why do I smell exhaust when I turn on the heater
Because you have an Exhaust leak and that is dangerous so either find it and fix or have ir repaired .
+1 to Volvo V70’s comment.
Most likely the exhaust leak is under the hood–from the exhaust manifold–but if the manifold/manifold gasket prove to be okay, then further checking of the entire exhaust system is a must.
Ideally, until the source of the exhaust leak has been located, the car shouldn’t be driven. If it must be driven, then it should be done only with all of the windows open.
You could have oil from the valve cover dripping on the exhaust manifold which many people mistake for an exhaust smell