The heater system in most older car designs, including all of the water cooled rear engined VW’s, used a valve which controls the flow of coolant through the heater matrix to control the temperature i.e. fully open = maximum heat, fully closed = no heat. This is fundamentally incompatible with the cooling system on Subaru engines – especially most of the normally aspirated EJ series engines. As can be seen in the schematics shown in How the Cooling System in a Subaru Works, if you blocked all flow through the heater circuit by fitting this type of heater system and turning the heater(s) off when the engine is cold, not only are you trying to force all bypass flow through the throttle body / idle speed control valve (which is far too small a diameter, so may result in excessive coolant pressure or water pump cavitation), but you’re also preventing sufficient flow getting to the thermostat wax capsule to keep it hot. Therefore it cools, and the thermostat remains closed, regardless of coolant temperature. 

Similarly, if you are not fitting a coolant powered heater in your VW engine conversion, but do not understand the Subaru cooling system sufficiently, you could think all that you need to do is block off the heater feed and return pipes on the engine. This cannot work.

To use a heater which uses a coolant flow valve to control temperature, what is needed is an alternative route for the coolant when the heater valve is closed, so the returning coolant always flows over the thermostat wax capsule regardless of heater setting. Theoretically, the best way to do this is to use a diverter valve. This is an active device which splits the flow between the heater matrix the alternative route. Heater 100%, alternative route 0%, and vice versa. These exist, and were the first method we experimented with back in 2004. However, they have a downside too. They are not very practical to install in water cooled T25 / T3 / Vanagon models, and this cancels any benefit of using them.

Although in theory, a simple heater bypass, which is far easier to install, will compromise the heater performance, testing showed that when the heater bypass design is just right, any performance compromise is not noticeable, with the both heater in a high spec t25 / T3 / Vanagon still performing very well in the coldest UK weather. This has have proven entire trouble free through the next 19 years, but does rely on the heater bypass being designed correctly. Get it wrong, and it either won’t work, or the heater performance will be compromised more than is necessary (there are plenty out there like this – almost always attempts to copy our system be someone who doesn’t understand it as well as they think they do). The theoretically better diverter valve is absolutely not worth the extra effort, cost and complexity.

With the addition of a suitably designed heater bypass, the heater(s) in a T25 / t3 / Vanagon (or an air cooled VW which has had a heater system fitted in which the heater temperature is controlled with a coolant flow valve), the heater system becomes compatible with the Subaru engines cooling system. A heater bypass is included by default in all of our coolant plumbing kits, as most customers do need one.

If you have installed a heater system in which the heater matrix remains hot all the time that the engine is running (i.e. a ‘blended air’ heater, like all Subarus and most other more modern cars use), you so not need a heater bypass. Please specify this if ordering one of our coolant plumbing kits, and well substitute the heater bypass with straight joiners.