To much water flow will cause over heating! runnig without a thermostat lets the water pass thru the radiator to fast to have time to cool as a result over heating I speak from experience here.the thermostat slows the flow of coolant thru the system (when its open )and allows it time to cool as it passes thru the radiator.a 180 degree thermostat is a good starting point. if the heater doesnt get warm enough in the winter you might want to change to a 195
