[todd8]

In grad school, many years ago, I would plug the drain and let the tub fill while I showered. I would pull the plug and let the tub drain once the water had cooled. This way my bathroom would warm up a bit. I suppose this might have saved a few cents.

[turbonaut]

These days you can get passive ‘waste water heat recovery’ systems that sit beneath a bath or shower tray - warming your inbound cold water with your outflow, so you use less new hot water (or have to heat it less).

[euroderf]

I'd be curious to know how these systems manage with dissolved soap, partially dissolved skin cells, hair, etc.