I saw this device a couple of days ago, and thought it was an awesome idea. Then I started doing some calulations.
Average shower time: 6 minutes @ 2 showers per day
Flow rate of the shower head: 2.5 gallons per minute
Hot water temperature: 55 decrees celcius
Cold water temperature: 10 degrees celcius
For a 40 degree shower, we will use 1.22 gpm of hot water. So, the total hot water usage per year is 5343 gallons.
Assume that the water loses 25% of its heat to the air and the bathtub, the drain water is 30 decrees celcius.
Also assume that the heat exchanger is 75% efficient, so that we transfer 75% of the heat from the drain water to the incoming water. The temperature different between the two is 20 degrees, so we will increase the incoming water from 10 degrees to 25 degrees (10 + .75*20). I might be wrong in how I am calculating this heat transfer.
Now, the energy required to heat 5343 gallons of hot water from 10 degrees to 55 degrees is 1054 kWh. The energy required to heat this same water from 25 degrees to 55 degrees is 703 kWh. So, over the course of a year, we will save about 350 kWh, or $35 on a grid connected system.
The model of heat exchanger I looked at was between $300 and $400 dollars. At that price, and assuming my other assumptions are right, it will take about 10 years to recover this cost.
Now, on an RE system, a savings of 1 kWh per day might be very significant. However, it seems unlikely that users of an RE system would be taking 2 6-minute showers per day, using electric heated hot water at 40 degrees. So the savings might be far less than 1 kWh per day.
Am I thinking correctly here? Am I missing something?
Chris