Improperly designed or maintained HVAC systems create multiple building science failures including pressure imbalances that affect air flow patterns and infiltration rates. Inadequate ventilation fails to remove indoor contaminants while excessive ventilation can create moisture problems and energy waste. Poor moisture control from mechanical systems leads to condensation issues and mold growth. Design conflicts between different mechanical systems can create competing pressure effects that compromise building envelope performance. Distribution problems result in uneven temperatures and air quality throughout buildings. Understanding these mechanical system interactions requires building science expertise to diagnose problems and design solutions that work with the building as a complete system.