Why your AC is struggling in GTA heat waves — and what to do

It's 34°C outside. You've had the air conditioner running since morning. The thermostat is set to 21°C and it hasn't budged from 27°C. You walk upstairs and it somehow feels hotter up there than it does outside.

If this sounds like your summer, you're not alone. Every heat wave season, homeowners across Markham, Vaughan, Scarborough, Mississauga, Hamilton, and Richmond Hill call us with the same complaint. The AC is on. It's running. It just isn't working.

The frustrating part is that this problem rarely has a single obvious cause. More often it's a combination of factors — some you can fix yourself in five minutes, some that need a professional, and a few that point to a bigger conversation about whether your current system is actually right for your home.

Let's go through all of it.

First: Understand What Your AC Is Actually Up Against

Here's something most homeowners don't realize. Air conditioners don't cool air from scratch — they move heat from inside your home to the outside. To do that efficiently, there needs to be a meaningful temperature difference between the indoor air and the outdoor air.

On a typical summer day in the GTA — say, 26°C or 27°C — your system handles this with room to spare. But when temperatures push into the mid-30s and the humidity climbs, the physics of the whole process gets harder. The gap narrows. The system has to work longer and harder to move the same amount of heat. Eventually, it just can't keep up.

This is completely normal. But it also means that any other problem your system has — dirty coils, low refrigerant, a clogged filter, poor airflow — gets amplified during a heat wave. A minor inefficiency that goes unnoticed on a regular summer day becomes a real problem when outdoor temperatures hit 36°C.

The Most Common Reasons Your AC Is Falling Behind

Your Air Filter Is Overdue

Start here before anything else. A clogged air filter is one of the most common reasons an AC underperforms, and it's also the easiest to fix. When the filter is choked with dust, the system can't pull enough air through the evaporator coil to cool it properly. Everything slows down. Cooling capacity drops.

Pull the filter out and hold it up to the light. If you can't see through it, swap it for a new one. Filters should be replaced every one to three months depending on your home, and during peak summer months it's worth checking them more frequently.

This is a five-minute fix that occasionally solves the problem entirely.

The Outdoor Unit Is Struggling

The condenser — the unit that sits outside your home — is responsible for releasing heat from your refrigerant into the outdoor air. It needs good airflow all around it to do its job. When shrubs grow up around it, debris collects against the fins, or direct sunlight beats down on it all day, the unit has to work significantly harder.

Walk outside and take a look. Clear any vegetation that's within two feet of the unit. Gently rinse the fins with a hose if they look dirty. Make sure nothing is blocking the top of the unit where the fan exhausts air upward. These are all things you can do safely without calling anyone.

The System Is Overdue for a Professional Tune-Up

Beyond what you can check yourself, there are several things that only a certified technician can properly assess. Refrigerant levels are one. Low refrigerant — whether from a slow leak or original undercharge — severely limits cooling capacity and isn't something you can spot or fix on your own.

Dirty evaporator coils are another. These are inside the air handler, and when they're coated in grime, the heat transfer efficiency drops. The same goes for a clogged condensate drain, worn capacitors, and other components that degrade quietly over time.

Annual maintenance catches all of this before heat wave season, not during it. If your system hasn't been serviced in the last 12 months, that's probably part of what you're dealing with right now.

Your Insulation Is Working Against You

This one surprises people. Your air conditioner doesn't operate in isolation — it's fighting to keep heat out of your home at the same time it's trying to cool the air inside. If your attic insulation is thin or your home has significant air gaps around windows and doors, heat pours in faster than your AC can deal with it.

Attic insulation deserves particular attention in GTA homes. In summer, a poorly insulated attic acts like a heat battery — it absorbs radiant heat from the roof all day and radiates it down through your ceiling into the living spaces below. This is almost always the reason second floors are dramatically hotter than ground floors during heat waves.

Improving your insulation doesn't just make your home more comfortable in summer. It reduces the load on your AC, cuts your electricity bills, and makes your heating system work less hard in winter. It's one of the highest-return upgrades a GTA homeowner can make, and it's often overlooked because it isn't as visible as a new unit.

The System Is Simply Too Small for Your Home

If your AC struggles every single summer regardless of maintenance — not just during extreme heat events — there's a chance the unit was never properly sized for your home. HVAC systems require a load calculation to determine the right capacity, accounting for square footage, ceiling height, window placement, sun exposure, insulation levels, and other factors. When that calculation is skipped or done poorly, you end up with equipment that's perpetually behind.

An undersized air conditioner will run almost continuously without ever catching up. If that pattern sounds familiar, it's worth having a professional assess whether your current equipment matches your actual home.

Parts of Your Home Just Aren't Covered

Even a properly sized and well-maintained central AC has limits when it comes to home additions, finished basements, third-floor bedrooms, or sunrooms. These spaces often sit at the end of long duct runs — or outside the duct system entirely — and they suffer for it.

A ductless mini-split heat pump is the most practical solution for these situations. It's a self-contained unit that mounts on the wall and provides both cooling and heating to a specific zone without needing ductwork. Installation typically takes a single day. The systems are quiet, efficient, and remarkably effective at handling the spaces your central system can't reach.

Ontario currently offers up to $5,000 in rebates for qualifying ductless heat pump installations, which changes the cost equation considerably. If you've been tolerating a hot upstairs for years, this summer might be the right time to actually fix it.

What to Do Today If Your Home Won't Cool Down

Walk through this in order.

Replace the air filter if it's overdue. Check the outdoor unit for debris and clearance. Close blinds and curtains on south- and west-facing windows between noon and 6 p.m. — direct sun through glass adds substantial heat to a room. If you have ceiling fans, make sure they're running counterclockwise in summer to push air downward.

If the problem persists after those steps, call for a professional inspection. Low refrigerant, dirty coils, and failing components are root causes that don't resolve themselves. Catching them in July is better than having the system fail on the hottest day in August.

If Your System Is Getting Old, Now Is the Time to Think Ahead

Air conditioners typically last 15 to 20 years with proper maintenance. As they age, efficiency drops and repair costs climb. If yours is on the older side and you're already spending money on service calls, the math on a replacement starts to look different.

This is also when it's worth considering a heat pump rather than a straight AC replacement. A modern heat pump cools your home exactly the way a central air conditioner does — but in winter, it reverses to heat your home using electricity rather than burning gas. Cold-climate models work reliably down to -25°C, which makes them genuinely practical for Ontario winters.

Ontario's Home Renovation Savings Program currently offers up to $7,500 in rebates for qualifying air-source heat pump installations, with combined provincial and federal savings potentially reaching $14,000 for some homeowners. If a new AC is already in your near-term plans, the rebate program makes a heat pump worth a serious look.

We Know GTA Summers. We Know GTA Homes.

Constant Home Comfort has been serving homeowners across Markham, Richmond Hill, Scarborough, Durham, Mississauga, Vaughan, Newmarket, Hamilton, Burlington, Barrie, Ottawa, Waterloo, London, and Kanata for years. Our licensed technicians handle everything from a seasonal tune-up to a full system replacement — and we handle all the rebate applications on your behalf so you're not left navigating government paperwork on your own.

We carry top brands including Lennox, Goodman, KeepRite, and Gree. We offer $0 down, 0% interest financing on approved credit. And we give you full pricing upfront — no surprises when the job is done.

If your AC is struggling right now, call us at 1-888-675-5907 or book an appointment at constanthomecomfort.com. We'll figure out what's actually going on and tell you exactly what it'll take to fix it.

Your house should be the one place you can escape the heat — not another place you're stuck sweating through it