In alt.solar.thermal Robert Gammon wrote:
Nick's heat exchanger can be installed in almost any orientation, but
horizontal seems to be his desire.

Horizontal is how all of my waste plumbing is arranged. I don't have any
place to put a vertical drop without adding a pump.

His heat exchanger will need to be cleaned out periodically of gunk,
especially if toilets drain thru the same heat exchanger.

I have never cleaned out the drains in this house.

The beauty of the GFX, is that it is metal, it can be stacked or daisy
chained with pumps to extract even more heat in a smaller space.

"stacked" implies a lot of vertical drop. Pumping requires energy.

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Clarence A Dold - Hidden Valley (Lake County) CA USA 38.8,-122.5