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Critical Heater Circuit Design Info for Subaru Engines

     There is a fundamental difference between the heater circuits of Subaru’s and some other vehicles, T25’s included. In Subaru’s coolant flows through the heater matrix whenever the heater is on, regardless of whether or not the heater is on or off, or set to hot or cold. This is quite common in modern cars, and less so in older ones, many of which control the heater temperature by throttling the coolant flow through the matrix.
     Subaru’s use the coolant flow through the heater matrix to heat the thermostat. Therefore, if the coolant flow is stopped, or reduced too much, as would be the case when turning the heater down in a system which throttles coolant flow through the heater, the thermostat sees the resulting reduced heat input as a decrease in engine temperature. This results in the thermostat closing as if the engine is cold, regardless of it’s actual temperature, and WILL RESULT IN AN OVERHEATED ENGINE.

     There are a number of possible solutions to this problem. Each is listed below, with info on their advantages and disadvantages:

    1.   Install a heater system which controls the temperature by blending hot and cold air, rather than throttling the coolant flow through the matrix. This is how Subaru heaters work, allowing coolant flow though the heater matrix at all times. This is not practical in vehicles which already have a heater system which throttles heater matrix coolant flow (such as T25’s), but may be possible, for example, in ex-air cooled VW’s into which a heater matrix is being installed as a part of the engine conversion.

    2.   Install a valve which provides a bypass to the heater matrix, either inversely proportional to flow through the heater matrix, or proportional to the pressure rise created by decreasing the flow through the heater matrix. This is the ideal solution, as it has no disadvantages, and is relatively easy to install into either new or existing systems. However such valves are difficult to get hold of.

    3.    Install a bypass pipe which partially bridges the flow between the heater feed and return pipes. This is a fairly crude solution, but one which can be simple and effective. Heater efficiency will be reduced as a result. However, if the optimum bore of tube is used as the bypass between the feed and return pipes, it is possible to provide adequate flow to keep the thermostat hot when the heater valve is closed without having too big an effect on the heater output.

    4.    Prevent the heater being turned right off. This is more of a way of avoiding the problem than a solution. It may be practical in cold climates, where the heater is always needed, but is definitely not practical in hot weather. Some sort of mechanical stop is needed on the heater lever or valve, rather than relying on remembering not to turn it off. Sooner or later, you’ll forget, and overheat as a result.

Thank you.

Site last updated on 15/08/08

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