Thank you @ashmccormick for the detailed explanation, much appreciated.

I am just flushing my TD42 cooling system and I don't currently have a heater core in place, as my GQ interior is fully stripped out at the moment. I have "looped" the thermostat housing in/out ports towards the heater with a piece of hose. I am wondering if this will prevent the thermostat from working properly, any comments on that?

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I think this would have the same effect as having the heater circulating water as in the procedure, but I'd rather be safe than sorry.