Rinnai Director Chris Goggin establishes the importance of heat pump system congruity and the different methods of measurement that reveal heat pump performance. This article will explain two relevant questions: Why is heat pump system and component harmony vital towards customer understanding of system performance? And what are the separate metrics used to provide an accurate representation of system performance?
Once a UK customer of decarbonising technology decides to invest in a heating and hot water system, how do they identify the optimum, correct option? Furthermore, how does the customer also recognise the overall ability of a system to perform at an efficient and high level of performance that also reduces costs?
There are several established methods for expressing the performance of heat pump heating and hot water systems. Seasonal Coefficient of Performance (SCOP) is widely used as a seasonal efficiency rating for the heat pump unit under defined test conditions. However, SCOP may not reflect installed performance because it does not account for site-specific factors such as local climate, the application, controls strategy, distribution losses, or the energy use of auxiliary components.
As a result, published ratings can sometimes appear higher than the efficiency achieved in real operation. SCOP is calculated using standardised seasonal climate profiles and a defined test flow temperature (commonly 35C for space heating). For UK commercial hot water applications, it is often more helpful to evaluate performance using assumptions that match the installation site temperature profile and the higher flow temperatures required for domestic hot water.
A SCOP approach focuses on the heat pump unit’s seasonal efficiency rather than the performance of the complete installed system. Where the overall system includes back-up heaters (for example gas-fired water heaters or electric immersion/cylinders) and auxiliaries such as secondary return pumps, controls, or circulation losses, the realised system efficiency can be significantly lower than the units SCOP.
Coefficient of Performance (COP) is another common way to express efficiency, but it is a single-point value measured at specific test conditions rather than over a whole season. COP does not, by itself, capture weather variation, load variation, or controls behaviour across the year. In addition, published COP values are dependent on the test boundaries used: they may exclude some system-wide electrical loads and distribution losses (such as pumps, fans, controls, or circulation losses). For this reason, COP should not be treated as a complete measure of overall system congruity or real-world running cost.
Another area that can be overlooked in headline efficiency discussions is part-load operation. In practice, heat pumps spend much of their time operating at part load, and efficiency can improve or deteriorate depending on modulation range, control strategy, and whether the unit cycles on and off. Single-point COP values do not describe these behaviours in sufficient detail. Additional influences include temperature lift and system design factors that affect mechanical, thermal, and friction losses across the wider system.
Finally, a high COP value does not necessarily guarantee low operational costs if the heat pump system is poorly sized, poorly maintained, or required to operate with an unnecessarily high temperature lift. COP is also commonly quoted at a limited number of operating points, so it provides less information about seasonal performance than SCOP or a system performance factor such as SPF.
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