AC,
Sounds like you think I built that device -- I wish!
Just a nifty idea I found on the web.
The commercial heat recovery (HR) units that do measure recovery report 50-60% -- that was the basis for my comment. I don't know if the DIY version does as well. I imagine the flows should be counter-current to maximize heat recovery. I'm planning to read up on flow physics a bit to see if I can come to any reasonable conclusions how to match unit geometry with flow rates. I do know that the water has to spread itself out as a film on the tube to optimize heat transfer. This is the reason the HR units are typically installed vertically.
Tempting to try, isn't it ?
I don't know how much the parts would cost, but I'll be very surprised if it reaches $100.