 2.5 - 7 years. That's in the US, imagine how much of a slam dunk it is in countries with carbon prices and higher energy prices.

https://www.energy.gov/energysaver/drain-water-heat-recovery

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