Help us help you. Where does the coolant leak from? How much coolant did you lose? What temperatures did you see?
The AC and Heat are two completely unrelated systems - the AC uses a compressor, condenser, and evaporator to cool the air while the HEAT uses a mini-radiator-like heater core behind the dash to heat the air. The AC system does not directly use the coolant and therefore should not make it leak. The only thing I can think of is that your expansion tank cap is failing causing the system to either lose or gain pressure when your'e using the HVAC system.