Frozen Coil — Commercial Air Conditioning

Ice on an evaporator coil seems paradoxical in the middle of a heat wave, yet it is one of the most common air-conditioning failures in commercial buildings. The mechanism is simple: if the coil drops below 0°C — for lack of air to cool or lack of refrigerant — the moisture in the air deposits as frost, which blocks the airflow and makes the freezing worse.

What makes an evaporator coil freeze?

Two families of causes. Air side: clogged filters, a failing supply fan, closed dampers or blocked diffusers — airflow drops and the coil overcools. Refrigerant side: insufficient charge (usually a leak), a misadjusted expansion device, or prolonged operation at a very low setpoint. The frost is never the failure itself: it is the symptom of an imbalance between airflow and cooling capacity.

What can you safely do when you find ice?

Stop the call for cooling (fan-only mode if available) while keeping the fan running to speed up the thaw, plan for the melt water (drain pan, ceilings below the unit), and replace the filter if it is accessible without tools or working at height. Never scrape the ice and never open the refrigerant circuit: pressurized fluids and sharp surfaces are for trained personnel.

When should you call an HVAC professional?

If the ice returns after a complete defrost and a fresh filter, if the unit serves critical spaces, or if melt water has reached ceilings or equipment. Montréal Combustion diagnoses frozen coils, refrigerant charges and airflow on commercial air conditioners and rooftop units across Greater Montréal: 450-473-0909.

Frequently Asked Questions

Can we scrape or break the ice to speed up defrosting?

No. The aluminum fins and coil tubes bend or puncture easily — a refrigerant leak would cost far more than the thawing time. Stop the cooling, keep the fan running and let the ice melt on its own.

Can a dirty filter really freeze a coil?

Yes, it's the most frequent cause: reduced airflow drops the coil temperature below freezing and the moisture in the air builds up as ice. Regular filter replacement is the cheapest prevention there is.

The ice comes back after every defrost: what does that mean?

A quick recurrence despite clean filters points to an insufficient refrigerant charge (likely a leak), a failing fan or a misadjusted expansion device. These causes require measurements by a qualified technician — recharging without repairing fixes nothing.