Here is the real problem. The main return air duct is improper. The vertical stack or "J"-chanel is attached to 2 joists of which there is only approximately. 2.5 inches on either side.
Coming Soon...
Just so you can see where the pictures are taken. Also the humidifier is of very poor quality. We do not recommend the installation of this type of humidifier to your HVAC system.
Alarm siren removed. See how a lot of the vent is blocked by a support beam? Even if the siren wasn't there, there is not enough room to allow sufficient air to return to the furnace.
Another small return air vent blocked by a support beam. No air is flowing through this vent.
A third return air vent on the main floor which is blocked. Why make a large opening but only a small vent? Crazy!!!
Small return on main floor blocked by alarm siren. No air is able to pass through.