How often should you clean your outdoor AC unit?

Short answer

Clean and check the outdoor AC unit once a year, before cooling season. The outdoor unit is the box with the big fan on top, and its job is to dump the heat pulled out of your house into the air outside. It does that through hundreds of thin metal fins, and when they're packed with grass clippings, leaves and cottonwood fluff, the heat has nowhere to go. A rinse with a garden hose and a careful look at the fins takes 45 to 90 minutes and costs $0 to $30 in supplies.

Why it matters

Refrigerant carries heat out of your rooms to the outdoor coil. Air pulled through the coil's fins by the fan takes that heat away. Block the fins and the heat stays in the refrigerant, so the compressor, which is the expensive heart of the system, runs hotter and longer to get the same cooling done. The house takes longer to cool, the electric meter spins faster, and on the hottest days the system may not keep up at all.

A compressor that runs hot for a summer wears out early. Replacing one costs a large share of what a whole new system costs, which is $5,000 to $12,000 for a matched furnace and air conditioner. A yearly rinse is the cheapest insurance that system has.

When to do it more often

If you have cottonwood trees nearby, or the mower throws clippings toward the unit, check the fins every month in summer. Fluff and clippings can coat a coil in a couple of weeks of heavy growth. A unit sitting under a shedding tree, next to a dryer vent, or beside a gravel driveway deserves the same monthly look. A glance is enough: if the fins look furry or gray instead of metallic, rinse them.

What you need

• A garden hose with a spray nozzle
• A screwdriver or nut driver, to remove the top grille on units that need it
• A soft brush, for debris that won't rinse off
• A fin comb, a cheap plastic tool that straightens bent fins
• Coil cleaner, a spray that foams away grime the hose leaves behind

Steps at a glance

1. Shut off power to the unit at the disconnect box on the wall beside it, or at the breaker.
2. Clear leaves, grass and anything growing within arm's reach of the unit.
3. Lift out the top grille and fan if the unit allows it, and scoop debris out of the bottom.
4. Rinse the fins from the inside out with the hose, so dirt pushes out the way it came in.
5. Spray coil cleaner on stubborn grime, wait as the label says, and rinse again.
6. Straighten any bent fins with the fin comb.
7. Put the grille back, restore power, and run the system to check that the fan spins and air blows out the top.

You're done when you can see light through the fins from the inside and the fan runs without rubbing.

Signs it's overdue

• The AC runs longer than it used to for the same thermostat setting.
• Air from the vents isn't cold, or isn't as cold as it was last summer.
• Ice on the refrigerant lines where they enter the house, which can come from poor airflow at either coil.
• Fins packed with debris, or a layer of fluff that looks like felt.

DIY or pro

Cleaning the outdoor unit is a moderate job that most people can do. The only tools are a hose and a screwdriver, and the main skill is being gentle with the fins, which bend if you lean on them. Use the hose at normal pressure, not a pressure washer, which folds fins flat.

A technician does the same cleaning as part of a spring tune-up, for $75 to $200. If you're paying for a tune-up anyway, let them do it. If you're not, do it yourself and save the visit for problems the hose can't fix, like a unit that hums without the fan turning or lines that keep freezing after the fins are clean.

Safety

• Electricity: shut off the disconnect or breaker before you take the grille off or put water anywhere near the unit. The fan can start on its own when the thermostat calls for cooling, and the unit's capacitor holds a charge even with the power off, so don't reach into the electrical compartment.

FAQ

Can I spray my outdoor AC unit with a hose?

Yes, with the power off. A garden hose at normal pressure is the right tool. Spray from the inside of the coil outward where you can, and don't use a pressure washer.

Should the outdoor AC unit be covered in summer?

No. It needs open air on every side to shed heat. Keep plants and fences well back from it.

What if the fins are bent?

Straighten them with a fin comb, working gently in the direction of the fins. A few bent fins don't matter, but a large flattened patch blocks airflow.

Does cleaning the outdoor unit lower my electric bill?

A clean coil sheds heat more easily, so the compressor runs less to reach the same temperature. The difference is biggest on a unit that was badly clogged.