The 1275cc heater system existed in two distinct configurations during production, and the correct one must be identified before ordering components. The early system (fitted from 1967 to approximately mid-1970, covering GAN4 to approximately GAN5-91407 and HAN9 to approximately HAN10-86377) used a square-box heater case with a separate external blower motor.
The later system (from approximately GAN5-91408 and HAN10-86378 onward) used a longer heater case with an integral fan, eliminating the separate blower unit entirely.
Early Square-Box System
The early square-box system draws air from the wheel arch area through a wire-reinforced flexible hose, pushed by the separate blower through the heater case and across the heater matrix. Within this early configuration, there is a further distinction: 1967-built cars had the air control valve mounted in the front panel of the car, requiring a 48-inch control cable, while subsequent cars relocated the valve to the blower assembly, shortening the cable to 28 inches. The blower motor and fan assembly, intake flange, and wire-reinforced hoses are all specific to this early configuration.
Later Integral-Blower System
The later integral-blower system simplified the arrangement by incorporating the fan motor within the heater case itself, reducing the number of separate components and hose connections. The air intake hose changed from wire-reinforced to moulded PVC, and the control cable was standardised at 28 inches. The motor and fan assembly is a replaceable unit within the heater case.
Heater Matrix
The heater matrix is the core heat-exchange component, through which hot engine coolant flows while the blower pushes air across its fins and into the cabin. The early square-box system uses its own matrix specification, while the later integral-blower system uses a matrix that is also shared with the 1500 heater system. An uprated matrix with improved heat output is available for the early system. The matrix seal around the heater box should be renewed whenever the matrix is replaced, as a deteriorated seal allows air to bypass the matrix rather than passing through it, reducing heat output significantly.
Heater Tap and Water Circuit
The heater tap on both 1275cc systems mounts on the cylinder head via a plinth, gasket, and studs, controlling the flow of hot coolant to the heater matrix. The tap washer is a serviceable item that should be renewed if the tap drips or fails to shut off completely, a leaking tap can cause coolant loss and a persistent smell of antifreeze in the cabin. Water hoses run from the tap to the matrix inlet, and a return pipe carries cooled water from the matrix outlet back to the radiator bottom hose area, secured by a clip to the inlet manifold.
Heater Box Seal
The heater box seal between the heater case and the battery platform prevents air leaks and water ingress. This seal is common to all heater system configurations. A perished seal allows cold air to be drawn into the heater circuit from the engine bay rather than through the intake hose, reducing the heater's effectiveness and potentially introducing exhaust fumes into the cabin.