Last Updated: October 2026
Switching from AC to heat is about a 30-minute fall job in most Ontario homes: shut down cooling, set the thermostat to Heat, and confirm warm air reaches every room before the first hard frost. What you do outside depends on your system. A cooling-only central AC can be cleaned and shut down for winter. A heat pump stays powered and uncovered, because it is your heater.
This AC to heat pump switchover checklist covers central AC with a gas furnace, all-electric heat pumps, and dual-fuel systems that pair the two. Find your system in the first table, work through the 8 steps, then use the troubleshooting sections if something doesn’t heat the way it should this fall.
| Your System | What to Do for Winter |
|---|---|
| Central AC + gas furnace | Stop cooling, clean the outdoor unit, cover the top only if you choose to, replace the filter and test the furnace. |
| Heat pump (all-electric) | Keep the outdoor unit powered and uncovered, clear debris, replace the filter and test heating. |
| Dual-fuel (heat pump + furnace) | Treat the outdoor unit as a heat pump: powered and uncovered. Ask your installer at what outdoor temperature the furnace takes over. |
If your system isn’t heating properly after the switch, the Heat Pump Troubleshooting Wizard can point to the likely cause before you book a service call.
The outdoor units look almost identical, so check the thermostat and wiring before you touch anything. One of the costliest fall mix-ups is a heat pump owner following AC winterization advice, wrapping the unit in a tarp, and then wondering why the house won’t heat in December.
| Clue | Central AC + Furnace | Heat Pump (or Dual-Fuel) |
|---|---|---|
| Thermostat shows “Aux Heat” or “Em Heat” | No | Usually yes |
| An O, B or O/B wire on the thermostat | No | Yes, it controls the reversing valve |
| Outdoor unit runs in January | No, it sits idle | Yes, it is moving heat indoors |
| Steam from the outdoor unit on frosty days | Never | Normal during defrost |
| Can you cover it for winter? | Optional, top only | Never |
Still unsure? The model number on the outdoor unit’s data plate will tell you. Search it with the brand name, or ask your installer. If your cooling system was replaced recently, check the paperwork too: a dual-fuel system looks almost the same from the backyard as a central AC.
Applies to: all systems. Step 3 depends on your system.
Work through these in order on a mild day, ideally before overnight lows settle below freezing. On a system with no faults, it takes about half an hour.
Doing the matching job in spring? Our AC spring tune-up checklist for Ontario reverses this process and covers the start-up checks your AC needs after a winter outside.
Applies to: cooling-only central AC. Heat pump owners can skip to the box at the end of this section.
Winterizing a central AC means cleaning it, keeping falling ice off the top, and making sure no water can freeze around it. Our air conditioner maintenance tips cover the rest of the yearly care routine.
A cover is optional. Outdoor units are built to sit outside year-round, and some manufacturers advise against covering them, so check your installation manual first. If you do cover it, switch off power at the outdoor disconnect, rinse leaves and cottonwood fluff out of the fins with a gentle stream from a garden hose (never a pressure washer), and let the unit dry. Then fit a breathable cover or a piece of plywood weighted on the top only. Never wrap the sides in a plastic tarp. A fully sealed unit traps condensation, rusts from the inside, and gives mice a warm place to chew wiring.
Leaving the disconnect off for the winter is also optional for a cooling-only AC, but it stops the unit starting on a mild November afternoon. If you do, write a note on your spring calendar: restore power about a day before the first start-up, because many compressors rely on a crankcase heater to warm the oil before running. Your installation manual will confirm whether yours has one.
The water you need to deal with is condensate, not refrigerant. All summer, the indoor coil above your furnace drips into a pan and a small drain line. Pour a cup of warm water into the pan or the line’s access tee and confirm it flows to the floor drain or pump. A slow drain in October becomes a musty smell and a clog next June.
If you have a high-efficiency furnace, its own condensate line keeps working all winter. Check that the trap isn’t cracked and the condensate pump (if you have one) switches on when you lift its float.
A central AC has no water supply of its own, which is where a lot of online advice gets muddled. The two water items to check at switchover time are near it, not in it:
Applies to: all systems. The O/B setting applies to heat pumps only.
Most changeover problems are settings, not broken parts. Work down this list before you call anyone:
If your thermostat is more than ten years old or doesn’t understand heat pump staging, an upgrade often pays for itself. Our guide to smart thermostat savings in Canada breaks down what’s realistic.
Applies to: gas furnaces and heat pumps.
Start by deciding whether the air is actually cold or just cooler than you expect. It’s the quickest way to tell a heat pump that’s working normally from one with a real fault.
According to Natural Resources Canada, a ducted heat pump moves a larger volume of air at roughly 25 to 45°C, while a furnace delivers less air at around 55 to 60°C. Skin temperature is around 33°C, so a hand over a heat pump register can feel lukewarm even when the system is working perfectly. Measure with a cheap probe thermometer instead of your palm.
| What You Notice | Normal? | Likely Cause | What to Do |
|---|---|---|---|
| Heat pump air feels lukewarm but the house reaches setpoint | Yes | Lower supply temperature by design | Nothing. Measure at the register to confirm. |
| Cool air for a few minutes, steam outside | Yes | Defrost cycle | Wait. Normal heating resumes on its own. |
| Cold air whenever the fan runs, burners or compressor off | No | Thermostat fan set to On instead of Auto | Set the fan to Auto. DIY fix. |
| Heat pump blows cold air in Heat mode constantly | No | Wrong O/B setting or a stuck reversing valve | Check O/B first, then call a technician. |
| Furnace fan runs but air never warms up | No | Ignition, flame sensor or gas supply fault | Check the filter and that the furnace power switch is on, then call a licensed technician. |
| Weak, cool air and the outdoor coil is caked in ice | No | Failed defrost control, low refrigerant or blocked airflow | Book service. Do not chip the ice. |
NRCan notes that time-based defrost controls start a cycle every 30, 60 or 90 minutes, while demand-defrost models defrost only when frost actually builds up. If you’re getting cool blasts more often than about once an hour in mild, dry weather, that’s worth flagging to a technician.
Applies to: heat pumps and dual-fuel systems.
Every air-source heat pump loses capacity as the outdoor temperature drops, and below a certain point it needs help. That point is called the balance point. NRCan puts the minimum operating temperature of newer models at roughly -15°C to -25°C. Below that, the backup heat carries the load.
So on a -20°C January night in Kitchener or Brantford, a mid-range unit running its backup heat is doing exactly what it was designed to do. Real faults look different: the house falls behind at -2°C, the outdoor fan never spins, or the outdoor unit sits encased in ice after a storm.
| Aux Heat | Emergency Heat (Em Heat) | |
|---|---|---|
| Turned on by | The system, automatically | You, manually |
| Heat pump running? | Yes, backup tops it up | No, backup only |
| When it’s used | Defrost, very cold nights, big setpoint jumps | Outdoor unit failed, iced solid or shut down by a technician |
| Running cost | Moderate | Highest with electric backup; depends on the fuel in dual-fuel systems |
Use Em Heat only to keep the house safe while you wait for a repair, then switch back to Heat once the outdoor unit is fixed. In some dual-fuel systems, Emergency Heat activates the designated backup heating source, which may be a gas furnace. The exact behaviour depends on the equipment and thermostat configuration, so check your system manual or ask your installer before changing modes.
Here’s an illustrative example using the Ontario Energy Board’s Tier 1 price of 11.2¢/kWh, in effect from November 1, 2026. Assume an all-electric home needs 60 kWh of heat on a -5°C day; your home’s real figure depends on its size and insulation. A heat pump working at a COP of about 2.4, the middle of NRCan’s published range near -8°C, uses roughly 25 kWh. Electric strips on Em Heat use the full 60 kWh.
That’s about $2.80 a day versus $6.72 on the electricity line alone, before delivery, regulatory charges, HST and the Ontario Electricity Rebate. Over a 10-day wait for a part, Em Heat adds roughly $39 to the commodity portion of your bill. Annoying, but far cheaper than frozen pipes.
Want to see what your own system costs through a whole Ontario winter? Plug your numbers into the Heat Pump Operating Cost Calculator.
| Safe for Homeowners | Leave to a Licensed Technician |
|---|---|
| Replacing the filter | Opening electrical access panels or replacing a capacitor |
| Changing thermostat modes and settings | Anything involving refrigerant, including recharges and leak repairs |
| Clearing leaves and snow from the outdoor unit | Gas valves, burners, ignitors and flame sensors |
| Rinsing an AC coil with the power off | Defrost controls and circuit boards |
| Resetting a tripped breaker once | A breaker that trips again, or an outdoor unit encased in ice |
If you’ve worked through the checklist and something still isn’t right, use our free troubleshooting wizard to identify likely causes and understand your next step. It points to whether the problem looks like a DIY fix or a service call. It can’t replace a technician’s diagnosis, but it can stop you paying a call-out fee for a thermostat setting.
Tell us what your system is doing, from cold air in Heat mode to ice on the outdoor unit, and get the most likely cause plus your next step.
Answer 5 quick questions about your heat pump symptoms and get a likely diagnosis with recommended next steps.
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Note: This diagnosis is based on common symptom patterns and is for informational purposes only. A certified HVAC technician should inspect your system for an accurate diagnosis.
Connect with certified local contractors who can diagnose and fix your heat pump — for free quotes.
GET FREE QUOTESStill running a cooling-only system with a cooling-side problem you noticed before shutdown, like short cycling or water around the furnace? The AC Troubleshooting Wizard handles those.
If the wizard points to a service call, the ranges below give you a starting point before you phone anyone. They are indicative estimates from our Oakville HVAC repair cost guide (March 2026), not a province-wide survey. Prices vary by city, brand, parts availability and after-hours timing.
| Repair | Indicative Cost (CAD, parts + labour) | Switchover Symptom |
|---|---|---|
| Diagnostic service call | $80 to $150 (often credited to the repair) | Any unclear fault |
| Thermostat repair or replacement | $120 to $300 | Won't change modes, blank screen |
| Capacitor replacement | $150 to $350 | Outdoor unit hums but won't start |
| Furnace ignitor replacement | $150 to $300 | Furnace clicks, no flame |
| Refrigerant recharge (finding and fixing a leak costs extra) | $200 to $500 | Weak heat, iced coil |
| Circuit board, including a defrost control board | $400 to $900 | Stuck in defrost or never defrosts |
Source: ACPrices.ca Oakville HVAC repair cost guide, March 2026. Estimates are not a substitute for an on-site diagnosis and a written quote.
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Only weigh replacement once you know what's actually wrong. If a technician has confirmed a major fault, or your AC needed an expensive repair this summer, fall is a sensible time to compare options. NRCan points out that the extra cost of moving from a cooling-only system to a heat pump is often quite low, and a heat pump paired with your existing furnace covers the shoulder seasons on electricity while the furnace handles the deep cold.
Rebates make a real difference here. Ontario's Home Renovation Savings program offers up to $7,500 for a cold-climate air-source heat pump on NRCan's qualified list, with the amount based on the unit's size. It lists no rebate for a cooling-only central AC, which can narrow the price gap between replacing like-for-like and upgrading. Confirm current amounts with the program before you sign a contract. Our Oakville HVAC rebates guide walks through how the program stacks up.
For real installed pricing, start with our heat pump installation cost guide for Ontario, then compare local numbers:
No. A heat pump is your heater in winter, so a cover blocks airflow and traps defrost meltwater. Keep it uncovered and powered, and clear snow from around, under and on top of it.
Switch once overnight lows stay consistently cold, which in Southern Ontario is usually late September or October. Avoid Auto mode during big day-to-night swings, since it can flip the system between modes.
Some coolness is normal. Heat pumps deliver air at roughly 25 to 45°C, cooler than a furnace, and briefly blow cool air during defrost. Constant cold air in Heat mode is a fault.
Only when the outdoor unit cannot run, such as when it is iced solid, damaged or shut down by a technician. With electric backup, emergency heat costs much more to run than the heat pump.
Both are optional for a cooling-only AC; check your manual. If you cover it, cover the top only. If you switch off the disconnect, restore power about a day before spring start-up.
Three things matter most when switching from AC to heat: know which system you have, keep a heat pump powered and uncovered all winter, and set the thermostat to Heat and watch one full cycle while it's still mild outside. If something doesn't heat the way it should, run the troubleshooting wizard before you book a service call.
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