John wrote:
The main return air duct mounted next to my upflow furnace flexes
inward when the furnace is operating. After the blower shuts off,
the sheet metal pops back out (oil canning) with an annoying bang
sound. It looks like a brace was created by the installer halfway
up. It needs another brace or bracket between this brace and the
humidifier above it so the sheet metal can't flex.

Where can I find such a brace? Is something ready made for this
purpose?

I use light weight angle iron.

--
Joseph Meehan

Dia duit