Robert Gammon wrote:

You might still reclaim heat from an upstairs shower, or add a pump...
If we pump 10 gpm at 100 F with 100 watts and heat 10 gpm from 55 to 95 F,
what's the net energy gain?

586:1

Single story home, so NO opportunity there.

Why not?

For slab type construction, with heavy clay soils, the easiest install
is to dig a pit down to where the outgoing sewer can be intercepted,
routed to a sewage ejector which pumps to the top of GFX (or your
alternative exchanger)...

.... which wouldn't need a pump.

Seems like separating gray water from black water is impractical, EXCEPT
in two cases.

1. New construction
2. Retro fit in a house with a basement and NO downstairs plumbing, i.e.
you can easily identify and separate toilet drains form sink/shower drains.

My house has a washing machine and pressure tank in the basement, but I can
easily identify pipes from various fixtures under the open basement ceiling.

... I am coming to the conclusion that I should route the output of the
heat exchanger to both shower cold taps and to the hot water heater.

Or lower the water heater temp to the shower temp.

Nick