The Thermodynamics of Heat Pumps: Coefficient of Performance (COP)
A heat pump is an electrical device that moves heat from one location to another using a refrigerant cycle, operating like a refrigerator in reverse. Unlike combustion furnaces (which burn gas or oil to create heat) or electric resistance heaters (which convert electricity directly into heat), heat pumps simply transfer existing thermal energy from the outdoor air or ground into your home. This makes them highly efficient, frequently exceeding 100% efficiency.
The efficiency of a heat pump is measured by its Coefficient of Performance (COP). The formula is: COP = Q_(heat)/W_(input) where Q_(heat) is the thermal energy delivered and W_(input) is the electrical energy consumed. A COP of 3.0 means the heat pump delivers 3 units of heat for every 1 unit of electricity it consumes, achieving a nominal efficiency of 300%. Standard electric resistance heaters have a fixed COP of 1.0 (100% efficiency).
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The thermodynamic cycle relies on a refrigerant fluid with a very low boiling point. The fluid is evaporated outdoors at low pressure, absorbing ambient heat from the air. A compressor then squeezes the vapor, raising its pressure and temperature. The hot vapor is routed indoors, where it condenses back to a liquid, releasing its heat into the home. An expansion valve then drops the pressure of the liquid, cooling it further, and the cycle repeats.
Additionally, heat pumps exhibit a property known as temperature dependency. As the outdoor temperature drops, the density of the outdoor air decreases, meaning there are fewer air molecules from which to extract heat. This causes both the heating capacity (BTUs/hour) and the operating COP of the heat pump to decline, making it critical to analyze seasonal averages rather than relying solely on a single, fixed peak COP rating.