As you've successfully changed your thermostat you obviously know what you're doing. But, for the edification of others lets think about how this works. The heater system allows coolant to circulate all the time which is what happens from the moment you start the engine. On the back end of the cylinder head is the thermostat (as you know) which will be closed when the engine is cold. When the engine heats the water circulating in the heater system to "working temperature" (that is the temperature the manufacturer wants the engine to run at) the thermostat will open and hot water will flow out to the radiator header tank, letting cool water flow from the bottom of the radiator into the engine. The engine coolant temperature will then drop so the thermostat will close and so the whole cycle goes on (in practice the thermostat spends it's life "floating" - more or less open depending on heat generated by the engine so that a more or less constant temperature is maintained - commonly around 90 degrees centigrade.) So if your thermostat is defective often it stays open too long and lets water circulate through the radiator too soon so the whole system runs cool.
For this to work properly the radiator has to be able to, under all operating circumstances, shed more heat than the engine can ever generate. It has to be realized that coolant circulates all the time through the heater matrix so it's cooling the engine too, but how much heat it can shed (in heating you inside the car) depends on how "hot" you set the heater control. So on a really cold day, when idling for lengthy periods with the heater fan on and your temperature control set to full hot it's quite conceivable that there might be sufficient cooling capacity in the heater matrix to satisfy the cooling needs of the engine on it's own. I've never owned a car where the heater matrix on it's own could actually exceed the cooling (at idle) requirements of the engine until now! Our Ibiza 1.0 3 cylinder TSI seems to be able to achieve this "trick". On very cold days when idling for prolonged periods (there's one particular set of traffic lights which invariably take about 3 or 4 "goes" to get through) If we have the heater on full hot with the fan on the temp gauge falls to about 85 degrees - from it's normal 90 - It never does it when driving or for short halts and she comes up to temp quickly from cold with the top hose staying cold until full heat is reached. Maybe yours is doing something similar?
