Why Is My AC Set to 72 but Reads 78?

Short answerWe read a 6-degree gap as the AC either not keeping up with the heat or something limiting it. Bryant lists extreme heat, poor insulation and an undersized unit. DOE and Bryant also list a clogged filter, leaky ducts, dirty coils and low refrigerant. If the gap stays on a mild day, we recommend an HVAC technician.

What this costs

Our national estimates, not quotes.
Option	Job	Low	High
Replace	AC Replacement	$3,800	$9,050

First, ask whether it's the weather

American Standard's rule of thumb is to keep the setting no more than 20 degrees below the outdoor temperature. By our arithmetic, on a 98-degree day, that means 78. Bryant says an AC may run 70 to 80 percent of the time on very hot days, so it can take a while to reach the setting. In our view, a gap on the hottest afternoon isn't always a failure, and the same gap on a mild day is more telling.

Things you can check safely

Filter: DOE says a clogged filter restricts airflow and reduces cooling. Replace a dirty one.
Thermostat: make sure it's on Cool, set below room temperature, and reading correctly. DOE says it may need new batteries. Bryant says to set the fan to Auto.
Heat near the thermostat: DOE says to keep lamps and TVs away from it, because they make the AC run longer.
Outdoor unit: Bryant says to keep at least 2 feet of clearance around it.
Sun: Bryant suggests closing blinds and cutting heat sources.

When it's a job for a technician

DOE lists low or leaking refrigerant and leaking or clogged ducts as problems to take to a professional. It says air loss through ducts accounts for about 30 percent of a cooling system's energy use. We don't recommend refrigerant as a do-it-yourself job. If a technician says it's low, ask where the leak is. Bryant also lists an undersized unit, and we expect a sizing check is how to confirm that.

Don't turn it lower to catch up

American Standard says a system will only cool as fast as it's able, so a lower setting doesn't speed it up. It only wastes energy and adds strain.

AC Not Cooling? What to Check Before You Call

Common questions

How far off is too far?

We didn't find an official number for how many degrees is too many. Go by the pattern: if your AC runs constantly and can't close the gap on a mild day, call a technician.

Could my thermostat just be wrong?

It could. DOE says to make sure it's reading the correct temperature. Our own tip: a separate thermometer next to it is a simple way to compare.