When Should You Turn Off Your AC in Ontario?

Ontario weather doesn't really do "smooth transitions." One week you are sweating through high humidity, and the next you are hunting for your favourite hoodie. It happens fast.

If you live in Markham, Mississauga, Ottawa, or any of our other service locations, you are probably asking yourself the same question every autumn: when should I actually turn off my AC?

Turning off the cooling isn't just a quick way to drop your hydro bill. It actually protects your expensive outdoor equipment from getting wrecked during our freezing winters. If you do it too late—or do it the wrong way—you could end up facing a massive repair bill next spring.

Here is how to get the timing right, winterize your system, and get your indoor air ready for the cold months ahead.

The 10°C Rule: When to Flip the Switch

Don't rely on the calendar for this. Rely on the weather forecast.

The best time to shut down your AC is when nighttime temperatures consistently stay below 10 degrees Celsius. In places like Richmond Hill, Waterloo, or London, we usually hit this sweet spot around early to mid-October.

Sure, you might want to keep the system on standby through the Thanksgiving weekend just in case a surprise warm spell rolls through. But after that, it's time to retire the cooling.

Why does the temperature matter so much? Running a standard central AC when it's too cold outside can seriously damage the system.

• Your compressor needs warm oil to run. Cold temperatures make the lubricant inside your outdoor unit incredibly thick and sluggish. AC units are built for summer heat. If you run the system when it is under 15 degrees Celsius, that cold, thick oil can't lubricate the compressor properly. This causes heavy friction and can seize the whole engine.
• You risk freezing your coils. The water your system extracts from your indoor air can easily freeze on the cold outdoor coils. That ice buildup blocks airflow, strains your blower motor, and can destroy your coils.

Why "Auto" Mode is a Bad Idea in Autumn

It is incredibly tempting to set your thermostat to "Auto" and forget about it during the fall. But this is a mistake.

When the outdoor temperature swings wildly—freezing mornings followed by warm, sunny afternoons—your system gets confused. It will constantly toggle back and forth between heating and cooling.

This constant switching is called short-cycling. It drains a massive amount of hydro, wears out your electrical components, and forces your AC to start up in the freezing morning hours when the compressor oil is cold.

Once autumn arrives, take your system off "Auto" and manually switch your thermostat to "Off" or "Heat".

Step-by-Step: Winterizing Your AC Unit the Right Way

Taking fifteen minutes to prep your outdoor condenser before the snow starts piling up will save you a fortune in spring repairs. Here is how to do it safely:

1. Shut Down the Power Completely

Don't just turn it off at the thermostat. If someone accidentally bumps the thermostat on a mild November day, the unit will try to kick on, which can instantly ruin your compressor.

• Walk outside, open the grey metal disconnect box next to your unit, and flip the switch to "Off".
• For extra safety, head to your basement and turn off the dedicated AC breaker on your electrical panel.

2. Get Rid of Summer Grime

Your AC spent all summer sucking in pollen, dirt, and grass clippings. If you leave that damp organic paste on your unit all winter, it will corrode the metal and cause refrigerant leaks.

• Clear away any weeds or leaves within a foot of the unit.
• Grab a garden hose on a low-pressure spray to wash down the vents.
• Note: Never use a pressure washer—the intense spray will instantly bend the soft aluminum fins and ruin your system's airflow. Let it dry completely.

3. Protect the Top (But Don't Wrap It)

A lot of homeowners make the mistake of wrapping their entire unit in a tight plastic or vinyl cover. Don't do this.

• Tight covers trap moisture rising from the ground, which rots the electrical terminals and rusts the metal.
• It also creates a cozy, wind-free nest for mice to move in and chew through your wires.
• Instead, just lay a piece of plywood over the top grate to protect the fan from falling icicles and heavy snow. Secure it with bungee cords or bricks, and leave the sides wide open so moisture can evaporate naturally.

Two Crucial Exceptions to Watch Out For

There are two major situations where you need to throw the standard AC rules out the window.

If You Have a Heat Pump, Ignore This Advice

If your home uses a modern, cold-climate air-source heat pump, do not cover the unit or cut the power.

These systems handle both heating and cooling. They run all winter long, extracting heat from the freezing outdoor air. Cutting the power will shut down your heating completely, leaving your home freezing.

For heat pumps, winter maintenance simply means brushing off heavy snow drifts and keeping the sides clear of ice blockages.

Air Cleaning During Wildfire Smoke

Wildfire smoke has become a regular issue in Ontario. When outdoor air is thick with dangerous particulate matter, opening your windows is a bad idea.

Keep your windows closed and keep your AC running. While standard central systems don't bring in outdoor air, they constantly pull your indoor air through your furnace's filter, cleaning the air inside your home.

Don't Forget to Prep Your Indoor Heating Systems

Transitioning your home for the winter means coordinating your outdoor shutdown with your indoor furnace prep. Make sure to take care of these steps:

• Warm up your home slowly: When you turn on your furnace for the first time, don't crank the thermostat up five degrees all at once. Raise it in steps of two or three degrees. This lets the metal heat exchanger expand slowly, protecting it from thermal stress and cracks.
• Adjust your humidifier damper: Your whole-home humidifier has a manual damper. In the summer, you closed it. Now, turn it to "Open" or "Winter," open the copper line water valve, and set your humidistat to 30% to 40%. Don't set it higher, or you'll get moisture running down your windows, which causes mold.
• Replace the air filter: A dirty filter blocks airflow, causing the furnace to overheat and shut down in the middle of a cold night. Swap fiberglass filters every month, pleated filters every two months, and deep media filters every three to six months.
• Check the side-wall PVC vents: High-efficiency furnaces vent through white plastic pipes on the side of your house. Peer inside with a flashlight to ensure leaves or animal nests aren't blocking them. Make sure to remove any fine mesh screens before winter, as moisture in the exhaust will freeze on the mesh and choke the system.

Get Professional Help Before the Cold Hits

Prepping your home for winter takes time. While cleaning and powering down your AC are easy DIY steps, a professional furnace check-up is the best way to ensure safe, reliable heating.

Our team at Constant Home Comfort offers complete seasonal diagnostics across Ontario. We will polish your flame sensors, flush out your condensate drains, and run safety checks on your heat exchanger to keep your family safe. Plus, we can help you apply for major HVAC rebates in Ontario—saving you up to $12,000 on new high-efficiency heat pumps, with 0% interest financing.

Give Constant Home Comfort a call today at 1-888-675-5907 to book your autumn furnace tune-up or get a free quote!