What is the 20 degree rule for air conditioners?

What is the 20 degree rule for air conditioners refers to two common rules of thumb used by HVAC technicians for performance checking and sizing expectations. Most common 20 degree rule states central AC should be able to maintain indoor temperature no more than 20 degrees below outdoor temperature, and temperature difference between return air and supply air at air handler should be 18 to 22 degrees Fahrenheit in cooling mode, meaning if return air entering indoor coil is 75 degrees, supply air leaving coil should be 53 to 57 degrees, indicating proper refrigerant charge and airflow of 400 CFM per ton. If difference is less than 15 degrees, AC may be low on refrigerant, have dirty filter, or dirty evaporator coil blocking heat transfer, requiring service. If difference is more than 25 degrees, airflow may be too low. Second 20 degree rule is AC should be sized so that on hottest design day, it can keep indoor at 75 degrees when outdoor is 95 degrees, a 20 degree difference, per Manual J design. Some interpret 20 degree rule as AC can only cool 20 degrees below outdoor, so if outdoor is 100 degrees, indoor may only reach 80 degrees if undersized, which is normal on extreme days. This rule helps diagnose problems: if outdoor is 95 and indoor cannot get below 80, AC may be undersized, low on refrigerant, or duct leak. Always check filter and coils first before calling technician.