Arrived to do a performance maintenance. Set system for cooling. Measured the systems static pressure measured a temperature split between return and supply air. Went outside and verified refrigerant levels. Verified operation of all electronic and mechanical components. Rinsed coil free of debris. Went back inside and checked interior of the furnace for cleanliness. Found that the fan for heating and cooling is heavily coated in dust debris. Spoke with homeowner about the system. Air conditioning system is operational at this time.