Here's a counter-intuitive thought (although I'm sure others have had it before me)
In the summer, to get more engine cooling, open the water valve so the heater matrix acts as an extra radiator.
In the winter, to warm the engine up quicker, close the water valve thus removing the cooling effect of the heater matrix.
Might not be very popular with passengers though.
Well the good state of the engine is more important than the passengers comfort
