The core idea
Kitchen, bathroom and laundry exhaust can pull replacement air through unintended gaps and transport odors or combustion products.
How the mechanism works
Removing air lowers indoor pressure unless planned makeup air or leakage supplies an equal flow.
When it becomes relevant
Typical trigger: Fan operation, closed windows, tight construction or multiple exhaust devices running together.
Evidence that supports this interpretation
- Odor or draft begins when a fan starts
- A door becomes harder to open
- Nearby combustion equipment is present
Evidence that weakens it
- The symptom occurs with all fans off
- Opening an interior door has no effect
- The source is a direct liquid leak
What this concept cannot prove alone
A single observation, image or meter reading cannot confirm the complete failure pathway. Use this concept to form better questions, compare alternatives and decide which safe observation would be most informative next.
Safety and professional boundary
Stop testing and seek qualified help if combustion odor, soot, alarms, dizziness or carbon-monoxide concern is present.
Primary references for this topic family
These references ground the general building-science or safety context. They do not diagnose a specific house, replace local requirements or certify a repair.
