A dirty evaporator in an air conditioning system will cause these symptoms...

• A. High suction pressure and high discharge pressure
• B. Low suction pressure and high discharge pressure
• C. High suction pressure and lower than normal discharge pressure
• D. Low suction pressure and low discharge pressure

Answer: (D)

When the evaporator coil is dirty, airflow across the coil is restricted and heat transfer from the air to the refrigerant drops. Because less heat is absorbed, the refrigerant boils less vigorously and the evaporating temperature drops. That means the low-pressure side (the suction pressure) falls below normal. With the refrigerant vapor generation reduced, the compressor has less work to do, so it moves less refrigerant and the high-pressure side (the discharge pressure) also drops. In short, a dirty evaporator reduces both sides of the pressures because it lowers the system’s heat transfer and cooling capacity.