Coolant in expansion tank stays cold

Hey folks,

technical question about my 45 1.8: just came home from a 100 mile drive and noticed that the water in the expansion tank is cold, but heater blows lovely hot air, car drives as good as always, oil and water level are fine, no bubbles, no oil in water... anything to worry about?

I am thinking the reason might be the intensive use of the heater (getting pretty cold over here one).

What do you think?

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