when using air conditioner, coolant smell comes through vents, why?

Pressure test the cooling system for leaks.

It would all depend on what type of smell is being noted. sometimes there may be a musty or mildew smell that is the a/c core having "growth" on it that may be needing to be cleaned. also could have issue with system leakage or a dirty cabin filter could even cause a smell concern

Your heater core could be going out as well. Do the windows fog?