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A comment on Hitachi launches CO2 heat pump water heaters with solar-friendly tariff controls. Parent comment

Re:Hitachi launches CO2 heat pump water heaters with solar-f... by gandreani on Thursday September 10, 2026 @09:39AM (#49643552)
Holy crap that re-circulation feature is luxurious!

I'd love to have something like that!

Whenever I take a hot shower I fantasize about putting in a liquid-to-liquid heat exchanger so the hot water can heat up the cold water and you can run the hot water at a much lower temperature.

This application with the bath tub feels way more justified.

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Re:Hitachi launches CO2 heat pump water heaters with solar-f... by bluGill on Thursday September 10, 2026 @10:33AM (#49644404)
Many people have thought about the drain heat exchanger. However drains need to be way oversized for various reasons and so you can't get good contact with the water: it is mostly air in the drain pipes.
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Re:Hitachi launches CO2 heat pump water heaters with solar-f... by shellfishgene on Thursday September 10, 2026 @11:12AM (#49645152)
That does exist. Simple versions just have a large, deep drain with coils the cold water runs through. Fancier ones have a proper heat exchanger next to the shower (requires a pump) or in the basement. Water sanitation rules probably make it non trivial to get a design certified.
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Re:Hitachi launches CO2 heat pump water heaters with solar-f... by gandreani on Thursday September 10, 2026 @01:40PM (#49647546)
Is the deep drain to diminish the air pockets at the exchange surfaces?
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Re:Hitachi launches CO2 heat pump water heaters with solar-f... by shellfishgene on Thursday September 10, 2026 @04:31PM (#49649765)
It's just so the cold water pipes sit in a pool of warm water. https://www.meanderhr.com/wp-content/uploads/2012/11/HXdrain...

This is even simpler: https://warmduscher.de/produkt/warmduscher-original/

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Re:Hitachi launches CO2 heat pump water heaters with solar-f... by kalleboo on Thursday September 10, 2026 @09:45AM (#49643642)
And this is the builder-grade model we got from the house renovation company. There are higher grade ones with fancier stuff...
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Re:Hitachi launches CO2 heat pump water heaters with solar-f... by londons_explore on Thursday September 10, 2026 @10:30AM (#49644364)
Presumably you need a bath with extra plumbing? What happens when the heat exchanger gets clogged with soap and hair?
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Re:Hitachi launches CO2 heat pump water heaters with solar-f... by quickthrowman on Thursday September 10, 2026 @05:51PM (#49650626)
Here you go: https://www.homedepot.com/p/Hydro-Systems-Fusion-Low-Voltage...

Drain heat exchangers are really expensive, this 3” by 48” copper one costs over $1000: https://www.homedepot.com/p/Power-Pipe-3-in-x-48-in-Drain-Wa...

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