COP / EER (instantaneous performance)

The COP (Coefficient of Performance) and EER (Energy Efficiency Ratio) measure the instantaneous performance of a thermodynamic system. The COP applies to heating: it is the ratio between the heat output (kW) and the electrical input (kW). The EER applies to cooling: it is the useful cooling capacity divided by the electrical input. These values ​​change in real time depending on external conditions, water/air temperatures, and load. They differ from seasonal indicators (SCOP/SEER) which incorporate weather profiles from a typical year and partial-load operation.

COP/EER: Operational definition and interpretation

COP (heating) = kW of heat delivered / kW of electricity consumed. Example: a COP of 3 means that 1 kW of electricity produces 3 kW of heat.

EER (cooling) = kW cooling delivered / kW electricity consumed. Example: an EER of 3 means 1 kW of electricity for 3 kW of cooling.

In practice, these ratios depend on:

- source temperatures (outside air, groundwater, geothermal probes),

- flow/return temperatures on the emitter side (floors 30–35 °C vs radiators 55–70 °C),

- of the partial load (compressor modulation),

- Auxiliaries (pumps, fans) included or not depending on the measurement method (manufacturer's data sheet, test standard, in situ measurement).

Key takeaway : a standard COP/EER at a fixed point (e.g., 7°C air / 35°C water) does not predict actual performance during harsh winters or heat waves. SCOP/SEER and building management system (BMS) monitoring (load curves, logs) are necessary to accurately reflect annual performance.

Interpreting COP/EER: strengths, limitations and areas for improvement

Strengths

• Immediate indicators to compare settings or variants (water law, DHW temperature, pump speed).
• Good indicators for diagnosing a deviation (fouling of heat exchangers, insufficient flow rate, overheating/subcooling).
• Useful for optimizing hydraulics (target ΔT), aerodynamics and compressor modulation.

Boundaries

• Very sensitive to context: an excellent COP in mid-season does not guarantee performance in extreme cold.
• Comparison between brands is difficult if the auxiliary components are not counted in the same way.
• Do not include defrosting periods (air/water heat pump) or frequent stops/restarts.
• Not very representative of the annual economy: prefer SCOP/SEER, IPLV, in situ measurements.

Points to consider (design & operation)

• Lowering the starting temperatures (low temperature emitters, optimized water law) → COP/EER increases.
• Ensure flow rates (minimum heat pump flow rate, balancing, clean filters) → avoid cycling.
• Clean the heat exchangers and take care of the air/water quality (frost, fouling = immediate drop).
• Monitoring in BMS: log kW hot/cold and kW elect for a COP/EER curve vs weather.
• Anticipating defrosting (air/water heat pump): dry installation, management of critical time slots.
• Check what the instrumentation measures: include/exclude pumps/ventilators according to the objective (internal comparison vs contract).

Anecdote — “A COP that was melting in the Montpellier sun”

In Montpellier, a reversible heat pump in an office building showed a decent EER in the spring… but plummeted during heat waves. Building management system (BMS) monitoring revealed: roof ducts exposed to the sun, insufficient temperature difference on the chilled water side, and a dirty heat exchanger. Simple solutions: insulation of the hot sections, flow rate adjustments, and cleaning. Result: EER +14% during hot periods, without any work on the compressor. The moral of the story: before blaming the unit, check the peripherals—that's often where the COP/EER is determined.