I just happened to be looking over all the paperwork I received after I had a
new furnace installed and I don't see where this "solid metal base" (please see below) is installed. The furnace is sitting directly on the floor and there's nothing added on the inside bottom (in the fan area).
The furnace has the left side opened where the return vent comes in.
And the top has the conditioned air going to the supply ducts. The bottom is
the way it came.
* From the installation instructions- Upflow Furnace: A solid metal base plate must be installed in the furnace bottom when using side return. Failure to install a base plate could cause the products of combustion to circulate into the living space and create potentially hazardous conditions including carbon monoxide poisoning or death.