The 20-degree rule for air conditioning says your thermostat should never be set more than 20°F below the outdoor temperature. A standard residential AC is designed to produce about a 20-degree temperature differential between outdoor air and indoor air, push it past that, and the system runs continuously without ever reaching the setpoint, spiking your electric bill and straining the compressor. On a 100°F afternoon in the Valley, setting the thermostat to 78–80°F keeps the system within its designed capacity; setting it to 72°F forces nonstop runtime and premature wear. The rule is also a useful diagnostic: a healthy AC should easily hold a 20-degree split on a normal day, so if yours cannot, something is wrong, low refrigerant, a dirty coil, or undersized equipment are the most common culprits. Call LA Heating and Air at (818) 660-1062 for a diagnostic.
