In alt.home.repair, on Sat, 30 Dec 2017 00:30:54 -0500, micky
wrote:

Separate oil (or gas) furnace question.

Everyone says that if too many of the heating ducts are closed off,
probably at the grill where the heat comes out of the wall or floor,
it's bad for combustion, so don't close too many. But why should that
be?

Closing off heating ducts lowers the volume of air that goes through the
furnace, (and I guess it slows down the air as it passes, which means
the air gets a little hotter than otherwise) and that means the firebox
gets a little hotter, but the fire still has plenty of air. Is a
firebox that's a little hotter really enough to impair the fire?