The Great Ontario Climate Flip: How to Safely Switch Your GTA Home From AC to Furnace

That weird week in October is always the same. You wake up freezing, turn the heat on, but by 3:00 PM you're sweating in your kitchen and kicking the cooling back on. It is classic Southern Ontario weather. From Mississauga and Scarborough up to Newmarket and Barrie, our fall weather has zero predictability.

But running your heating and cooling on the same day—or just blindly sliding your thermostat over when the first real frost hits—is a great way to stress your equipment. Taking twenty minutes to walk through your house and prep your system right now keeps your monthly bills down and protects you from waking up to a freezing living room and a brutal emergency repair bill.

At Constant Home Comfort, we’ve spent more than ten years keeping families warm across Ontario—everywhere from Ottawa and Kanata down to London, Waterloo, Hamilton, Burlington, and the entire GTA (including our home base in Markham, plus Vaughan and Richmond Hill). Here is a practical, completely restructured checklist our local techs use to get residential systems ready for the winter freeze.

Inside the House: Thermostats, Filters, and Humidifiers

Grab your phone's flashlight and let’s do a quick walk-through of your hallways and basement.

Start with the thermostat

Walk over to your main control. Go ahead and switch the setting from "Cool" or "AC" over to "Heat".

If you want to protect your equipment, avoid two classic mistakes:

• Don't leave it on "Auto" during fall. It's incredibly tempting to set it and forget it. But when the temperature bounces between a sunny afternoon and a freezing night, "Auto" forces your system to constantly fight itself, cycling back and forth between heating and cooling. This wastes electricity and wears out your mechanical parts before winter even gets started.
• Don't shock your furnace. If your house is sitting at 16 degrees and you want it at 21, don't crank the dial up five degrees all at once. Raise it gradually—by two or three degrees at a time. This helps your system warm up smoothly and protects your heat exchanger from sudden thermal stress.

Check on your filter

Head down to your furnace. If you haven't changed your air filter since the start of summer, it’s probably packed with dust, pollen, and pet hair. Trying to push hot winter air through a dirty, clogged screen is the number-one reason furnaces shut down in the middle of a cold snap.

Without proper airflow, your furnace's heat exchanger will rapidly overheat. This trips a safety limit switch, forcing the system to shut down completely (this is called short-cycling). Over time, this constant overheating can actually crack your heat exchanger, which is a massive safety hazard and an incredibly expensive fix.

• If you use standard 1-inch fiberglass filters, swap them out every 30 days.
• For standard 1-to-2-inch pleated synthetic filters, aim for every 60 to 90 days.
• If you have a deep 4-to-5-inch media cabinet, you can usually go three to six months between changes.

Locate that humidifier switch

While you're standing in front of your furnace, look at the metal ductwork directly above or beside it. If you have a flow-through whole-home humidifier, you'll see a manual metal lever or a rotating dial.

During the summer, this damper should have been closed to keep your AC from freezing up. For winter, you need to manually turn this lever to the "Winter" or "Open" position. This allows the warm furnace air to flow through the humidifier pad.

Don't forget to open the small water valve on the copper line feeding the unit, and set your humidistat to between 30% and 40%. Setting it any higher during a deep freeze will just cause water to run down your cold window panes, leading to mold and wood rot.

Heading Outside: PVC Pipes and the AC Condenser

Slip on some shoes and head into the backyard. We have a few quick checks to make out here before the snow starts piling up.

Clear the side-wall vents

If you have a modern, high-efficiency gas furnace, it vents through two white plastic PVC pipes sticking out the side of your house.

Take a flashlight and peer inside both pipes. Leaves, grass clippings, and nesting birds or mice love these warm, dry spots in the autumn. If either pipe is blocked, the furnace's inducer fan can’t establish proper pressure, and your system will go into a hard lockout and refuse to ignite.

A quick warning: Do not leave fine mesh screens over these pipes in the winter. The moisture-rich exhaust from your furnace will freeze instantly on the cold metal mesh, creating a solid block of ice that chokes your heating system.

Prep your AC (And stop wrapping it in plastic)

If you have a standard central air conditioner, it’s going to sleep for the winter. First, locate the metal disconnect box on the exterior wall near the unit and pull the plug or flip the switch to "Off". This stops the AC from turning on accidentally if someone plays with the thermostat on a mild day.

Now, clear away any leaves, twigs, or dirt from the base of the unit. When it comes to covering it, never wrap your outdoor unit tightly in a vinyl or plastic tarp. Tight wraps trap condensation against the metal, which accelerates rust, ruins the circuit boards, and provides a cozy, wind-protected home for mice to chew through your wiring. Instead, simply lay a piece of plywood on the top of the unit to block falling icicles or heavy snow, and secure it with a couple of bungee cords. Leave the wire sides completely open to breathe.

Note for Heat Pump Owners: If you run an air-source heat pump, ignore the cover advice. Your heat pump runs year-round and needs completely unrestricted airflow to heat your home. Covering it or cutting its power will shut down your heating completely.

The First Ignition: Testing Things Safely

Don't wait for a freezing night in December to see if your system actually works. Run a quick 15-minute test in early autumn while the weather is still mild.

With a fresh filter installed and all your indoor registers fully opened, switch your thermostat to heat and raise the temperature a few degrees.

• What's that smell? When the furnace fires up for the first time in months, you're probably going to notice a dry, dusty burning smell. Don't panic—this is completely normal. It is just summer dust burning off the hot surfaces of your heat exchanger, and the odor should blow away and disappear entirely within 5 to 10 minutes.
• What's that noise? Stand near your vents and listen. You should hear a steady, gentle hum. If you hear loud metallic clanging, screeching, grinding, or if the furnace shuts down repeatedly within a few minutes, power the system down at the main switch and call in a professional.

The 2026 Free Money Play: Ontario Rebates Explained

Note: Rebate amounts and eligibility can change over time. For the latest incentives, visit our Promotion page.

If you already know your older furnace or AC is on its last legs, upgrading this fall can save you thousands of dollars through Ontario’s Home Renovation Savings Program (HRSP). Backed by Save on Energy and Enbridge Gas, the program offers massive incentives for energy-saving upgrades:

• Up to $7,500 for Cold-Climate Heat Pumps: If you currently heat with electricity, oil, propane, or wood, you can qualify for up to $7,500 back on a certified cold-climate air-source heat pump.
• Up to $2,000 for Enbridge Gas Customers: Natural gas homes can claim up to $2,000 when installing an eligible heat pump alongside their gas furnace, creating a highly efficient hybrid heating system.
• $1,250 for Standalone Attic Insulation: Boosting your attic insulation to at least R-50 can get you a standalone rebate of up to $1,250.
• $100 for Smart Thermostats: You can claim a direct $100 rebate for installing a qualified smart model.

The big administrative win for 2026 is the "No-Audit Path". If you are only installing individual heat pumps, smart thermostats, or attic insulation, you no longer need to shell out $600 and wait weeks for a pre-upgrade home energy assessment. You can go straight to installation with an approved, registered contractor like Constant Home Comfort and still claim your full rebate.

Let Constant Home Comfort Handle the Details

Getting your home comfort system ready for winter shouldn't be a stressful chore. If you're unsure about adjusting your humidifier, cleaning out your vents, or if you want a professional eye to make sure your furnace's heat exchanger is safe, our team is always ready to help.

We serve homeowners across Ontario—including Markham, Richmond Hill, Scarborough, Durham, Mississauga, Vaughan, Newmarket, Ottawa, Waterloo, London, Barrie, Hamilton, Burlington, and Kanata. As a registered and approved contractor under the Home Renovation Savings Program, we verify your eligibility and handle all the complicated rebate paperwork and filings on your behalf at no extra cost. Plus, we offer flexible $0-down, 0%-interest financing on approved credit to make your home upgrades completely stress-free.

Don't wait for the freezing temperatures to catch you off guard. Call Constant Home Comfort today at 1-888-675-5907 for a free, competitive quote or to book your seasonal furnace tune-up! Our phone lines are open 24/7 to keep you warm and cozy.