On Feb 1, 1:13*am, z wrote:
i just noticed, a hole about an inch in diameter in the bottom of the
hot air return ducting in the basement, just before it enters the
plenum. not sure where the duct is coming from, presumably from the
return register upstairs in line with the duct where it vanishes into
the finished part of the basement. no return register in the basement
itself, although there is one outlet which is closed pretty much all
the time. it's an oil furnace with central air also, if that makes a
difference.

is this hole there for a purpose? prevent small-sized floods from
flowing down the duct into the plenum? suck up carbon monoxide from
the oil burner to circulate around the bedrooms? does it make any
sense to plug it?

Adding numerous cold air returns to my basement was the best thing I
ever did in my house in terms of HVAC. Made that space feel very
liveable, and really opened up the furnace in terms of its ability to
get a lot of input air easily. Much of the main floor air return flow
is now down the basement staircase as that stairway gets more dust
pulled into it I noticed.

The small hole could be from a humidistat as someone mentioned or
there could have been an in-duct damper there that somebody removed
(the hole would be from the rotating control shaft of the damper
plate). In any case that hole is too small to be effective, tape it
up or it will become a whistle.