A heater bypass adaptor allows the correct amount of coolant to bypass the heater system when the heat is turned down by throttling the coolant flow through the heater matrix. Such a system is used in all water cooled T25 / T3 / Vanagon’s, and many other cars. This system is not compatible with Subaru engines, which have a ‘blended air’ temperature control. Without a heater bypass adaptor a Subaru conversion which uses a throttled coolant flow heater system WILL OVERHEAT when the heater is turned off.