Turn the system off right away. That’s the first thing. If frost or ice starts forming anywhere on the unit–especially on the evaporator coil–you’re not getting proper cooling, and the longer it runs like that, the more likely you’ll end up with serious damage. Let it thaw, then take a closer look (or call someone who will).
One issue I’ve seen more than once is blocked airflow. It sounds simple, but a dirty air filter or closed-off vents can mess with how air moves over the coil. When there’s not enough warm air passing through, the coil gets too cold and freezes. Swapping out a filter takes two minutes and might save you from a service call.
Low refrigerant levels? That’s another one. If there’s a slow leak in the system–and these do happen–it reduces the pressure inside the coil. That drop in pressure causes the temperature to fall below freezing. Then condensation forms, freezes, and before you know it, there’s a chunk of ice where there shouldn’t be. You might hear a hissing sound. Or maybe the AC just never quite hits the temperature you set. That’s usually a red flag.
Thermostat settings can trip you up too. If someone cranks the temperature way down trying to “speed up” cooling on a hot day, the system might not shut off properly. It keeps running, the coil gets too cold, and–yep–more ice. Sometimes it’s not even the setting, but a faulty thermostat giving false readings. Hard to tell without testing, though.
It’s also worth thinking about weather. On cooler summer nights or during those odd temperature dips (and we get those here), running the AC might push the system into conditions it wasn’t really designed for. It seems counterintuitive, but an air conditioner running when it’s already chilly outside can actually increase the chance of icing up.
How Low Outdoor Temperatures Lead to AC Icing
Shut off the cooling function when nighttime temperatures dip below 15°C. Most air conditioners aren’t built to handle operation in cooler ambient conditions. If they keep running, the drop in pressure inside the evaporator coil can cause condensation to freeze on contact.
This usually happens during spring or early fall, when the weather flips quickly and the thermostat is still set to cool. The system pulls in cold outdoor air, which reduces the coil temperature too far. Suddenly, you’ve got ice buildup choking airflow and stressing the compressor.
Why the Coil Matters
The evaporator coil relies on a balance between refrigerant pressure and surrounding air temperature. When that balance shifts–say, because it’s only 11°C outside at night–the coil drops below zero and moisture in the air turns to frost. Left unchecked, the system strains to maintain the setpoint, using more energy and risking damage.
Simple Steps to Avoid It
If you’re running the AC on a cool evening, watch for signs like weak airflow or unexpected humidity. Better yet, use a programmable thermostat to shut the unit down automatically when the temperature drops. Some modern models have a low-ambient lockout feature–worth enabling if available.
I’ve seen systems in northwest neighbourhoods seize up completely in September just because someone forgot to adjust the schedule. A minor oversight, but it can lead to a service call. Prevention’s easier than dealing with a frozen unit and a few hours without cooling on a warm afternoon when it rebounds.
What Dirty Air Filters Do to Your AC’s Airflow and Coil Temperature
Replace the air filter monthly–maybe every two months if the system runs less often. That’s not an optional task. A clogged filter restricts airflow across the evaporator coil, and when airflow drops too low, the coil temperature drops below freezing. Ice forms. Then more ice. Eventually, airflow is nearly blocked, and now the unit struggles, wasting power and cooling almost nothing.
This isn’t just a matter of temperature. When the coil ices over, refrigerant pressure shifts. The compressor might overheat or short cycle. Some techs have found fan motors burned out after running against resistance for too long. It doesn’t take years. One particularly dusty spring can do it.
What Happens Inside the System
As the filter gets dirty, less warm indoor air reaches the coil. Since that air isn’t flowing past the coil to absorb the refrigerant’s cold, the temperature of the coil keeps dropping. Once it hits the frost point, moisture from the air begins to freeze on the fins. Not all at once, of course. But over hours or days, a thin film of frost becomes a thick crust. Cooling output drops sharply–often before the homeowner notices anything more than a slight change.
Why This Often Gets Missed
Many assume the unit’s just “getting old” or needs more refrigerant. It’s rarely that simple. In dozens of service calls, the root cause was just a filter clogged with pet hair, drywall dust, or cottonwood fluff. Even brand-new systems can ice up if the airflow is choked off.
If the coil’s already frozen, turn the system off and let it thaw naturally. Replacing the filter alone won’t fix it until the ice is gone. But after it melts, a fresh filter and routine checkups will usually keep the issue from coming back–unless there’s a deeper airflow problem hiding in the ductwork.
Why Low Refrigerant Levels Cause Ice Buildup on AC Coils
Check refrigerant levels immediately if you see frost or ice forming on the evaporator coil–this isn’t just cosmetic. Low pressure inside the coil causes the refrigerant to expand more than it should, dropping its temperature below freezing. When that happens, moisture in the air hitting the coil turns to ice instead of draining off as condensation.
This process doesn’t take long. Even a small leak can trigger it. You might notice weaker airflow first, or maybe warmer air from the vents. Then the coil ices over, airflow drops even more, and eventually, the entire system shuts down or short-cycles to avoid damage.
What causes the refrigerant to run low? Usually, it’s a leak–tiny pinholes in the lines, bad fittings, even corrosion from Calgary’s fluctuating indoor-outdoor temperatures. Recharging it without fixing the source won’t solve anything. It’ll just keep happening, maybe worse next time.
If you’ve topped off the refrigerant more than once in the past year, that’s a red flag. Pressure testing and UV dye tracing can help locate the exact spot. Don’t wait. Coils aren’t cheap, and neither is a burnt-out compressor.
One more thing: the coil needs warm indoor air to stay just above freezing. Low refrigerant disrupts that balance. The colder it gets, the more frost forms, and it becomes a feedback loop. Catching it early can save thousands–or at least spare you from being stuck without cooling during a July heatwave.
Contact “Calgary Air Heating and Cooling Ltd” For More Information:
Address
95 Beaconsfield Rise NW, Calgary, AB T3K 1X3
Phone
+1 403 720-0003
Hours of operation
Open 24 hours 7 days a week