Heating in van not working properly

Coolant full and void of air bubbles?

how can i know that?

A visual inspection is the first step. See if reservoir is at the full line. Not 100% full up to the neck but up to the fill line. Run vehicle and see if level goes down in which radiator could be low. If car is 6 hours cold, open radiator cap (if it has one) and see if you can see fluid. Then fill reservoir and run vehicle. After you shut off car, see how much was taken into system. Wait to refill because it is under pressure and you can get burned and sprayed in face.