I've wondered more than a few times why drain water heat recovery units are not built with a pump for installation in one story homes. Seems like activation could be either manual or by water weight.
By the way, I'm willing to wager that the least expensive, best savings/dollar on water heat is to first change out the shower head to a low flow, 1.2 or 1.8 gallon/minute type.
Here are a
few reviewed;
And the type
I am going to buy.