AC6 min read

AC Not Cooling During an Edmonton Heat Event: 7 Causes to Check First

Updated: October 23, 2026•EdmontonHVAC.ca

Quick Answer

An Edmonton AC running but not cooling has seven likely causes: a clogged air filter restricting airflow (free fix, 30% of cases), low refrigerant from a leak (not consumed — leaks must be found and repaired, 20%), a failed capacitor on the outdoor unit (cheap fix, 15%), a frozen evaporator coil (shuts system down, 10%), an undersized system that can't keep up with current heat events (sizing issue, 10%), dirty outdoor condenser coils (easy clean, 8%), and compressor problems (expensive, 7%). The first three cover two-thirds of all cases. During peak heat weeks, response times stretch to 2-3 days, so catching problems early matters.

First, confirm the AC is actually running

Walk outside and listen at the condenser unit (the large outdoor box). If running, you'll hear the compressor and fan. If silent, the outdoor unit isn't getting power — check the dedicated breaker in your electrical panel, the disconnect switch on the wall next to the condenser (usually a pull-out lever), and the thermostat is set to Cool below current room temperature.

If outdoor unit is running but no cold air is coming out of indoor vents, see Cause #4 (frozen coil). If outdoor unit is running normally and some cool air is coming out but it's not enough, see Causes #1, #2, #3.

Cause #1: Clogged air filter (30%)

Same cause as half of furnace short-cycling issues — a clogged filter restricts airflow across the indoor evaporator coil. The coil can't absorb heat from the air passing through it, so cold air production drops. In severe cases, the coil freezes solid, which shuts the system down entirely (Cause #4).

The 60-second AC check

Pull the filter out and look through it against a light. If you can't see light, replace it. Edmonton homes typically need filter changes every 60-90 days during cooling season. If it's been more than 3 months, that's the first fix to try.

Cause #2: Low refrigerant (20%)

Refrigerant is not a consumable. If your AC is low on refrigerant, there's a leak somewhere — through a pin-hole in a copper line, through a failed valve seal, through a coil crack. 'Topping up' the refrigerant without finding the leak just leaks out again within weeks and damages the compressor along the way.

The symptom of low refrigerant: AC runs continuously, outdoor unit sounds normal, indoor air is cool but not cold, and cooling capacity dropping over weeks or months. Often paired with ice forming on the refrigerant line going into the house.

Watch out for 'top-up only' service

If a technician quotes refrigerant top-up without leak detection, decline the service and find a different technician. Proper repair requires: electronic leak detection (sometimes UV dye), repair of the leak, then recharge to manufacturer specification. Total cost: $500-$1,500 depending on leak location. Compressor damage from running low adds another $2,000-$4,500 if the leak was ignored too long.

Cause #3: Failed capacitor (15%)

The outdoor unit uses a capacitor to help start the compressor and fan motor. Capacitors fail with age and heat — Edmonton summers of mid-30°C stress them. Symptom: outdoor unit humming but not actually running, or starting but shutting off within seconds.

This is the cheapest common AC repair: $200-$400 fully installed. 15-minute replacement for a trained technician. Usually possible same-visit.

Cause #4: Frozen evaporator coil (10%)

If restricted airflow (clogged filter) or low refrigerant goes uncorrected, the evaporator coil (inside the indoor unit) can drop below freezing. Moisture in the air freezes on the coil, forming an increasingly thick layer of ice. The ice insulates the coil from the air passing through it, cooling production drops to near zero, and the system shuts down to prevent damage.

Turn the AC off at the thermostat (don't just raise the setpoint — fully OFF).
Turn the Fan to ON (which runs the blower without cooling), so warm indoor air flows across the frozen coil to accelerate thawing.
Wait 4-8 hours for the coil to fully thaw. You may see water around the indoor unit — have towels ready.
While waiting, replace the air filter.
After thawing, turn the AC back on with the fresh filter. If it cools normally, the problem was filter-related.
If it freezes again within 24 hours, the underlying cause is low refrigerant — book a service call.

Cause #5: System undersized for current heat events (10%)

Edmonton heat events have gotten longer and more intense. ACs that were adequately sized when installed 10-15 years ago may not keep up with current peak weeks. Symptom: AC runs continuously during peak afternoon heat but can't quite get the house below 24-25°C.

This isn't a repair situation — the system is working as designed; the design is just no longer adequate for the climate. Options: live with it (many Edmonton homes do), add supplemental cooling (ductless mini-split for the hottest rooms), replace with a larger unit at next replacement cycle (not immediately — $5,000-$8,000 project).

Cause #6: Dirty outdoor condenser coils (8%)

The outdoor unit has coils that release heat to outside air. Over years, those coils accumulate pollen, grass clippings, dirt, cottonwood fluff — anything the fan has pulled in. Dirty coils can't release heat efficiently, which drops cooling capacity.

The fix is a condenser coil cleaning — easy DIY with a garden hose spraying from the inside out, or part of an annual tune-up. Pricing: $150-$250 for professional cleaning. Dramatic improvement if coils were severely clogged.

Cause #7: Compressor issues (7%)

Rare, expensive, and usually the diagnosis of exclusion. A failing compressor can run but produce reduced cooling, overheat, or shut off from internal protection. Symptoms: outdoor unit louder than normal, breaker tripping, uneven cooling that doesn't improve with filter/refrigerant service.

Compressor replacement: $2,000-$4,500. On an AC past 10 years old, replacement of the entire outdoor condenser usually beats compressor-only replacement in both cost and warranty.

Edmonton heat event timing matters

Edmonton's peak heat weeks (typically mid-June through late August) create massive AC service demand. Reputable dispatchers prioritize no-cool emergencies, but typical response during a 30°C+ heat wave stretches to 1-3 days. If your AC is showing weakness in late spring, book service in April or May — before the peak-season rate increase and the backlog.

Annual spring tune-ups ($150-$300) include refrigerant check, capacitor test, coil cleaning, and filter replacement — catching most of the common failures before they strand you during a heat wave.

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Frequently asked questions

How much does it cost to fix an AC not cooling in Edmonton?

Capacitor replacement: $200-$400. Contactor replacement: $200-$400. Refrigerant leak repair + recharge: $500-$1,500. Fan motor: $400-$800. Evaporator coil replacement: $1,500-$3,000. Compressor replacement: $2,000-$4,500 (often replacement of the whole outdoor unit makes more sense). Diagnostic service call: $100-$200, often waived if you authorize the repair same visit.

Why does my AC work in the morning but not the afternoon?

Classic undersized or borderline-sized system symptom. In the morning when outdoor temperature is 20°C and the house is cool, the AC easily maintains temperature. By mid-afternoon when outdoor temp hits 30°C+ and solar heat gain is at peak, the system can't produce enough cooling to overcome the heat input. Either an undersized system, or a system limping along with a refrigerant leak or dirty coils (making a correctly-sized system behave as if undersized).

Should I turn my AC off when I leave or just raise the setpoint?

For Edmonton summer, raising the setpoint 3-5°C saves energy without creating recovery problems when you return. Turning the AC fully off during peak heat means the house heats up significantly, and the AC runs at maximum capacity for hours when you return — stressing equipment and sometimes failing to catch up in extreme heat. Programmable or smart thermostats make this trivial to automate.

Is it safe to run my AC continuously during a heat event?

Yes — modern AC units are designed for continuous operation during peak demand. In Edmonton's longer summer heat events (3+ consecutive 30°C days), ACs will run most of the day without issue. If an AC can't maintain setpoint even running continuously, that indicates undersizing or system weakness, not that the system is being overworked by running continuously.

What's the best thermostat setpoint for Edmonton summer?

Comfort setpoints typically run 22-24°C daytime, 20-22°C overnight. Energy-saving setpoints (24-26°C) reduce AC runtime significantly. Setting much below 22°C in peak heat often has the AC running continuously without achieving setpoint — which uses more energy and stresses equipment compared to accepting a slightly higher indoor temperature.

Can wildfire smoke affect my AC performance?

Smoke itself doesn't affect the AC's cooling capacity, but ash and particulate matter clog air filters much faster than normal. During Alberta wildfire smoke weeks, change your air filter every 2-4 weeks instead of 60-90 days, and keep windows closed to prevent smoke infiltration (which overloads the filter further). MERV 11-13 filters handle smoke particles better than lower MERV ratings.