The problem maybe in two places. Your coolant should not need a fill if you do not have a leak. The heater should work if you do have low coolant. You might have a vacuum leak affecting the heater door that moves the hot air. The coolant may be leaking slowly, too slowly to see it in a quick pressure test. You need an overnite pressure test. In this test pressure is maintained for a long period showing leaks that are not easily detected. My treatment of these leaks is a common copper based additive to the coolant. You need to check your coolant every gas fill or at least every month to catch problems like this.