Is its a genuine dual stage mazda themostat ? If yes, I probably agree with Wibla, could be a blockage in one of the pipes....
and the GD heaters dont have a valve, the water flows through the core all the time, GCs had a valve that shut off the flow when the heater was not needed, so it cant be that.
Probably what I would do, is drain the coolant, and flush the system, then hook up a garden hose to the heater return to the water pump, then see how much water flows from the heater out onto the grass (might be a bit difficult to do in a Canadian winter (or nearby)).....you could probably achieve the same results without going outside, by using a large bucket (20Litres) and a drill pump, to force some water through.
Its not a problem we really face here in Australia, or at least not for any length of time.
And as for flushing, no I never flush with distilled water, I use tap water for that, and where I live its fairly clean, however I do use demineralised water when I mix up the coolant (which I like to do myself, so I know what I am getting, I never trust premix)
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