I need my old lennox hvac system replaced which is in the attic. There used to be an attic fan opening in the ceiling that was closed in before I bought the house. So, the only opening into the attic is a small scuttle (24 inch x 24 inch). I want to get folding attic stairs installed. But, there are different finished widths available. I don't know how these attic hvac systems are installed. Are they brought up into the attic in pieces and assembled in the attic or is it already assembled before it is brought up into the attic? The largest part of the old unit appears to be the evaporator (24 inch x 28 inch). If the unit is brought into the attic already assembled I think I will need at least an attic stair unit 24 inch clearance in width. Any help would be appreciated.