1. Relationship Between COP and Control Systems
COP (Coefficient of Performance) represents the ratio of heat energy produced by a heat pump to the electrical energy consumed. However, this ratio is not limited to the compressor and refrigerant cycle alone. COP is directly influenced by operating conditions and control strategies.
COP values measured in laboratory conditions are based on steady-state operation. In real-world applications, systems operate under continuously changing loads. At this point, the room thermostat becomes the primary control element defining the heat pump’s load profile.
Incorrect thermostat control can cause a heat pump with a theoretically high COP to perform poorly in real operation.
2. Key Parameters Affecting Heat Pump COP
Instantaneous COP depends on the following parameters:
Heat source temperature (air, ground, water)
Supply water temperature
Return water temperature
Compressor speed
Operating time and start-stop frequency
Part-load operating ratio
Most of these parameters are indirectly influenced by the room thermostat, as it determines when the system operates, stops, and at what capacity.
3. Impact of On-Off Thermostats on COP
On-off thermostats force heat pumps into two operating states: full capacity operation or complete shutdown. From an engineering standpoint, this control method is disadvantageous for heat pumps.
Key reasons include:
Increased compressor inrush current during each start
Low COP during start-up
Frequent cycling reduces average COP
The system shuts down before reaching stable operating conditions
Especially at low outdoor temperatures, on-off control can cause severe COP losses. Often, the perceived inefficiency originates not from the heat pump itself but from the control strategy.
4. COP Advantage of Modulating Thermostats
Modulating room thermostats allow the heat pump to adjust capacity according to demand. This enables:
Long-duration low-capacity operation
Stable supply water temperature
Continuous compressor speed control
Minimal start-stop cycles
From an engineering perspective, heat pumps achieve their highest COP values at partial load conditions. Full-capacity operation is often the least efficient point.
Modulating thermostats keep the heat pump within this efficient operating range, improving both instantaneous COP and seasonal performance factor (SCOP).
5. Relationship Between Setpoint Temperature and COP
There is a direct relationship between thermostat setpoint and COP. Higher indoor temperature demand requires higher supply water temperature, which:
Increases compressor pressure ratio
Raises electrical consumption
Reduces COP
Engineering best practice recommends avoiding unnecessarily high indoor temperature setpoints. High measurement accuracy and narrow hysteresis control are critical for maintaining COP.
6. Start-Stop Frequency: The Most Critical Factor Affecting COP
Frequent cycling is one of the most damaging factors for COP:
Reduces compressor efficiency
Increases electrical consumption
Accelerates mechanical wear
If the thermostat reacts to minor temperature fluctuations, the heat pump never reaches optimal operating conditions.
Therefore, thermostats used in heat pump systems must:
Be heat pump compatible
Support modulation
Use extended time-based control logic
7. Hydraulic Balance and Indirect COP Effects
While the thermostat does not directly control hydraulics, it influences flow via zone valves and circulation pumps. Frequent zone opening and closing:
Reduces flow rate
Destabilizes return water temperature
Causes incorrect load detection
Lowers COP
Thus, thermostat selection should not be separated from hydraulic design.
8. Proper Thermostat Integration to Preserve COP
From an engineering perspective, the ideal thermostat:
Matches the heat pump’s modulation range
Prevents frequent cycling
Supports long-duration low-load operation
Avoids excessive supply water temperature increases
This approach allows real-world performance to approach theoretical COP values.
Therefore, in heat pump projects, the room thermostat should not be considered a secondary accessory, but rather a performance-defining system component.
9. Engineering Conclusion
When a room thermostat operates together with a heat pump, COP:
Can decrease by 20–30% with improper control
Can reach full system potential with correct control
Can closely match theoretical values in optimized scenarios
Therefore, improving COP is not only about selecting a better heat pump—it requires implementing the correct control strategy.