How the vapour-compression cycle moves heat, why temperatures affect COP, and what monovalent and bivalent systems mean.
Core idea
A heat pump transfers energy from a lower-temperature source to a higher-temperature heating circuit. In a vapour-compression cycle, evaporation absorbs heat, compression raises refrigerant pressure and temperature, condensation releases heat to the system, and expansion lowers pressure before the cycle repeats. Electricity drives the process; it is not converted one-for-one into all of the delivered heat.
What to recognise
- COP compares useful heat output with electrical input at a stated test condition.
- SCOP represents performance across a season and is more useful for broad annual comparisons.
- A smaller temperature lift usually helps efficiency, so lower flow temperature and suitable emitters matter.
- Defrost and colder outdoor conditions can reduce immediate output and efficiency.
- A monovalent design relies on one main heat source; a bivalent design deliberately combines sources.
Apply it
If a unit delivers 8 kW of heat while drawing 2 kW electrically, its instantaneous COP at that condition is 4. That figure will change with source temperature, flow temperature, cycling, pumps and defrost. Use it to understand a measured point, then use design and seasonal data for annual expectations.
Keep the boundary clear
COP is not a percentage efficiency and one catalogue value does not predict every dwelling. System sizing, emitters, controls, hot-water temperatures and fabric heat loss all affect the result. Do not infer refrigerant-side competence from understanding the cycle.
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