How Air-to-Air Heat Pumps Work: A Simple Explanation
Understanding the science behind one of the most efficient heating and cooling systems available
Air-to-air heat pumps seem almost magical: they can heat your home in winter and cool it in summer, all while using far less energy than traditional heating systems. But how do they actually work? In this guide, we'll break down the science behind heat pumps in plain English, so you can understand exactly what's happening inside these efficient systems.
The Core Principle: Moving Heat, Not Creating It
The key to understanding heat pumps is this: they don't generate heat—they move it from one place to another.
Think of your refrigerator. It doesn't "create cold"—it removes heat from inside the fridge and releases it into your kitchen. That's why the back of your fridge feels warm. An air-to-air heat pump works on exactly the same principle, but it can reverse the process.
Why This Matters:
Moving heat requires much less energy than creating it from scratch. This is why heat pumps achieve 300-400% efficiency—for every 1 kW of electricity used, they can move 3-4 kW of heat. This is physically impossible with traditional heating methods like gas boilers or electric radiators.
The Refrigeration Cycle: The Heart of a Heat Pump
Heat pumps use something called the refrigeration cycle (or vapour-compression cycle). This involves four main components working together:
1. Evaporator (Outdoor Unit)
Absorbs heat from outside air. The refrigerant inside evaporates (turns from liquid to gas) as it absorbs this heat, even when it's cold outside.
2. Compressor
Compresses the refrigerant gas, increasing its temperature dramatically. This is where the electrical energy goes—running the compressor.
3. Condenser (Indoor Unit)
Releases the heat into your home. The hot refrigerant gas condenses back into a liquid, giving up its heat to the indoor air.
4. Expansion Valve
Reduces the pressure of the refrigerant, cooling it down before it returns to the evaporator to start the cycle again.
Heating Mode: Step-by-Step
Let's walk through what happens when your heat pump heats your home on a cold Manchester winter day:
Outdoor Unit Absorbs Heat
Even when it's 5°C outside, there's still heat energy in the air. The outdoor unit (evaporator) contains refrigerant at an even lower temperature (maybe -10°C). Heat naturally flows from warmer to cooler, so heat from the outdoor air flows into the cold refrigerant.
Refrigerant Evaporates
As the refrigerant absorbs heat, it evaporates (boils) and turns from a liquid into a gas. This happens at very low temperatures because the refrigerant is specially chosen for this property.
Compressor Heats the Gas
The compressor squeezes the refrigerant gas, which dramatically increases its temperature—often to 60-70°C or higher. This is the main energy-consuming step, but it's highly efficient.
Indoor Unit Releases Heat
The hot gas flows to the indoor unit (condenser). A fan blows air over the hot coils, and the refrigerant gives up its heat to warm your room. As it cools, the refrigerant condenses back into a liquid.
Expansion Valve Resets Pressure
The liquid refrigerant passes through an expansion valve, which drops its pressure and temperature, preparing it to absorb heat again. The cycle repeats continuously.
Cooling Mode: The Process Reverses
In summer, your heat pump simply reverses the flow direction. Now it works exactly like an air conditioner:
- •The indoor unit becomes the evaporator—it absorbs heat from your room
- •The outdoor unit becomes the condenser—it releases that heat outside
- •A reversing valve switches the refrigerant flow direction
This reversibility is what makes air-to-air heat pumps so versatile—one system handles both heating and cooling throughout the year.
Why Heat Pumps Are So Efficient
Heat pumps achieve efficiency levels that seem to break the laws of physics. Here's why they're so much better than traditional heating:
| Heating Method | How It Works | Efficiency |
|---|---|---|
| Gas Boiler | Burns gas to create heat | 90-95% |
| Electric Radiator | Converts electricity to heat | 100% |
| Air-to-Air Heat Pump | Moves existing heat | 300-400% |
The Efficiency Paradox Explained:
Heat pumps can exceed 100% efficiency because they're not creating energy—they're using a small amount of electrical energy to move a larger amount of heat energy. For every unit of electricity used to run the compressor, 3-4 units of heat are transferred into your home. This is called the Coefficient of Performance (COP).
Understanding COP (Coefficient of Performance)
COP is the key metric for heat pump efficiency. It's calculated as:
COP = Heat Output ÷ Electrical Energy Input
A COP of 3.5 means 3.5 kW of heat for every 1 kW of electricity
Real-World COP Values
- •Mild weather (7-10°C): COP of 4.0-4.5 (highly efficient)
- •Cold weather (0-5°C): COP of 3.0-3.5 (still excellent)
- •Very cold (-5°C): COP of 2.0-2.5 (still better than electric heating)
Even in the coldest conditions, heat pumps remain more efficient than traditional electric heating, which has a maximum COP of 1.0.
Key Components of an Air-to-Air System
Outdoor Unit
Contains the compressor, outdoor heat exchanger coil, and fan. This is where heat is absorbed in heating mode (or expelled in cooling mode).
Typically mounted on an external wall or ground-standing. Built to withstand UK weather.
Indoor Unit(s)
Wall-mounted units containing the indoor heat exchanger and fan. These distribute conditioned air into your rooms.
You can have multiple indoor units connected to one outdoor unit for zone control.
Refrigerant Lines
Copper pipes carrying refrigerant between outdoor and indoor units. Insulated to prevent heat loss.
Professionally installed through small holes in external walls.
Inverter Technology
Modern heat pumps use inverter-driven compressors that vary their speed. This allows precise temperature control and better efficiency.
Unlike on/off systems, inverters run continuously at varying speeds, saving energy and maintaining comfort.
Common Technical Questions
How can heat pumps extract heat from cold air?
All air contains heat energy as long as it's above absolute zero (-273°C). The refrigerant in the outdoor coil is colder than the outdoor air, so heat naturally flows from the air into the refrigerant. Even at 0°C, there's plenty of heat available to extract.
Do heat pumps work in Manchester winters?
Absolutely. Modern heat pumps work efficiently down to -15°C or lower. Manchester's average winter temperature is 3-7°C—well within the optimal range. Even during the coldest days, heat pumps remain effective.
Why is refrigerant important?
Refrigerant is chosen for its ability to change phase (liquid to gas and back) at useful temperatures. Modern refrigerants like R32 are environmentally friendly and highly efficient at absorbing and releasing heat.
How does defrost mode work?
In very cold conditions, frost can build up on the outdoor coil. The heat pump automatically reverses into cooling mode for a few minutes to melt the frost, then switches back to heating. This is normal and doesn't significantly impact efficiency.
Why Air-to-Air Heat Pumps Excel in Manchester
- •Mild climate: Manchester's 3-7°C winters are perfect for heat pump efficiency. COP stays high throughout winter.
- •Consistent performance: No extreme cold snaps that would reduce efficiency significantly.
- •Year-round value: Increasingly warm summers (25-30°C) mean the cooling function gets real use.
- •Humidity control: Dehumidification helps with Manchester's damp conditions.
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