... 15gallons x 8.33lb/gal x (110-60) = 6.2K Btu, about 2 kWh, worth about
20 cents at 10 cents/kWh? That's about $0 per year, which points out the
need for cheap parts. Then again, some people have 4 teenagers at home.
The heat exchanger we are exploring stores 12.5 gallons of fresh water and
54 gallons of greywater, so it should be very efficient with hot water use
in bursts of up to 12.5 gallons. A 10-minute shower with my 1.25 gpm head
uses 12.5 gallons.
That's good heat recovery, altho it might increase if your bathroom were
as cold as your cold water, and letting tub water cool may leave your feet
slightly less clean and require more frequent tub cleaning than real-time
You might reduce the shower temp and the difference between the shower
and drain temps with a low-thermal-mass fully-enclosed shower like this:
Sounds a lot more expensive, with less savings, since this heat exchanger
uses no electrical energy, but a typical heat pump needs about 1 kWh of
electrical energy to move about 3 kWh of heat.
Then again, Florida ACs with desuperheaters used to heat water, and
Powertech Ltd sell ACs that heat water in Cyprus, and PE Drew Gillett
often imagines that a kitchen fridge could heat water, and I often
think about pumping cold water through a $0 SEER 10 window AC and
back into a hot water tap.