Should You Cover the Outdoor AC Unit for Winter?

The leaves are coming down, the AC has been off for weeks, and you're looking at that metal box beside the house and wondering if it needs a coat for winter. Hardware stores have covers on the shelf, and the house you're buying may even have one draped over the condenser. For most cooling-only central air units, the answer is no. A full cover often does more harm than good. If you want some protection, cover only the top. And if the unit is a heat pump, don't cover it at all while it's in use.

01The short answer

Outdoor condensers are built to sit out in the weather all year. Lennox answers "no" when asked whether you need to cover the outdoor unit, and says modern units "typically don't require winterization." HVAC.com says manufacturers generally advise against covering and leave the final call to the homeowner. HVAC pros quoted by AOL also advise against fully covering an AC for winter.

There's one exception for cooling-only systems. If your yard drops a lot of debris on the unit, a top-only cover can help keep it out. The Farmers' Almanac suggests checking your owner's manual and asking your installer whether conditions at your house justify one.

Here's what that means in practice:

• Cooling-only central AC: no full cover. A breathable cover or a board on top is optional, and the sides stay open.
• Heat pump or heating mini-split: never covered while it's running. It works all winter and needs open airflow.
• Buying a house with a covered unit: take the cover off before anyone runs the system, and look inside for rust, nests and chewed wiring.

That last point is about more than looks. A full cover causes real problems.

02Why a full cover causes problems

Trapped moisture. Rain, snowmelt and condensation get under a cover, and the cover keeps them from drying out. Lennox warns that full covers, especially plastic or vinyl, can trap moisture and invite rust, mold and rodents. HVAC.com says the trapped moisture can cause mold and damage wiring, connections and circuit boards. Matt Gorbacz of Clean Air Technologies told Hunker that plastic or tarps "trap moisture and leave you with rust, corrosion, and even critters nesting in the unit."

A winter home for animals. A covered condenser is dry, dark and out of the wind, which makes it a good shelter for mice and squirrels. HVAC pros quoted by AOL say animals nest in covers, chew wiring and block drains. You often won't find the damage until the first hot day in spring, when the unit won't start.

Startup damage. Covers get forgotten. Randall Boutte of HomeServe told AOL that running the AC with a cover still on can damage the equipment or cause refrigerant leaks. If you do use a cover, put a note on the thermostat as a reminder.

Your warranty. Travis Arnesen of Service Experts told AOL that damage from improper covering will likely void most warranties. Check your warranty paperwork before you buy anything.

A "waterproof" label doesn't fix any of this. The Farmers' Almanac notes that "waterproof" on the package doesn't mean a cover suits your unit, and a cover that keeps water out can also keep it in.

03When a top-only cover makes sense

Lennox says a cover may be worth considering if the unit sits under trees, collects falling debris or leaves, or if your technician recommends one. Oaks, maples and pines dropping into the top grille all fall into that category. A top cover keeps leaves, twigs and seed pods out of the fan opening while the sides stay open so air can move and moisture can dry.

You have three common options:

• A breathable top cover. Lennox suggests a "lightweight, breathable cover" that won't collect water. Hunker names mesh covers as one choice. AOL's sources describe a top-only cover of waterproof, flexible plastic that sits over the fan and leaves the sides open.
• A sheet of plywood. Lennox says plywood secured on top is fine as long as it doesn't restrict airflow. Weigh it down or strap it so wind can't lift it. AOL warns that in snowy areas, the weight of ice and snow can bend or crack covers, so check the top after big storms.
• A manufacturer accessory. Trane sells a WeatherGuard Top accessory listed under its heat pump products, in two kits: BAYTWGR0003 for a 33" x 30" base and BAYTWGR0004 for a 37" x 34" base. Trane's product page doesn't say whether it should stay on through winter, so ask a dealer before you leave it in place.

Clean the unit before you cover it. Lennox says to clean the unit and clear debris from inside it first. Hunker recommends hosing off the unit and clearing away leaves before winter. A late-October afternoon, once most of the leaves are down, is a good time to do it:

1. Turn off power at the outdoor disconnect.
2. Pull leaves and twigs off the top grille and out of the base.
3. Rinse the fins gently from the inside out with a garden hose at low pressure, not a pressure washer.
4. Let the unit dry, then put on the top cover.

All of this applies only to cooling-only units. Heat pumps follow different rules.

04Heat pumps and mini-splits

A heat pump's outdoor unit is the heart of your heating system in winter. It pulls heat from the outdoor air, so it has to move a lot of air all season. HVAC.com says you "should never cover the outdoor unit" of a heat pump, and cites fire hazard, blocked airflow and overheating. Lennox says heat pumps that run year-round should never be covered. The Farmers' Almanac reports that Trane advises against a solid cover and recommends clearing snow away instead.

Many mini-splits heat as well as cool. The Farmers' Almanac says to check whether yours does. If it heats, it needs the same open airflow plus room to drain water during defrost.

Frost is normal, but heavy ice is not. Heat pumps build up frost on the coil in cold, damp weather. Lennox says light frost on the coils is normal, and that the unit should defrost itself about every 30 to 90 minutes. During defrost you may see steam and hear the fan stop for a short time. These are signs of trouble:

• Thick ice on the cabinet or coils
• Ice blocking the fan
• Icicles hanging from the unit
• Ice that doesn't melt within 6 to 8 hours, or that refreezes quickly (Lennox says ice should thaw in about 2 to 4 hours once it's above freezing)

Never chip the ice. Lennox warns against chipping at ice or pouring hot water on the unit, because either one can damage the coils, fins and fan blades. A snow guard on top is fine, but don't block the sides.

Keep 2 feet clear. Lennox and AOL's sources both recommend at least 2 feet of clearance on all sides. That means shoveling snow away, keeping drifts off, and making sure the roof drip line doesn't drop water onto the top.

05Winter checklist and when to call a pro

Before winter (a good job for the weeks before Thanksgiving):

1. Clear leaves and debris from the top and base, and rinse the fins gently.
2. Trim shrubs and move stored items so there's about 2 feet of space on all sides.
3. Cooling-only AC: add a top-only cover if debris is a problem. Never wrap the sides.
4. Heat pump: no cover. Put a snow shovel somewhere you can reach it easily.
5. Book a professional inspection. AOL's sources recommend one every year, before winter.

Signs you need a technician. Lennox says to call a pro if:

• The outdoor fan won't spin after the ice melts
• The refrigerant lines ice over
• You hear hissing
• You hear grinding, squealing or banging
• Ice won't clear within 6 to 8 hours or keeps coming back

Refrigerant work and electrical repairs inside the unit are jobs for a licensed HVAC technician. Your part is limited to turning off the power and clearing debris and snow.

What damage costs. If rust, ice or chewed wiring ruins the outdoor coil, replacement gets expensive. PickHVAC puts condenser coil replacement at about $900 to $2,300 installed, with labor of roughly $400 to $1,000 (2 to 4 hours at $75 to $150 an hour). Other estimates reach about $800 to $3,000, so treat any single figure as a rough estimate and get local quotes. Check the warranty first. PickHVAC notes that a manufacturer's warranty on the coil may cover the part, leaving you to pay for labor and refrigerant. That's one more reason not to void the warranty with the wrong cover.

Buyers: if the inspection report mentions a cover on the condenser, ask how long it has been on. Then have the inspector or an HVAC technician look for corrosion on the coil, rodent nesting, and damaged wiring before you close.

Sources (9)

• Lennox: Outdoor Unit Care FAQ
• Lennox: Frozen Heat Pump
• HVAC.com: Should You Cover Your Outdoor Unit in Winter?
• Farmers' Almanac
• AOL: Should You Cover Your AC Unit in Winter? HVAC Pros Weigh In
• Hunker: Should You Cover Your Air Conditioner in Winter?
• Trane: WeatherGuard Top Accessory
• PickHVAC: AC Coil Replacement Cost
• HomeAdvisor: AC Condenser Repair and Replacement

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