How Heat Pumps Work: Complete Technical Explanation (2026)
Heat pumps seem almost magical—they extract heat from freezing outdoor air and use it to warm your home. But there's no magic involved, just clever application of thermodynamics and refrigeration principles. This guide explains exactly how heat pumps work, in terms accessible to homeowners and technical enough for those who want the details.
The Core Principle: Moving Heat, Not Creating It
Key insight: Heat pumps don't generate heat through combustion or electrical resistance. Instead, they move heat from one place to another using a refrigeration cycle.
This is why they're so efficient:
- Electric resistance heater: 100% efficiency (1 kW electricity → 1 kW heat)
- Gas furnace: 80-95% efficiency (some heat lost in exhaust)
- Heat pump: 200-400% efficiency (1 kW electricity → 2-4 kW heat)
How is >100% efficiency possible? The electricity doesn't directly create heat—it powers a compressor that moves heat that already exists in the outdoor air. Even at -20°C, outdoor air contains significant thermal energy.
The Refrigeration Cycle (Heat Mode)
Heat pumps use the same basic technology as your refrigerator or air conditioner, just reversed to provide heating.
Four Main Components
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Evaporator (outdoor coil in heating mode)
- Refrigerant evaporates here, absorbing heat from outdoor air
- Even at -20°C, outdoor air has usable heat energy
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Compressor
- Compresses refrigerant vapor, increasing pressure and temperature
- This is where electricity is consumed
- Variable-speed compressors adjust output to match heating demand
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Condenser (indoor coil in heating mode)
- Hot, high-pressure refrigerant releases heat indoors
- Refrigerant condenses back to liquid
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Expansion Valve
- Reduces refrigerant pressure before it re-enters evaporator
- Regulates refrigerant flow rate
Step-by-Step Process (Heating Mode)
Step 1: Absorb heat from outdoor air
- Cold liquid refrigerant (around -10°C to -20°C) flows through outdoor coil
- Outdoor air (even at -10°C) is warmer than the refrigerant
- Heat transfers from air → refrigerant
- Refrigerant evaporates (becomes vapor)
Step 2: Compress the vapor
- Low-pressure vapor enters compressor
- Compressor squeezes vapor, dramatically increasing pressure and temperature
- Vapor exits compressor at 70-90°C (much hotter than needed for heating)
Step 3: Release heat indoors
- Hot vapor flows through indoor coil
- Indoor air (20°C) is cooler than refrigerant (70-90°C)
- Heat transfers from refrigerant → indoor air
- Refrigerant condenses back to high-pressure liquid
Step 4: Expand and repeat
- High-pressure liquid refrigerant passes through expansion valve
- Pressure drops dramatically
- Temperature drops to below outdoor air temperature
- Refrigerant returns to evaporator to repeat cycle
This cycle repeats continuously while heat pump is running (60-3,000+ times per minute for vapor circulation).
Cooling Mode (Air Conditioning)
Heat pumps can reverse this cycle to provide cooling in summer.
Reversing valve switches the refrigerant flow direction:
- Indoor coil becomes the evaporator (absorbs heat from indoor air)
- Outdoor coil becomes the condenser (releases heat outdoors)
Result: Heat is moved from inside → outside, cooling your home.
This is why heat pumps are called "air conditioners that can run backwards."
Key Technologies Explained
Variable-Speed Inverter Compressor
Old technology: Single-speed compressor (on/off only)
- Runs at 100% or 0%
- Frequent cycling (inefficient)
- Temperature swings
Modern technology: Variable-speed inverter compressor
- Adjusts speed from 10% to 100% based on demand
- Runs continuously at lower speeds
- Maintains consistent temperature
- 30-50% more efficient than single-speed
How it works: Electronic inverter converts AC power to DC, then back to variable-frequency AC to control compressor motor speed.
Result: Better comfort, lower energy bills, longer equipment life.
Enhanced Vapor Injection (EVI)
Problem: Standard heat pumps struggle below -10°C because refrigerant can't absorb enough heat from cold air.
Solution: EVI injects additional refrigerant vapor mid-compression
- Increases refrigerant mass flow
- Boosts heating capacity at low temperatures
- Maintains efficiency even at -20°C to -25°C
Used in: Cold climate heat pumps (Mitsubishi Hyper-Heat, Daikin Aurora)
Result: 100% heating capacity maintained at -15°C (vs 60-70% for non-EVI models).
Defrost Cycle
Problem: When outdoor temps are 0°C to 7°C with high humidity, frost builds up on outdoor coil, blocking airflow and reducing efficiency.
Solution: Automatic defrost cycle
- Heat pump temporarily reverses to cooling mode
- Hot refrigerant melts frost on outdoor coil
- Takes 5-15 minutes every 30-90 minutes (when needed)
During defrost:
- Indoor fan may shut off (prevents cold air blowing indoors)
- Backup heat may activate briefly
- Steam visible from outdoor unit (normal)
Advanced defrost: Modern heat pumps use sensors to detect frost and only defrost when needed (not on a timer).
Refrigerants Used in BC Heat Pumps
R-410A (Puron):
- Most common in current systems
- Higher pressure than old R-22
- Non-ozone depleting
- Being phased out due to high global warming potential (GWP)
R-32:
- Newer refrigerant
- Lower GWP than R-410A (68% reduction)
- Better efficiency at low temperatures
- Used in many cold climate models
Future: R-454B, R-32 variants designed for lower environmental impact.
Important: Refrigerants must be handled by certified technicians only (TSBC Gas-2 or Gas-3 license in BC).
How Heat Pumps Work at Different Temperatures
Mild Conditions (+10°C outdoor)
- Efficiency: COP 3.5-4.5 (350-450% efficiency)
- Operation: Easy for heat pump, lots of heat available in outdoor air
- Capacity: Often running at 40-60% speed to match low heating demand
Typical Winter Day (0°C outdoor)
- Efficiency: COP 3.0-3.5 (300-350%)
- Operation: Still very efficient, some defrost cycles needed
- Capacity: Running at 60-80% speed
Cold Day (-10°C outdoor)
- Efficiency: COP 2.3-2.8 (230-280%)
- Operation: Working harder, less heat available outdoors
- Capacity: Running at 80-100% speed
- Note: Standard heat pumps may struggle; cold climate models excel
Very Cold (-20°C outdoor)
- Efficiency: COP 1.7-2.2 (170-220%) for cold climate models
- Operation: Maximum capacity, backup heat may activate
- Capacity: Running at 100%, using EVI technology
- Note: Standard heat pumps may shut down; cold climate models designed for this
Extreme Cold (-25°C outdoor)
- Efficiency: COP 1.5-1.8 (150-180%) for cold climate models
- Operation: At design limit, backup heat likely needed
- Note: Dual fuel systems (heat pump + gas boiler) recommended for areas with frequent temps this low
Key takeaway: Even at -20°C, a heat pump is still 170-220% efficient—nearly twice as efficient as electric resistance backup heat.
Air-to-Air vs Air-to-Water Heat Pumps
Air-to-Air (Ductless & Ducted)
How it works:
- Refrigerant absorbs heat from outdoor air
- Heat released directly to indoor air via fan coils
Distribution:
- Ductless: Individual wall units blow heated air
- Ducted: Central air handler blows heated air through ducts
Response time: Fast (feel heat within minutes)
Best for: Forced-air heating, quick temperature changes
Air-to-Water (Hydronic)
How it works:
- Refrigerant absorbs heat from outdoor air
- Heat transferred to water (35-65°C)
- Hot water circulates through radiant floors, baseboards, or radiators
Distribution: Hydronic piping system
Response time: Slow (thermal mass takes hours to heat up)
Best for: Radiant floor heating, replacing boilers, high comfort
Learn more: Air-to-Water Heat Pumps in BC
Heat Pump Efficiency Metrics Explained
COP (Coefficient of Performance)
Definition: Ratio of heat output to electricity input
Example: COP of 3.0 = 3 kW of heat for every 1 kW of electricity (300% efficient)
Use: Measures efficiency at a specific outdoor temperature
- COP at +8°C: 3.5-4.5 (very efficient)
- COP at -8°C: 2.5-3.2 (still efficient)
- COP at -15°C: 2.0-2.5 (cold climate models)
Learn more: Understanding Heat Pump Ratings
HSPF (Heating Seasonal Performance Factor)
Definition: Average heating efficiency over entire heating season
Units: BTU output per watt-hour of electricity
Typical values:
- Standard heat pump: HSPF 8-10
- High-efficiency: HSPF 10-13
- Top cold climate models: HSPF 12-14
Use: Compare overall efficiency between models (higher = better)
SEER (Seasonal Energy Efficiency Ratio)
Definition: Average cooling efficiency over entire cooling season
Typical values:
- Minimum (Canada): SEER 13-14
- High-efficiency: SEER 18-22
- Top models: SEER 26-30+
Use: Cooling efficiency metric (higher = lower summer electricity bills)
Why Heat Pumps Are So Efficient
Thermodynamic advantage: Moving heat requires far less energy than creating it.
Analogy:
- Creating heat = building a sandcastle from scratch
- Moving heat = moving an existing sandcastle
Math:
- Electric resistance: 1 kW input → 1 kW heat (100% efficient)
- Heat pump: 1 kW input → 3 kW heat moved (300% efficient)
The catch: Efficiency decreases as outdoor temperature drops (less "free" heat available to move). But even at -20°C, heat pumps are still 170-220% efficient—far better than resistance heat.
Common Questions
"How can a heat pump extract heat from -10°C air?"
Answer: Temperature is relative. Even -10°C air contains significant thermal energy compared to absolute zero (-273°C).
How it works: Refrigerant in outdoor coil is even colder than -10°C air (around -15°C to -25°C), so heat naturally flows from air → refrigerant. The compressor then concentrates this heat to a useful temperature (70-90°C).
"Do heat pumps work in BC winters?"
Answer: Yes, especially cold climate models.
Coastal BC (-5°C to -8°C design temp): Standard heat pumps work great Interior BC (-15°C to -25°C design temp): Cold climate models required
Learn more: Cold Climate Heat Pumps in BC
"Why does my heat pump blow 'cool' air sometimes?"
Answer: Heat pumps deliver air at 35-45°C (vs 50-60°C for gas furnaces).
This feels "cool" compared to furnace heat, but it's still warming your home efficiently. Your body temperature is 37°C, so 40°C air feels only slightly warm.
Not a problem: Modern variable-speed heat pumps run longer at lower output for better comfort.
"What's the difference between a heat pump and an air conditioner?"
Answer: A heat pump IS an air conditioner with a reversing valve.
Air conditioner: Cooling only (moves heat outdoors) Heat pump: Heating + cooling (reversing valve switches refrigerant direction)
Cost difference: +$500-$1,000 for reversing valve and controls
Heat Pump Operating Modes
Heating Mode
- Indoor coil = condenser (releases heat)
- Outdoor coil = evaporator (absorbs heat)
- Refrigerant flows: outdoor → compressor → indoor → expansion valve → outdoor
Cooling Mode
- Indoor coil = evaporator (absorbs heat)
- Outdoor coil = condenser (releases heat)
- Refrigerant flows: indoor → compressor → outdoor → expansion valve → indoor
Auto Mode
- System automatically switches between heating and cooling based on indoor temperature and setpoint
Dry/Dehumidify Mode
- Runs cooling cycle at low fan speed to remove humidity without overcooling
Fan Only Mode
- Circulates air without heating or cooling
Next Steps
Now that you understand how heat pumps work:
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Related Guides
- Types of Heat Pumps in BC - Complete overview of all heat pump types
- Understanding Heat Pump Ratings - SEER, HSPF, COP explained
- Ductless vs Central Heat Pumps - Which system is right for you
- Cold Climate Heat Pumps - Performance in Interior BC winters