How does the cooling circuit ventilate?

Tach municipality! The Clio of my friend (Clio II, 1.2 16 V, year `03) received yesterday a new timing belt and a new water pump. At the pick-up the master said that we should check the cooling liquid level again today, because it has to be aired first and then replenished if necessary! This morning, however, the compensation tank was overcrowded with a cold motor, i.e. the liquid was in the container about 5 cm above the Min mark, so where it actually was should stand in case of a warm engine! Has there not been any venting or is there simply too much cooling fluid in it? Or is there a special procedure for venting the cooling system? Greeting Fly Death