Once a customer decides to invest in a heating and hot water system, how do they identify the correct option that can perform at an efficient and high level of performance while also reducing costs?
Seasonal Coefficient of Performance (SCoP) is widely used as a seasonal efficiency rating for the heat pump unit under defined test conditions. However, SCoP doesn’t 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 appear higher than the efficiency achieved in real operation.
SCoP is calculated using standardised seasonal climate profiles and a defined test flow temperature (commonly 35°C for space heating). For UK commercial hot water applications it is more helpful to evaluate performance using assumptions that match the installation site temperature profile and the higher flow temperatures needed.
A SCoP approach focuses on the heat pump’s seasonal efficiency rather than the performance of the complete system. Where the overall system includes backup 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 unit’s 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 doesn’t, by itself, capture weather variation, load variation, or controls behaviour across the year. 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 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. 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 doesn’t necessarily guarantee low operational costs if the heat pump system is poorly sized, poorly maintained, or needs 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 Seasonal Performance Factors (SPF).
Both SCoP and SPF can be used to discuss seasonal efficiency and operating cost, but SPF is often treated as the more representative metric because it can be calculated using measured or project-specific assumptions for the whole system.
www.rinnai-uk.co.uk