Do Heat Pumps Work in -20°C?

Every winter, the same question comes up for Toronto homeowners: do heat pumps really hold up when the temperature plunges to –20°C? It's an understandable concern. Toronto doesn't just experience cold snaps, –20°C is a perfectly normal part of our winter landscape, and any heating system has to be able to handle it without hesitation.

What often gets lost in the discussion is how far heat pump technology has come. The older models many people remember did struggle in harsh weather, but today's cold-climate systems are built specifically for deep-freeze performance. They can keep delivering heat at –20°C and often well below that, though the way they operate, and how efficiently they do it, depends on factors like proper sizing, installation quality, airflow, and the overall condition of the home.

Why People Think Heat Pumps Don't Work in Extreme Cold

For years, heat pumps had a reputation for underperforming in cold climates, and that perception didn't come from nowhere. Early-generation heat pumps from the 1980s through early 2000s were simply not designed for Canadian winters. They relied on older refrigerants and single-stage compressors that struggled once temperatures dipped below freezing. When the outdoor coil couldn't extract enough heat from the air, homeowners often ended up relying heavily on backup heat, which led to higher energy bills and frustration.

Adding to that misconception, many homeowners compare modern systems to what they or their parents owned decades ago. Others hear the term "heat pump" and assume all models work the same, not realizing that standard heat pumps and cold-climate heat pumps are entirely different technologies. Poorly sized or poorly installed systems from the past also contributed to the belief that heat pumps simply "stop working" in the cold.

Today's systems are not the heat pumps of the past. Cold-climate heat pumps are specifically engineered for sub-zero performance, thanks to major technological advancements:

• Variable-speed and inverter-driven compressors that ramp up or down based on demand
• High-performance refrigerants that extract heat even at extremely low outdoor temperatures
• Enhanced vapor injection (EVI) technology in many high-end models, designed to boost output at –20°C and below
• Smarter sensors and controls for faster adaptation during temperature drops
• ENERGY STAR Cold Climate certification that sets strict performance standards for Canadian conditions

These updates allow modern systems to deliver consistent, reliable heating well into the deep freeze, making them a practical, cost-efficient solution for Toronto homes facing long winters and fluctuating temperatures.

How Modern Cold-Climate Heat Pumps Perform at –20°C

One of the biggest advantages of today's cold-climate heat pumps is their ability to continue producing meaningful heat even when temperatures drop to –20°C, and often down to –25°C or lower. Unlike older models that would simply give up at sub-zero temperatures, modern systems are designed to extract heat from extremely cold air using advanced compressors, optimized refrigerants, and precise sensor controls.

A key concept is COP (Coefficient of Performance), a measure of how efficiently a heat pump operates. At 0°C, many cold-climate models can achieve a COP of 2–3, meaning they produce 2–3 units of heat for every unit of electricity used. As temperatures fall, efficiency naturally drops, but at –20°C, many systems still maintain a COP above 1, which is significantly more efficient than electric resistance heat.

Manufacturer Performance Curves

Every heat pump model comes with a performance curve that shows how much heating capacity it can deliver as outdoor temperatures fall. While specific numbers differ by brand and model, there are some consistent patterns:

• Heating capacity gradually reduces, typically staying 60–75% of its rated output at –20°C
• Systems don't shut off at –20°C, they continue running as long as they were designed for cold climates
• Variable-speed compressors automatically increase output when temperatures drop
• High-end cold-climate units can maintain full rated capacity down to –15°C, and strong output beyond that

This is why choosing a true cold-climate heat pump (not a standard unit) and properly sizing it for your home is important for Toronto winters.

Cold-Weather Performance in Toronto’s Housing Types

Heat pumps in Toronto face a unique challenge: older housing stock. The city is full of brick semis, older detached homes, and townhouses that vary widely in insulation quality. Here's how performance plays out in different scenarios:

• Older brick semis:
  Efficient heating at –20°C is achievable but often requires proper sizing and good air sealing.
• Townhomes with moderate insulation:
  Typically perform very well, with minimal reliance on backup heat.
• Newer detached homes:
  Strongest performance, cold-climate heat pumps can meet most or all heating needs down to –20°C.
• Homes with high heat loss:
  May still heat reliably but could require supplemental or dual-fuel backup during deep freezes.

In all cases, installation quality is as important as the unit itself. A well-installed, properly optimized system will consistently outperform a poorly sized or incorrectly installed one, especially at extreme temperatures.

What Happens During the Defrost Cycle

When temperatures drop below freezing, moisture in the air begins to accumulate on the outdoor coil of a heat pump. At around –5°C and below, this moisture can freeze, forming frost or even ice buildup. This is completely normal, and every heat pump is designed to handle it through a process called the defrost cycle.

During defrost mode, the system temporarily reverses operation to warm the outdoor coil and melt the frost. It may sound counter-intuitive, but it's essential for maintaining airflow and efficiency. Indoors, you might briefly feel cooler air at the vents or notice the system pausing, this is simply part of the system protecting itself and restoring performance.

How Often Defrost Happens at –20°C

The frequency of defrost cycles varies based on conditions such as humidity, temperature, and airflow around the outdoor unit. At –20°C, most cold-climate heat pumps:

• Enter defrost less often than they do around –5°C to –10°C
• Complete cycles quickly, often in just a few minutes
• Return to normal heating immediately afterward

The colder the air, the lower the humidity tends to be, meaning frost can actually form less frequently at –20°C than at slightly warmer sub-zero temperatures. Proper airflow around the outdoor unit also reduces frost buildup dramatically.

How Modern Systems Optimize Defrost

Older heat pumps used timed defrost cycles, whether the unit needed them or not. This led to wasted energy, unnecessary heat loss, and the old stereotype of "heat pumps blowing cold air in winter."

Modern cold-climate systems use intelligent sensor-driven defrost, which only activates when frost is actually detected. Advanced features include:

• Coil temperature sensors that monitor frost accumulation
• Adaptive algorithms that reduce the number and duration of defrost cycles
• Variable-speed compressors that minimize indoor temperature swings
• Shorter, more efficient cycles that conserve energy
• Improved refrigerants that handle low-temperature defrosting better

For homeowners, this means fewer interruptions in heating, more consistent indoor comfort, and better overall efficiency, even during Toronto's deepest cold snaps.

What Triggers Backup Heat on the Coldest Days

Backup heat is one of the most misunderstood parts of heat pump operation. Many homeowners assume that if a heat pump uses backup heat, it means the system "can't handle" the cold, but that's not true. Backup heat is a built-in, intentional part of a heat pump's design, meant to support the system during peak demand, not replace it.

At –20°C, a well-sized cold-climate heat pump can still provide the majority, and sometimes all, of your home's heating. However, depending on the home's insulation, layout, and heat loss, the system may use auxiliary heat to maintain comfort or support the heat pump during short intervals.

Backup heat is normal, not a failure. The key is whether it's used occasionally (normal) or constantly (sign of a problem).

Dual-Fuel (Hybrid) Heating Systems

For Toronto homes, especially older brick houses with higher heat loss, many installers recommend a dual-fuel system, also known as hybrid heating. This setup pairs a heat pump with a gas furnace and automatically chooses the most efficient and cost-effective heat source based on outdoor temperature.

Benefits include:

• Lower heating costs during milder weather
• Reliable high-output heating during extreme cold snaps
• Automatic switching for efficiency and comfort
• Redundancy if one system ever fails

Dual-fuel is particularly popular in Canada because it combines the affordability of gas during the coldest days with the efficiency of a heat pump the rest of the winter.

Electric Backup Heat

Homes without gas can use electric resistance backup heat, which kicks in only when needed. This supplemental heat may run during:

• Defrost cycles
• Deep freezes (typically below –20°C)
• Sudden cold snaps
• Recovery mode after long setbacks
• Periods of very high demand (e.g., poorly insulated rooms)

Electric backup heat is not meant to run constantly, and in a properly installed system, it usually doesn't.

How to Ensure a Heat Pump Performs Well at –20°C

Even the most advanced cold-climate heat pump can only perform as well as its installation and environment allow. To get reliable, consistent heat at –20°C, several factors must be aligned:

Proper Sizing for Toronto's Climate

Sizing is the single most important variable. A heat pump that's too small will rely heavily on backup heat, while one that's too large may short-cycle and struggle with efficiency and comfort. Cold-climate systems must be sized based on:

• The home's heat loss calculations
• Age and insulation quality of the house
• Window type and air leakage
• Toronto's average and extreme winter temperatures

A correctly sized system should maintain warm, steady output with minimal auxiliary heat, even during cold snaps.

Quality Installation

The installer's expertise affects nearly everything: airflow, refrigerant charge, placement, defrost efficiency, and long-term reliability. Poor installation is one of the top reasons heat pumps underperform at extreme temperatures.

Look for installers who specialize in cold-climate heat pump systems, such as the certified technicians at HeatPumps.ca, because the installer’s expertise makes a significant difference in performance, efficiency, and long-term reliability.

Air Sealing & Insulation

A heat pump can only compensate so much for a drafty home. Older Toronto homes often lose heat through:

• Window gaps
• Poor attic insulation
• Uninsulated exterior walls
• Air leaks around doors or chimneys

Improving insulation and sealing can dramatically enhance cold-weather performance and reduce backup heat usage.

Thermostat Settings & Smart Controls

Heat pumps operate best with steady, consistent temperature settings. Rapid temperature setbacks or aggressive changes can trigger unnecessary auxiliary heat, especially at –20°C.

Smart thermostats designed for heat pumps can optimize performance automatically.

Cold-Weather Installation Best Practices for Toronto

Cold-climate systems must be installed with local environmental challenges in mind. For Toronto homeowners, placement and airflow are especially important in winter.

Outdoor Unit Placement

Installers often aim to position the unit:

• Above typical snow accumulation levels
• Away from roof drip zones where meltwater can freeze into thick layers of ice
• With at least 12–18 inches of clearance around the unit for proper airflow
• Sheltered from high winds that can reduce efficiency

These details prevent ice buildup, reduce defrost cycles, and improve stability during cold weather.

Avoid Blockages Around the Unit

Items that may seem harmless in summer, like patio furniture, shrubs, or snow shovels, can restrict airflow in winter. Maintaining a clear, open space around the outdoor unit is important, especially at –20°C.

Keep Snow & Ice Away

A quick check after major snowfalls can prevent reduced airflow and keep the system running at peak performance.

Tip: Never chip ice off the coil, this can damage the unit. A warm defrost cycle will melt ice safely.

When a Heat Pump Might Struggle at –20°C

Even though modern cold-climate heat pumps are designed for extreme weather, there are certain conditions where a system may have difficulty keeping up. These issues don't mean the heat pump is failing, they highlight factors that affect performance in deep cold.

 Older or Poorly Insulated Homes

Many Toronto homes, especially pre-war brick houses and semi-detached properties, lose heat faster than a heat pump can replenish it during extreme cold.
If your home has:

• Single-pane or aging windows
• Missing or thin attic insulation
• Large drafts around doors
• Uninsulated walls

…the heat pump may need more help from backup heat during –20°C weather.

Undersized Heat Pump Systems

If a unit was sized for mild conditions rather than true Canadian cold, it may run constantly at –20°C and still struggle to hit the target temperature. 

This is one of the most common reasons homeowners feel their system is "not working" in extreme cold.

Incorrect Refrigerant Charge

If the refrigerant level is too low or too high, the system can lose efficiency, ice up faster, or run for long periods without delivering enough heat.

This is an installation or maintenance issue, not a limitation of heat pumps themselves.

Airflow Problems (Indoor or Outdoor)

Restricted airflow impacts performance significantly, especially during cold snaps.

Common causes include:

• Dirty indoor filters
• Blocked or poorly designed ductwork
• Snow buildup around the outdoor unit
• Ice from roof melt dripping onto the unit
• Shrubs or objects placed too close to the unit

When airflow drops, heating output drops with it.

Excessive Frost or Ice Buildup

A small amount of frost is normal, but thick or persistent ice indicates an issue. At –20°C, the unit can enter defrost more often if airflow is blocked or humidity levels surge.

This can cause:

• More frequent pauses in heating
• Longer recovery times
• Temporary reliance on backup heat

Most of these issues are preventable with proper installation and seasonal maintenance.

Warning Signs Something Is Wrong

A well-functioning cold-climate heat pump should still heat effectively at –20°C. If it can't, look for these red flags:

Backup Heat Runs Constantly

Occasional use is normal. Constant use suggests:

• Undersizing
• Poor insulation
• Incorrect settings
• Installation issues

Ice Completely Covering the Outdoor Unit

A thin layer of frost is fine, thick sheets of ice or a frozen fan are not.

Little or No Warm Air from Vents

This can indicate:

• Low refrigerant
• Compressor issues
• Severe airflow restriction
• A failing defrost mode

Short Cycling

The system turns on and off frequently, causing poor heating and higher energy bills.

Indoor Temperatures Not Reaching the Setpoint

If your thermostat is stuck several degrees below target for multiple hours, the system may be struggling.

If you're noticing any of these issues, it's a good idea to have a certified technician take a closer look. The team at HeatPumps.ca can diagnose the problem quickly and make sure your system is running safely and efficiently. Service Request

Heat Pump Reliability in Toronto Winters

Modern cold-climate heat pumps are built for exactly the kind of winters Toronto experiences, including those bitter stretches when temperatures plunge to –20°C and beyond. While older models struggled in deep cold, today's systems use advanced compressors, improved refrigerants, and smart defrost controls to keep homes warm, efficient, and comfortable even during extreme weather.

Performance will always vary based on the home, the installation, and the system's size, but a well-installed cold-climate heat pump can reliably heat most Toronto homes with only occasional support from backup heat. Understanding how these systems work, from low-temperature efficiency to defrost cycles and auxiliary heat, empowers homeowners to make confident decisions and avoid common misconceptions.

If you're considering upgrading your heating system or want to ensure your current heat pump is ready for winter, working with a certified Toronto installer is important. With proper sizing, placement, and maintenance, your heat pump can deliver dependable performance through every cold snap the season brings.