What You Should Never Do to Your AC in Winter

An outdoor HVAC unit covered with a protective cover surrounded by snow — Credit:
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When cold weather arrives, your air conditioning unit is likely the last thing on your mind. But the care you provide your unit during winter can make or break how efficiently it runs when it warms back up. Even when it's not in use, your AC is still vulnerable to damage from cold weather, debris, animals, and well-intentioned mishaps. Something as harmless as covering your unit can cause permanent damage come spring and summer. To avoid costly repairs and headaches down the line, we spoke to an HVAC expert who cautions against making these five common mistakes when caring for your AC unit in winter.

Devin Coyne, HVAC technician for Climate Solutions Services, with more than 10 years of experience

Don't Cover the Outdoor AC Unit

While you may be tempted to cover your outdoor AC unit in winter to protect it from snow and sleet, doing so can cause irreparable damage. Covering the unit restricts airflow, causing moisture to build up and creating perfect conditions for mold and rust.

Covered AC units also become a hiding place for animals seeking shelter from inclement winter weather. "Squirrels, mice, and other small animals create nests under the cover, using paper and wires inside the unit to build their homes, which can cause internal electrical damage," says Devin Coyne, HVAC technician with Climate Solutions Services.

Don't Run Your AC Unit in Winter

We understand the temptation to run your AC unit during holiday parties when the extra bodies warm your home, but our HVAC expert warns against it unless your unit is designed to run in colder weather. "When you run your air conditioner in cold weather, it operates at a much lower pressure, which can cause the evaporator coil to freeze," says Coyne. "When the coil freezes, air can't flow through the unit, which creates a block of ice." The evaporator coil sits on top of your furnace and runs through your ductwork to your outdoor AC unit. "When you turn your heater back on, all of the ice melts and can potentially damage your furnace," he says.

Don't Close Your Air Vents

If you close your air vents in winter and run your AC unit, you risk freezing the evaporator coil. "Your ductwork is designed to carry a certain amount of CFMs (Cubic Feet Per Minute) to each room in your home," says Coyne. "Closing the air vents limits the amount of air moving through the house, and restricting airflow will cause the evaporator coil to freeze." Similar to the issue above, when you turn your heater back on, the ice will melt onto your furnace.

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Don't Turn Off the Power to Your AC Unit

This isn't the case for all AC units, but certain models shouldn't be turned off in winter. "There's a compressor in the outdoor unit, and most of the time, the compressor has a crankcase heater that keeps the oil from solidifying," says Coyne. "When you turn the unit off, you de-energize the crankcase heater, which causes the oil to congeal." This is an issue because, when you turn the unit back on in late spring, the oil won't be able to lubricate the compressor, causing it to lock up. According to Coyne, this can cost thousands of dollars to repair.

While AC units without a crankcase heater can be powered down during winter, Coyne still advises against it. He notes that many people forget they turned their unit off in winter. When warm weather arrives, they think their AC unit is broken and call out a technician for repairs. Some companies will charge you for the time it took the technician to come to your home, only for them to tell you your unit isn't on.