Frozen AC Coil in Chicago? Here's What's Causing It

Common causes of frozen ac coil

1. Dirty air filter

The #1 cause. A blocked filter reduces airflow across the indoor coil. Without warm air moving across it, the coil's surface temperature drops below freezing and condensation turns to ice. A filter is only ever part of the story, though — a coil that freezes once will freeze again unless the reason airflow dropped is identified, and a coil that freezes with a clean filter points to a refrigerant or blower fault instead.

2. Low refrigerant

Low refrigerant lowers the coil's operating pressure and temperature, causing it to freeze. Refrigerant doesn't get consumed — if it's low, there's a leak that needs to be found and sealed before recharging.

3. Blocked vents or closed dampers

If too many vents are closed in your house, return airflow drops and the coil freezes. Registers blocked by furniture or rugs have the same effect. It is worth telling us how many rooms have vents closed when you call — it helps us separate an airflow problem from a refrigerant one.

4. Dirty evaporator coil

Over years, the indoor coil collects dust that insulates it. The coil can't transfer heat efficiently and starts to freeze. Annual maintenance includes coil cleaning.

5. Failed blower motor or weak blower

If the indoor blower runs slow or not at all, airflow drops and the coil freezes. The system tries to keep running anyway, building up ice.

6. Outdoor temperature too low

Running AC when outdoor temps drop below ~60°F can cause freezing. Save the AC for actual hot days; use the system fan instead for circulation in mild weather.

What to tell us when you call

Turn the system off first. Running an air conditioner with a frozen coil can send liquid refrigerant back to the compressor, which is the most expensive component to replace. Shut it down, then call. The more you can describe, the better the chance we arrive with the right part already on the truck. It helps to know:

1. Where is the ice — on the indoor coil, the copper lines outside, or both?
2. Is this the first time it has frozen, or has it happened before?
3. When was the air filter last changed?
4. Was it cooling normally before this, or gradually getting weaker?
5. How old is the system, and when was it last serviced?

When to call SAVA

If your AC freezes again after thawing + filter change, the cause is either low refrigerant (leak), a dirty coil, or a failed blower. All require diagnosis with proper tools. Call us — running a frozen system damages the compressor.

SAVA is a family-owned, EPA Section 608 Certified HVAC company serving Chicago and North Shore suburbs. We diagnose the exact fault before any work begins, carry common parts for same-day repairs, and give honest repair-versus-replace guidance. Learn more about our AC repair service or emergency AC repair.

Brands we service

Plus most other residential central AC brands — if yours isn't on the list, call and we'll confirm.

Frequently Asked Questions