Why Air Conditioners Freeze Up, and How to Stop It

It sounds backwards, but on a hot day you can find ice on your air conditioner. Ice on the copper lines outside, frost on the indoor coil, or water pooling around the furnace once it melts. Here’s why it happens.

How a coil ends up frozen

The indoor coil runs just above freezing. Warm house air moving across it keeps it there. If not enough air crosses the coil, or the refrigerant pressure drops too low, the coil temperature falls below 0°C and the moisture it pulls from the air freezes on the surface. Ice blocks more airflow, which makes it colder, and it snowballs.

Common causes

• Dirty filter: the number one cause.
• Closed or blocked registers and returns.
• Dirty indoor coil: dust that gets past a poor filter coats the coil over the years.
• Blower problems: a weak capacitor or failing motor, or the wrong fan speed set for cooling.
• Low refrigerant: which means there’s a leak. Refrigerant doesn’t get “used up.”
• Running the AC when it’s cool outside: below roughly 15°C outdoors, many systems aren’t designed to run and the coil can freeze.

How to thaw it safely

Turn the thermostat to Off and the fan to On. Let it run until all the ice is gone, which can take a few hours. Put a towel down near the furnace in case the drain pan overflows. Don’t chip at the ice; the coil fins and copper are easy to damage.

Prevention

Change the filter regularly, keep registers open, and have the system checked yearly. If it freezes again with a clean filter, it’s time for a technician to check refrigerant pressures and airflow. Topping up refrigerant without finding the leak is a short-term fix at best.

Need help? See our air conditioning services or contact us at 647-657-2977. We serve the whole GTA.

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