The cabin ventilation system draws fresh air through the scuttle vent ahead of the windscreen, passing it through the heater box where it is either heated by the matrix or passed through cold depending on the heat control valve setting, and distributing it through the demister outlets at the base of the windscreen and the face-level vents in the dashboard. The demister directs warm air across the base of the windscreen to clear condensation and frost, and on a car used in British weather it is one of the most regularly used functions of the heating system.
Demister Components
When the demister tubes harden, crack, or collapse with age, the airflow to the screen is reduced or lost entirely, leaving the screen slow to clear and the car impractical in cold or damp conditions. The demister tube routes warm air from the heater outlet to the windscreen base and is available as a replacement unit, secured by the demister tube clip.
Rubber bumper demister ducts are available in left and right hand fitments for 1975 to 1980 cars, reflecting the revised dashboard and vent arrangement introduced with the rubber bumper specification. The demister escutcheon finishes the vent aperture where it meets the dashboard, and demister nozzles are available in left and right hand fitments, with the escutcheon sealing trim available separately.
Fresh Air & Face-Level Ventilation
Face-level ventilation was added to the MGB at the 1972 interior revision, with fresh-air vents fitted to the centre of the facia for the first time, a significant improvement that the pre-1972 cars lack entirely, having only the scuttle air vent controlled by a lever under the dashboard with three stages of opening. Fresh-air vents draw air from a forward-facing intake, providing unheated airflow for ventilation independent of the heater, useful in mild weather, with adjustable louvres to direct the flow. Vent grilles cover the various openings, providing directional adjustment and protecting the ducting from debris, and are fascia-specific in style on the V8.
Sealing & Maintenance
The scuttle vent grille sits in the cowl panel ahead of the windscreen, and leaves, debris, and water that enter the scuttle area should be cleared regularly, as blockage restricts airflow and trapped moisture promotes corrosion in the scuttle panel and heater box area, one of the most common rot points on the body shell. Correct sealing of the heater box to the bulkhead is essential for effective air distribution, as deteriorated seals allow air to leak into the engine bay rather than reaching the cabin, significantly reducing both heater output and demister performance, a problem the heater box seal kit addresses comprehensively. Vent service typically involves cleaning accumulated dust from the vent and ducting, lubricating sticky control mechanisms, and replacing damaged grilles.