The heater is a Smiths unit mounted on the bulkhead behind the dashboard, drawing warm air through a matrix and distributing it across the windscreen and into the footwells via a system of ducts and outlets. The matrix is fed with hot engine coolant via the heater hoses, while the blower fan draws air from the engine bay through a fresh-air intake, passes it over the matrix, and distributes the heated air through the dashboard and footwell vents.
On a car of this age, the matrix is the most frequent source of heater problems. A leaking matrix will flood the footwells with coolant, while a blocked or partially blocked matrix delivers reduced or no heat. Matrix faults are typically internal corrosion or sediment blockage reducing flow and heat output, or external corrosion at the seams causing leaks. The blower motor, seals, and ducting all deteriorate with age and regular use, and the heater box seals in particular harden and allow warm air to escape into the engine bay rather than being directed into the cabin.
Matrix & Blower
The heater matrix is available in a high-efficiency specification that improves on the original thermal performance, and is a direct replacement that fits within the standard heater box without alteration. A combined two-speed heater motor and large impeller kit with heater matrix is available as a complete package for a comprehensive heater overhaul, converting the original single-speed blower to a two-speed unit with a larger impeller for improved airflow at the same time as renewing the matrix.
The standard single-speed heater blower fan is available as a direct replacement for the original Smiths unit. The blower fan is driven by a brushed DC electric motor, with fan speed controlled from a dashboard switch via a series of resistors, and brush wear is the most common cause of motor failure.
Seals & Ducting
The heater box seal kit supplies all the seals needed to make the heater casing airtight. The heater-to-bulkhead seal kit addresses the joint between the heater box and the bulkhead aperture, a common source of cold air ingress and water entry where the original seal has hardened. The heater cover seal and fan intake foam seal are available individually. The heater outlet door for pre-1970 cars and the revised outlet for 1972 onwards are both available, reflecting changes to the heat distribution arrangement.
The demister elbow, air tube, heater drain tube, and heater vent knob are all available individually. Significant heater work, such as matrix renewal or full box overhaul, typically requires dashboard removal for proper access, so it is best carried out before the cold season starts.