DIY is realistic  for: Adding or upsizing a return  in an accessible attic. Fixing one “hot room”  by improving supply size/route, sealing leaks, and balancing. Sealing and insulating  existing ducts, replacing grilles/boots, and adding dampers. Call a licensed HVAC contractor  when: You need load/duct redesign  across multiple rooms, or your static pressure is already high  (you’ll measure this shortly). You’re modifying the main trunk or plenum , relocating equipment, or ro