Stacked or daisy chained does NOT mean increased height. It merely
means we hook up two shorter GFX lengths in series, pumping effluent
from the outlet of the first one to the inlet of the second one.
Overall efficiency rises. For instance we could get two S4-40s and set
them on the wall parallet to each other. Inlet for the first one is
from house sewer. Outlet of first is pumped (100W power used) to inlet
of second, outlet of second is piped to city sewer/septic tank. One
BIG advantage of this system in a septic tank is that you can put
BOILING water down the kitchen drain, something you cannot do with a
plain ole septic tank.

One issue with this configuration is water pressure drop GFX Tech will
argue for manifolding, that is, connect potable water supply to coil
inlet on BOTH GFX units and tie the coil outlets from BOTH units to a
common pipe to hot water heater and cold side of showers, That
produces a pressure drop of about 2-3 psi A full series connection
with potable water to the coil inlet on the one connected to city
sewer/septic tank, its output then connected to coil input of the GFX
attached to the house sewer, and the coil outlet then connected to
house hot water and shower cold side. This produces a pressure drop of
5-6 psi.

In my configuration, the inlet water pressure will be a constant 65psi
So dropping to about 60psi is no big deal, and the water temp to the
house rises. Effluent temp in both series and parallel configs drops
by at least 20F, maybe much more

In 40 inches of vertical height, we get 80 inches of heat recovery.