Robert Gammon wrote:

Nick says GFX needs a toothbrush for cleaning.

Wrong again. Read much? :-)

... I have a slab foundation and NO basement (plumbing is buried
beneath the slab). The ONLY choice for me is a sewage ejector in the
corner of the garage, near where the main sewer line exits under the
slab with a GFX stack in the corner above the sewage ejector.

Au contraire. That way, you could easily use either heat exchanger,
and you might use the flat spiral version with no pump.

Nick