I get this one a lot in Calgary. You flip on your home cooling, you keep it set where you can actually sleep, and then your power charges show up and it feels like something is broken. Sometimes something is broken. Other times it is just how hard your system has to work in our dry heat, especially if your place bakes in late-day sun and the air leaks out faster than you think.

I have been on calls where the homeowner swore the outdoor unit was “going crazy,” and I walk around back and the condenser coil is basically wearing a sweater of cottonwood fluff. Or the filter looks like a felt pad because it has not been changed since last summer. Then the blower is starving, the coil gets too cold, things ice up, it cycles weird, and you pay for a lot of electricity without getting much comfort. It is not magic. It is just airflow and heat transfer, and both can get wrecked by small, boring stuff.

There is also the way people use their thermostat. Some houses do fine with a steady setpoint. Some do not, because they are drafty, or the attic insulation is thin, or the return air is in a bad spot. I have seen beautiful new equipment stuck with a supply duct that came loose in a ceiling space, cooling the attic instead of your bedrooms. You can imagine how that shows up on your meter.

So in this article I am going to walk through the usual suspects I see on service calls. Not theories. Real patterns. A few simple checks you can do yourself, and a few spots where it makes sense to get a tech out before you keep feeding money into a system that is struggling for no good reason. Well, usually anyway.

Calculate what your cooling system should cost per day (kWh, SEER/EER, thermostat setpoint, runtime)

If you want a rough daily number, think in kWh first. Your outdoor unit has a capacity (like 24,000 BTU/h for a “2‑ton”) and a rating, either EER (steady test) or SEER (seasonal average). The quick math is: watts ≈ BTU/h ÷ EER. Then kWh per day = (watts ÷ 1000) × hours of compressor time. Example I see a lot in Calgary: 24,000 BTU/h with an EER around 10 is about 2,400 watts. If the compressor logs 6 hours of actual time across the day (not 6 hours of fan, I mean the compressor pulling hard), that is 2.4 kW × 6 = 14.4 kWh for that day. If you only have SEER, it is a bit fuzzier because it depends on conditions, but using EER ≈ SEER ÷ 1.15 gets you close enough for a sanity check. I’ve had customers swear their system is “on all day,” then we pull runtime from a smart thermostat and it is 3.2 hours. Or it really is 14 hours because the filter looks like a felt blanket. Both happen. Most of the time, at least.

Your thermostat setpoint changes runtime more than people think, because your house is not a sealed cooler, it is leaking heat all day. A smaller gap between indoor setpoint and outdoor temperature usually means the unit cycles less, and a bigger gap means it just keeps calling. If you want to price that out for your own place, do it for two setpoints and compare. Use the same kWh formula, just swap in the runtime you see on your thermostat history. If you do not have that history, use a simple estimate: take total “calling” hours and multiply by a duty cycle, like 0.6 on a hot afternoon, 0.2 overnight, then average it. I’ve watched people chase a number by dropping the setpoint 3°C at 4 pm, then getting mad that nothing feels instant. It is not instant, it is just longer compressor time. Well, usually anyway.

Home and duct problems that stretch cooling cycles

If your place takes forever to cool down, a lot of the time it is not the outdoor unit at all. It is your house leaking air like a sieve, or the ductwork bleeding off cold air into a basement ceiling you never sit in. I see this all over Calgary, especially in older homes with renovated basements and a dozen new holes cut for pot lights and bath fans. The system keeps pushing, the thermostat keeps asking, and the cycle just drags on.

Air leaks are sneaky because you do not notice them until you feel that one hot spot in the hallway, or you hear a whistling at a door frame on a windy day. Check weatherstripping, attic hatch covers, and the top of the basement stairs. Then look at insulation gaps. I have opened attic hatches and found the insulation pulled back around a bathroom fan and never put back, so you are basically cooling the living space while heat pours down from above. You can also get a rough clue from dust streaks around baseboards or around ceiling registers, air is moving there, and it is not doing you any favours.

Duct leakage: cold air that never makes it to you

Leaky ducts are a big one. The joints at the plenum, the takeoffs, the elbows, the return-air box, all common spots. If the return side is pulling air from a dusty mechanical room, you get weak airflow and dirtier filters, plus the house never feels quite right. I have stuck my hand over a basement trunk line and felt cold air blasting out of an unsealed seam. That air was paid for, and it was going nowhere useful. Mastic or proper foil tape (not the cloth stuff) can help, but if the duct layout is a mess or undersized, sometimes the better fix is stepping back and planning it properly during a central air conditioning unit installation.

Dirty filters and blocked airflow

Filters. People either never change them, or they buy the most restrictive one they can find because it sounds “better.” Then the blower struggles, the coil can get too cold, and airflow at the registers drops off. The house feels warm, so you keep turning the thermostat down and nothing happens fast. If you cannot remember the last time you swapped the filter, that is your sign. Hold it up to a light. If you cannot see light, your system cannot see air.

One last thing I’ll toss in, because I keep seeing it: supply registers closed in spare rooms and returns blocked by furniture. It seems harmless, but it can mess with pressure in the duct system and push air out through any little crack it can find. Cooling equipment likes steady airflow, not a bunch of closed-off paths. Most of the time, at least.

Questions & Answers:

My AC is set to 72°F, but my bill jumped a lot this month. What usually causes that?

Most spikes come from run time. If the system runs many more hours per day, the meter moves fast. Common reasons: hotter outdoor temps, higher indoor humidity, and heat coming into the home through sun-facing windows, attic heat, or air leaks around doors and ductwork. A dirty air filter or clogged outdoor coil can also make the unit run longer to reach the same setpoint. Another frequent cause is “set it and forget it” cooling: holding a low temperature all day while the home is empty keeps the AC cycling for hours that don’t really benefit you.

I lower the thermostat at night to cool the house faster. Does that make the bill higher?

Lowering the thermostat doesn’t make most central AC systems cool faster; it just tells the unit to run longer until it reaches the new target. If you drop it from 76°F to 68°F every night, the AC may run deep into the night to pull down both the air temperature and the heat stored in walls, furniture, and floors. That longer run time adds up. A steadier setting (or a small nighttime setback) usually costs less than big swings. If sleeping comfort is the issue, try a modest change (1–3°F), use fans to improve comfort, and keep blinds closed on sunny windows during the day so the house doesn’t “bank” heat that must be removed later.

Why does my AC seem to run nonstop on really hot days, and is that normal?

On extreme-heat days, long run times can be normal—your system is fighting a larger heat load. If it’s 98°F outside and humid, the AC must remove both heat and moisture. It may run for long stretches and still only hold the indoor temperature a few degrees higher than your usual target. That said, “nonstop” can also point to issues: low refrigerant, a failing capacitor, blocked airflow (dirty filter, closed vents, matted indoor coil), or duct leaks dumping cold air into the attic. A quick check is whether the air coming from supply vents feels clearly cool and whether the indoor temperature slowly falls after sunset. If it never catches up even at night, schedule a service call.

I got a high bill but the AC doesn’t feel like it’s cooling well. What can I check myself before calling someone?

1) Replace or clean the air filter (a clogged filter is a common reason for poor cooling and longer run time). 2) Make sure all supply vents are open and not blocked by rugs or furniture. 3) Look at the outdoor unit: clear leaves and debris, and make sure there’s space around it for airflow. 4) Check doors and windows for warm drafts; a small gap can add hours of AC use over a month. 5) If you have accessible ductwork (basement/garage), feel for strong air leaks at seams. 6) Compare day vs. night performance—if it cools fine at night but struggles in the afternoon, solar heat gain (windows, attic) is often the reason. If you see ice on the refrigerant line or the indoor coil area, turn the system off and call for service; that can indicate airflow or refrigerant problems.