Is it bad that my furnace at home is sitting on an OSB platform? I am doing a change out in a couple weeks and during the process I was planning on sealing it from the inside. Its all original from 1993.The platform acts as the return plenum. Not nearly as bad as this one lol. 16'' flex duct from house goes straight into said plenum.
Personally I have built plenums out of osb and 2x4's before. They were for twinned furnaces usually in a commercial building like a bank. As long as its sealed, it works fine. But that's just my experience.