The outdoor unit rejects heat from your house. The condenser coil — the wall of aluminum fins around the outside of the unit — is where that heat exchange happens. When the fins are coated with dirt, pollen, grass clippings, or cottonwood fluff, the system works harder and costs more to run.
Cleaning the condenser takes about 15 minutes and a garden hose.
Step by step
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Turn the system off at the thermostat. Then turn it off at the disconnect box mounted on the wall near the unit.
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Clear debris from around the unit. Remove leaves, mulch, weeds, and any objects within two feet of the unit on all sides. Trim back any shrubs or plants encroaching on the clearance zone.
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Rinse from the inside out. Stand above the unit and angle the hose downward through the top grille, spraying outward through the fins. The goal is to push dirt out from the inside, not drive it deeper into the coil from the outside. Work your way around the unit.
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Use a regular garden hose. A pressure washer will flatten the fins. Flattened fins block airflow and reduce efficiency. A standard hose with a spray nozzle provides enough force to remove surface debris without damage.
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Do not bend the fins. If you see fins that are already bent or flattened, a fin comb (available at hardware stores for a few dollars) can straighten them. But be gentle — the aluminum is soft.
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Let it dry, then restore power. Wait 15 to 20 minutes for the electrical components to dry before switching the disconnect back on and turning the thermostat to "cool."
How often
Once or twice during the cooling season is usually enough. If you have trees shedding pollen or cottonwood directly over the unit, check it monthly.