This unit is 24 years old and I dont know how long the plywood has been used. I’m putting in the report that HVAC system is at the end of its useful life and she should budget for new units in the very near future. Would it be a health issue with the plywood? (foreclousure)
What makes you think it 24 years old? When was the yellow stickers first used?
Inside the wooden box is most likely lined with sheet metal just like a wall cavity.
Most areas allow wood on the return side of a system but never on the supply side. (Temp differential)
Or not…it may be better practice to have a metal plenum or metal liner but if just wood with that moisture staining + rust on the unit, I’d have concerns.
At 24 years of age my guess would be the A-coil drain pan was leaking down into the return plenum
Phil would be nice if you had a pic of the whole HVAC . So we could see more about the moisture staining . An if there is no air into that box other than what we see , it would be a good place for spores to collect and start to grow (with the moisture leaking down). The set up looks mickey mouse !