Supply Air Plenum and Humidifier Installation

Help Support House Repair Talk:

thapranksta

Home DIY Newbie
Joined
Oct 8, 2011
Messages
95
Reaction score
16
I had purchased a whole house humidifier to install on my furnace. When I went to measure everything and figure out what the best installation technique would ultimately be in my attic, I discovered that the supply plenum above my furnace and AC coil was not a rigid rectangular duct as I have often seen them depicted in my research. The supply plenum is made of some soft material (cardboard maybe?) wrapped in aluminum tape. Also to add on to that, my furnace has a return supply duct going into each side so there is no true return plenum. This was installed 6 years ago. Is this normal?

Shot of side of furnace, AC coil, and the "faux" supply plenum
PXL_20211203_190830590.jpg

Faux plenum only
PXL_20211203_190844992.jpg

Trane XV80 Furnace with return duct going in. The other side also has a return duct.
PXL_20211203_190836201.jpg
Outside of that, if this is normal practice where would I install a bypass or powered humidifier? I'll save my other questions around water supply for later.

Thanks.
 

Guzzle

Well-Known Member
Joined
Nov 24, 2021
Messages
565
Reaction score
189
Location
Maryland
Use an ice pick or a magnet to check if there is metal ductwork under that insulating wrap. Almost certainly there is.

If not, if you cut large pieces of cardboard or Masonite to exact size, a sheet metal shop can make you one or two pieces that cover the wrap & anchor to existing metal ductwork, to support the humidifier.
The flanges to fasten these can be on the sheets or you can use aluminum extruded angles.

Cut the humidifier holes first.
 

Sparky617

Member
Joined
Nov 3, 2014
Messages
1,957
Reaction score
794
Location
Cary NC
It is likely a fiberglass duct. I had them in my first house. It is a foil wrapped rigid fiberglass board about 1" thick.

 

thapranksta

Home DIY Newbie
Joined
Oct 8, 2011
Messages
95
Reaction score
16
Yeah, it's definitely not metal. I took a magnet which stuck to everything nearby made of metal but not the supply plenum. I would think it is fiberglass board as suggested.
 
Top