When I was building (1993) airplane the firewall map was not available , and
we located items on the firewall at our own discretion. I located my air mix
valve so that the engine air to the cabin was between the two support ribs
(aft side of firewall) on the right side of center. Then I encapsulated in
that area between the ribs with fiberglass board (as used in making
air-conditioning ducts and plenums). I tapped into that box for two outlets
to go to the rear seats/two outlets to go to the pilots feet/two outlets to
go to the defroster diffuser. The I have no control over these taps. But it
works. It's very quiet-he seems to be distributed fairly well. The windows
are cold , but the windshield defrost nicely. I've had as much as a quarter
inch ice on the windshield and it came off in about three minutes using full
heat.

Charlie K.

PS

My flapper valve is permanently closed.