Why Is My Air Conditioner Running All Day? Eight Reasons Alberta Homes Struggle To Cool

It is 31°C in Spruce Grove, the air conditioner has not shut off since breakfast, and you are wondering whether it is dying or just losing. Here is the reassuring part: on a genuinely hot Alberta afternoon, a correctly sized air conditioner is supposed to run for long stretches. The worrying version is different — the unit runs all day and the house still never reaches the temperature you set. That is almost never a dead compressor. Nine times out of ten it is airflow, and most of the causes are things you can check yourself in ten minutes.

First: Long Run Times Are Not Automatically A Problem

An air conditioner does not cool a house the way a kettle boils water. It removes heat at a steady rate, and it has to remove heat faster than the sun, your appliances and your walls put it back in. On a 32°C day, that balance point often means the system simply runs — continuously, quietly, and correctly.

In fact, a long, steady cycle is more efficient and more comfortable than a system that blasts for six minutes and shuts off. Short-cycling wastes energy on repeated start-ups, strips out less humidity, and wears out the compressor faster. If your neighbour brags that their A/C “barely runs,” they may just have an oversized unit.

The real question is not how long the system runs. It is whether it is gaining ground. Set the thermostat, wait an hour, and watch the indoor temperature. If it is slowly falling, your system is working. If it has been flat for hours, keep reading.

Reason 1: A Clogged Filter Is Strangling The System

This is the single most common cause we find in Greater Edmonton homes, and it is also the cheapest to fix. Your furnace blower pushes air across the indoor evaporator coil; if the filter is packed with dust, pet hair and wildfire-smoke particulate, far less air crosses that coil. Less air means less heat carried away, which means the house cools slowly — and the system runs and runs trying to catch up.

ENERGY STAR’s guidance is blunt: check the filter every month during heavy-use months, and change it at least every three months. In an Alberta summer that overlaps smoke season, monthly is often the real interval.

Reason 2: The Outdoor Condenser Cannot Dump Its Heat

Everything your A/C removes from inside has to be rejected outside, through the condenser coil. If that coil is furred with cottonwood fluff, grass clippings, dryer lint or dust — and in this part of Alberta, late June cottonwood is practically a seasonal hazard — the unit cannot shed heat. Head pressure climbs, capacity falls, run times stretch out, and the compressor works harder for less result.

Give the unit room to breathe. Keep at least half a metre of clearance on all sides, do not stack patio furniture or bikes against it, and with the power off at the disconnect, rinse the fins gently from the inside out with a garden hose. Never use a pressure washer — the fins bend, and bent fins block airflow permanently.

Reason 3: The Evaporator Coil Has Frozen Solid

It sounds absurd during a heat wave, but a block of ice on the indoor coil is a classic mid-summer failure. Any condition that reduces airflow across the coil — a filthy filter most often, but also a failing blower or closed registers — drops the coil surface below freezing. Condensation turns to frost, frost becomes ice, ice blocks what little airflow remained, and the system now runs continuously while cooling almost nothing.

The tell is a cold supply register with barely any air coming out of it, or visible frost on the copper line at the furnace. The fix is not to keep running it. Switch the thermostat to OFF, set the fan to ON, and let the blower melt the ice for a few hours. Then replace the filter before restarting the cooling. If it freezes again, you have a refrigerant or blower problem that needs a technician.

Reason 4: Your Ductwork Is Working Against You

Alberta homes were designed around furnaces. Warm air rises on its own, so heating forgives a lot of duct sins that cooling does not. Add a decade of accumulated dust, a few crushed flex runs in the basement ceiling, and a handful of registers closed off “to save money,” and you have a system delivering meaningfully less air than its rating.

• Closed vents do not save money. They raise static pressure across the whole system, reduce total airflow, strain the blower, and can push the coil toward freezing.
• Blocked registers count as closed. A sofa, a rug or a stack of storage bins over a floor vent has the same effect.
• Dirty ducts cost airflow at every run. Buildup narrows the effective duct diameter, and what settles there gets recirculated into your rooms.
• Leaky ducts cool your basement joists. Air escaping an unsealed seam in an unconditioned space is cooling you paid for and will never feel.
• Undersized returns are the silent one. A system can only supply as much air as it can pull back. Too few returns caps everything downstream.

Reason 5: The Thermostat Is Asking For Something Impossible

A residential air conditioner is engineered to hold roughly a 20°C indoor temperature against a design outdoor temperature — not to produce arctic air on demand. Setting the thermostat to 17°C on a 33°C day does not cool the house faster; the system removes heat at exactly one rate. All you have done is guarantee it never reaches setpoint, so it never shuts off.

Natural Resources Canada suggests a summer setting between 22°C and 25°C, with the higher end of that range costing you noticeably less. NRCan also notes that running an ENERGY STAR certified ceiling fan alongside the A/C lets you set the thermostat two degrees higher while feeling the same — cutting cooling costs by about 14%. If nobody will be home for more than four hours, push the setpoint up to around 28°C rather than turning the system off entirely.

Reason 6: The House Is Gaining Heat Faster Than You Think

Once sunlight passes through glass and is absorbed by your floor, that heat is inside and your A/C has to pay to remove it. Blocking it before the glass beats blocking it after, every time. West-facing bedrooms in Spruce Grove’s newer subdivisions are the classic offender — large windows, light-coloured interiors, and full afternoon sun.

Close blinds before the sun hits the window, not after. Run the dishwasher and dryer at night. Cook outside on the worst days. And check the door to the garage: an unweatherstripped gap there is a direct pipeline from a 40°C box into your hallway.

Reason 7: Low Refrigerant — Which Means A Leak

Refrigerant is not fuel. It is not consumed, and a sealed system does not need topping up. If your charge is low, refrigerant left through a leak, and adding more without finding that leak is paying to fill a bucket with a hole in it. Symptoms overlap heavily with airflow problems — long run times, weak cooling, a frozen coil — which is exactly why you rule out the free stuff first.

Reason 8: Smoke Season Is Making Everything Worse

Wildfire smoke is now a recurring feature of Alberta summers, and Edmonton historically sees its smokiest skies later in the season. Smoke loads your filter far faster than dust alone, which quietly drags you back to Reason 1. If you have upgraded to a denser MERV 13 filter to capture fine particulate — which Health Canada recommends for smoke — verify that your equipment can handle the added resistance. Over-filtering an older furnace can starve the very airflow you are trying to clean.

When It Is Time To Call Someone

Call a professional if the coil re-freezes after a filter change, if the outdoor unit hums but the fan does not turn, if you hear the compressor start and immediately stop, if there is standing water or oily residue around the equipment, or if the system simply cannot hold setpoint after you have addressed filter, coil, registers and thermostat. Those are equipment problems, not maintenance problems.

Everything upstream of that — the filter, the coils, the ducts, the airflow — is maintenance. It is also, in our experience across Spruce Grove, Stony Plain and the Greater Edmonton Area, where the answer usually lives.

Quick Answers

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This article is general maintenance information for Alberta homeowners, not a substitute for a professional diagnosis. Efficiency and thermostat guidance is summarized from Natural Resources Canada and ENERGY STAR; heat warning thresholds from Environment and Climate Change Canada. Refrigerant work must be performed by a licensed technician. Always disconnect power before cleaning outdoor equipment.