HVAC Air Distribution, Cabin Comfort and Air Quality
HVAC air distribution, cabin comfort and air quality
This domain tests the non-refrigerant side of comfort: airflow, doors, filters, drains, ventilation and cabin air quality. Many poor cooling complaints are air-distribution faults, not charge faults.
Core rules
- Airflow path: outside or recirculated air, cabin filter, blower, evaporator, heater core, blend and mode doors, ducts and vents. Restriction anywhere reduces cooling.
- Blend doors control temperature; mode doors control face, feet and defrost. Actuator calibration and door sealing are critical.
- Evaporator drain must be clear. A blocked drain wets the casing, causes odour and fogging.
- Cabin filter condition affects airflow and air quality. Recirculation reduces outside odour and cooling load but can increase fogging if humidity is high.
- Comfort includes temperature, humidity, airflow, noise and stratification, not just vent temperature.
Worked example
A customer reports poor cooling, a musty smell and windscreen fogging. The refrigerant pressures are normal. Inspection finds a blocked evaporator drain, a saturated cabin filter and a recirculation door stuck closed. Clearing the drain, replacing the filter and recalibrating the door restores airflow and reduces humidity. The reasoning is that the refrigeration circuit was not the cause; the symptoms were air quality and distribution faults.
Traps
- Adding refrigerant for an airflow complaint.
- Ignoring a blocked drain.
- Fitting a filter backwards or omitting it.
- Forcing a blend door instead of recalibrating.
- Testing only at one blower speed.