The spacing is in case the furnace has a "hot spot" and
blows hotter air. Might damage the coil, melt plastic, etc.
Which is why the oil furnace coils have to be higher up.
Hotter heat exchanger.

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Christopher A. Young
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No clue there. But in most cases, cased is the way to go.
It also
takes
care of the spacing reqt between coil drain and furnace,
etc. You
just
sit it on top of the furnace and you're done.