Tight closets and “creative” vents don’t make comfort—they make risk. In this episode, Jason breaks down the air-in / air-out / room-pressure triad and the simple clues he looks for on real walk-ins: white powder (zinc oxide) on vents, melted plastics, rust trails, and fried igniters. We stay owner-safe and brand-neutral: what to ask, what to photograph, and why “repeated safety shutdowns” are a signal to fix the air path—not swap parts. Quick code touchpoint: NFPA 54/IFGC for fuel gas and IMC for combustion air/venting—plus manufacturer instructions.