One thought is that if you live in a cold climate it might be easier to recover the heat from the showers and other hot water use for space heating rather than warming the incoming cold water.

Something like an extended drain pipe or holding tank that keeps the water inside the house long enough to get the heat out of it?

This might also be part of a grey water recycling system?  That is, hold the water in a tank long enough to lose it heat, then let it flow or pump it out for plant watering.

You would want a way to bypass this in the summer when you don't want the heat.

If you had solar water heating, you would essentially be getting three  uses out of the  solar heated water -- shower heat + house heat + grey water for plants?