Auxiliary Heater water pump is mounted to the Webasto unit so the water pump is right near where it needs to be efficient to get the water flow up to the heater matrix .
The rest pump is there to allow the heater to give warm air when the engine has been switched off , but it must be able to get some flow through the aux heater and stopping the water from boiling up ....just some flow will be enough to stop the water from boiling and getting the flow into the heater matrix core
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