David Combs wrote:

What's an "A-coil"?

This discussion is about a combined forced air heating and cooling
system. In a typical installation the air first passes through a
squirrel cage fan, then the heat exchanger, then the cooling coil which
is often A-shaped. At the beginning of the heating season there is
frequently a burning smell from dust, etc. that have accumulated on the
heat exchanger or coil during the off-season.