The MGB heater system used a Smiths fresh air unit throughout production, a single casing mounted at the rear of the engine bay housing the heater matrix and fan, drawing air through an intake grille on the scuttle panel. A hot water valve on the rear right-hand side of the cylinder head controls heat output, with two cabin controls managing temperature and air distribution alongside a fan switch. No MGB was ever factory-fitted with air conditioning, though aftermarket systems are sometimes found on North American market cars. The key point to establish before ordering any heater component is which of the three distinct system specifications the car has, as parts are not interchangeable across all variants.
The system was revised at the MkII changeover in 1967 to 1968 and again for the 1977 model year, with changes to the motor, controls, and wiring at each stage.
Heater Matrix
The heater matrix is the component most likely to cause cabin water leaks or loss of heat output on an aged MGB. A matrix that is partially blocked or leaking will produce poor heat output, a coolant smell in the cabin, or damp carpets, all common complaints on cars where the cooling system has not been properly maintained over the years. A high-efficiency replacement matrix is available, offering improved heat output over the original specification and recommended for any car undergoing a heater system overhaul.
Heater Fan Motor
The fan motor specification changed during production. The single-speed motor used on 1968 to 1976 cars and the two-speed arrangement on 1977-onwards cars are not directly interchangeable. On 1977-onwards cars, the system uses a standard motor with an internal resistor to achieve the two speeds, confirm the model year before ordering. A two-speed motor and large impeller upgrade kit is available, providing noticeably improved airflow and heat distribution over the standard unit.
Heater Hoses, Valve & Controls
The heater valve, hoses, and control cables all deteriorate with age and are a frequent source of both poor heater performance and minor coolant leaks in the engine bay. A stiff or seized heater valve is the most common single cause of a heater that produces no heat despite the engine being at full operating temperature, the valve should move freely through its full range when the cabin control is operated.
Replacement valves, hoses, and control cables are available for all variants. The heater outlet arrangement on the cylinder head changed with the 1972 model year, so model year must be confirmed when ordering hoses.
Vents
Replacement scuttle intake grilles, demister vents, and the face-level fresh air vents introduced with the 1972 dashboard revision are all available.
Upgrades
The heater box seal kit is recommended for any car where the heater casing has been removed for matrix replacement, deteriorated seals allow air to bypass the matrix, reducing heat output regardless of matrix condition. A high-efficiency matrix combined with the two-speed motor and large impeller upgrade provides a genuine improvement over the standard system’s performance, which was always modest by modern standards. The MGOC Spares range covers heater matrices, fan motors, heater valves, control cables, hoses, scuttle grilles, demister vents, heater box seal kits, and all associated heating and ventilation components for every MGB variant.