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Why isn’t the whole shebang in the attic, centered over the main part of the house so you have short duct runs?

I know that ship has sailed. Duct work needs to be that large but doesn’t look nicely strapped. I hope the garage is still isolated from the house by a firewall.

There is no need to use flexible duct for the main supply and return. The installer did this to minimize his cost. Using hard duct would look a lot better and give better air flow.

Is setting the furnace on the platform something you requested? Is there a concern of flooding?

I don't like what I see for the condensate drain. How is it connected to the plumbing for drainage and how can it be cleaned in case there is a blockage?

I did not request the platform. I thought it was standard. the garage does flood in the winter months. At the risk of sounding stupid, what is the condensate drain?

The white plastic pipe to the left of the unit that drains the water away.