Central AC is freezing up (ice on the coil/lines)

Q: Central AC is freezing up (ice on the coil/lines)

In Canada, when a central AC freezes up, the first step is always to thaw it: switch the thermostat to cooling OFF but leave the fan ON so the blower melts the ice off the coil — you cannot diagnose a frozen system, and running it iced can damage the compressor. Once it's thawed, the most common cause to rule out is a dirty air filter or blocked vents starving airflow across the indoor coil, both free-to-cheap DIY fixes. A dirty evaporator coil is next. Persistent freezing can mean low refrigerant from a leak or a failing blower motor — both licensed-HVAC repairs. Refrigerant work is licensed-only across Canada. Never chip ice off the coil.

A frozen air conditioner looks alarming, but the first move is simple and it's something you can do right now: turn the cooling off, run the fan, and let the ice melt. Most of the time the cause underneath is a cheap airflow problem.

tuneWhat are you noticing?

Select anything that matches and the checklist below reorders to fit. Not sure? Skip it.

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Start here

In Canada, thaw the system first: set the thermostat to cooling OFF but leave the FAN ON, and give it a few hours to melt all the ice — you can't diagnose a frozen coil, and running it iced can wreck the compressor. Then pull the air filter. A grey, clogged filter is the most common reason an AC freezes up, and a fresh one is under $20. Replace it, then run cooling again and watch whether it re-ices.

Ranked from real-world failure data. The order updates as you select what you’re noticing above.

Likely causes, in the order to check them

Showing the typical order

Beginner-friendly Doable, with care Safety-critical Call a pro

handymanHow to fix it

After fully thawing the system, find the filter at the indoor air handler or the return grille, read the size printed on its frame, and slide in a fresh one facing the airflow arrow the right way. Then run cooling and confirm it no longer re-ices.

Make sure the system is fully thawed (cooling OFF, fan ON until the ice is gone) before you close it back up and run cooling again.
Locate the filter slot at the indoor air handler or the large return-air grille, slide out the old filter, and read the size stamped on its cardboard frame (for example 16x25x1).
Insert a matching new filter with the printed airflow arrow pointing toward the unit, restore cooling, and check the coil an hour later — if it stays ice-free, that was it. Set a reminder to check the filter monthly in cooling season.

schedule10 min savings$8–20

Pleated furnace/AC air filterMatch the size stamped on the old filter frame (e.g. 16x25x1) and the MERV rating your system is rated for

gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: homeowner may do this · PE: homeowner may do this · QC: homeowner may do this
Replacing an air filter and opening or clearing supply/return vents is routine homeowner maintenance in every province.

verifiedWhy this one goes to a pro

The refrigerant charge is part of the sealed system. In Canada, handling refrigerant requires certification, and a real fix means finding and repairing the leak, not adding gas. This is a licensed HVAC repair — a technician can confirm a low charge quickly and do it legally.

gavel Check your province first AB: licensed professionals only · BC: licensed professionals only · ON: licensed professionals only · PE: licensed professionals only · QC: licensed professionals only
Refrigerant handling requires certification across Canada; leak repair and recharge are licensed-only work in every province.

Your notes travel with the request — what you saw, what you ruled out, when it started — so the pro arrives with a head start instead of a mystery.

visibilityHow to check

warning Thaw it fully before you open or run anythingTwo things matter here. First, always thaw before diagnosing — cooling OFF, fan ON — because you cannot see a coil under ice, and running the compressor while the coil is frozen can slug it with liquid refrigerant and damage it. Second, when you do open the air handler, cut power at the thermostat and, if in doubt, the furnace/air-handler switch or breaker; the blower can start on its own. Never chip or scrape ice off the coil — the fins are thin aluminum over refrigerant tubing and puncture easily. If a light dusting doesn't restore it, a full coil clean is a fast job for an HVAC tech.

Thaw completely first: set the thermostat to cooling OFF, leave the FAN ON, and wait until every trace of ice on the coil and lines is gone — often a few hours. Do not skip this; a frozen coil can't be read and running it iced can damage the compressor.
Once thawed, turn the system fully off at the thermostat, remove the access panel on the indoor air handler, and shine a light on the evaporator coil (the A-shaped or slab coil of fins).
Look at the fins: clean aluminum with visible gaps means the coil is fine and the trouble is elsewhere; a grey, furry, or matted coat of dust means the coil is choking its own airflow and needs cleaning.

buildflashlight schedule20 min

verifiedWhy this one goes to a pro

Diagnosing and replacing a blower motor means working around line-voltage wiring, a run capacitor that can hold a dangerous charge, and a motor that must be matched to the system. This is a licensed HVAC job — describe the weak or straining fan to a technician and let them confirm it.

Your notes travel with the request — what you saw, what you ruled out, when it started — so the pro arrives with a head start instead of a mystery.

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Why the list is in this order

We don’t sort by likelihood alone. We sort the way a good, honest tradesperson thinks: rule out the free, safe, common causes first. That’s why the cheap checks outrank the expensive repairs — and why anything dangerous gets a hard stop instead of a pep talk. The order exists to save you money, not to sell you anything.