Probably a hole drilled when someone had the ducts cleaned. Look at other
ducts and see if there isn't a similar hole with a small plug in them as
well. They sometimes drill a hole and run a rod with a multi-sided nozzle
on one end and connected to an air compressor that blows the dirt and dust
lose. Their big vacuum then sucks the dust out, probably thru a hose that
was attached to a larger opening cut in the plenum. All these holes should
be plugged - but the plug may have fallen out of the hole you see in your
duct.


"z" wrote in message
...
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?