In the winter, I rather enjoy hot showers. I try to keep them short (5-6 min), but it's still a lot of heat going down the drain and out of the house - particularly with 3 other people who do not have the same timing compunctions. I was wondering if it would make sense to install a heat exchanger between the incoming and outgoing water lines (possibly with a valve to skip it during summer) to capture that heat.
I don't know enough about the efficiency of available heat exchangers to come up with a ballpark estimate for heat energy (and cost) saved.
Possibly relevant data: four adult household, roughly one shower/(day-person), average time 15 min. Water temperature of ~110ºF (~43°C), gas heated (recent heater).