Rinnai publishes white paper on seasonal performance factor

Rinnai is issuing a report that details what it describes as “the potential limitations of manufacturer-issued efficiency measures” for heat pumps, namely COP (coefficient of performance) and SCOP (seasonal coefficient of performance).

Manufacturers often measure heat pump efficiency using COP, which is the ratio of useful heat energy produced to the electricity consumed at a specific external air temperature and temperature rise required.

Rinnai states that due to the way in which efficiency is measured utilising a SCOP rating, commercial hot water systems (CHWS) will potentially show a lack of system congruity when considering the entire system, against the expected performance of the heat pump alone, which, as a consequence, affects product expectations and ultimately carbon reductions.

The Rinnai white paper has adopted the seasonal performance factor (SPF), which evaluates all components that use energy within a commercial hot water system, therefore, measuring the performance of the entire system as opposed to solely the heat pump.

The report also presents a detailed case study to underline the two systems of thought used within the hot water dispersal industry. These are used to inform potential customers of a system’s efficiency.

The case study presents two heat pump systems:

  • System 1 covers the DHW demand with 100% heat pump;
  • System 2 uses 40% heat pump and includes an electric cylinder to provide back-up heat.

The results show that when considering the entire commercial hot water system, including the buildings’ piping system, system 1 has a higher SPF (2.75) than system 2 (1.49), due to system 2 operating the back-up heater in peak times and to recover secondary system heat losses.

However, when observing the SPF of only the heat pump unit, system 2 is higher. This can mislead the customer as they are expecting an SPF of close to 3.33, but the system only performs to an SPF of 1.49, as seen in table 2.

The purpose of the white paper is to highlight the difference in performance between differing system boundaries and works towards informing the customer of the reality of their CHWS.

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