Heat Pump: How It Works

A heat pump delivers 3 units of heat to your home for every 1 unit of electricity it consumes. That is not a violation of conservation of energy. It is not creating energy from nothing. It is moving heat from outside to inside, and the electricity is paying for the move, not the heat.

A heat pump uses a four-step refrigeration cycle to move heat from a cold source to a warm destination. Step 1 (Evaporate): a refrigerant absorbs heat from outdoor air (even cold air) and evaporates into a gas. Step 2 (Compress): a compressor squeezes the gas, raising its temperature and pressure (this is the step that consumes electricity). Step 3 (Condense): the hot, compressed gas releases heat into your home and condenses back into a liquid. Step 4 (Expand): the liquid passes through an expansion valve, dropping in pressure and temperature, and the cycle repeats.

The coefficient of performance (COP) measures how much heat is delivered per unit of electricity consumed: COP = Q_delivered / W_input. A COP of 3 means 3 kWh of heat delivered per 1 kWh of electricity. The first law is satisfied: 1 kWh of work + 2 kWh absorbed from outdoor air = 3 kWh delivered indoors.

Worked Example

A heat pump has a COP of 3.5, and the home needs 10 kW of heating.

• Calculate electricity required. W = Q / COP = 10 / 3.5 = 2.86 kW.
• Heat absorbed from outdoor air. Q_outdoor = Q_delivered - W = 10 - 2.86 = 7.14 kW.

On a very cold day (-10°C), the COP drops to 2.0. How much electricity is needed for the same 10 kW of heating?

W = 10 / 2.0 = 5 kW. The COP drops in cold weather because the temperature difference between source and destination increases, requiring more work to move the same heat. This is why heat pumps are most efficient in moderate climates and may need backup heating in extreme cold.

Heat pumps are a heat engine running in reverse: instead of extracting work from a temperature difference, they use work to move heat against the temperature gradient.

---