How is the Seasonal Performance Factor of my heat pump calculated?

The seasonal performance factor of a heat pump system is not the easiest thing to work out. Read to find out more.

seasonal leaf

The seasonal performance factor (SPF) of your heat pump is the equipment’s average output of heat covering one full heating season. The factor is the figure which the Department of Energy and Climate Change (DECC) uses to calculate whether you are eligible to receive renewable heat incentive (RHI) payments when the domestic RHI scheme picks up early in 2014.

In July 2013, the DECC announced that the derivation of the seasonal performance factor will be based purely upon the operating temperature of the technology installed. What this means now is that all equipment is assumed to have equal efficiency at given flow temperatures, with the only differentiation being made between whether the heat pump is ground source or air source.

The Domestic Heat Emitter Guide (DHEG) is to be used to find the operating temperature of your system. The guide is published by the Microgeneration Certification Scheme (MCS) and states that any air source heat pump system with a top design flow temperature of 53 degrees C or above will be eligible for RHI, as will a ground source heat pump setup with a design flow temperature of more than 65 degrees C. This is because these temperatures are the points at which the DHEG states that the SPF lies below the EU minimum SPF standard of 2.5.

While this makes the analysis quite easy, it does bring up some issues in regards to trade associations such as the Heat Pump Association, which they are attempting to resolve with the DECC.

For example, one manufacturer might have a considerably different efficient heat pump capable of reaching an SPF of over 2.5 at 55 degrees C which would not qualify if used with 55 degrees C as its maximum design flow temperature. On the other hand, another manufacturer might have a heat pump which is not as high a quality at higher flow temperatures. As these increase it may even have a poorer SPF at 50 degrees C than the first manufacturer at 55, but the second heat pump will still qualify.

Additionally, the DHEG will not differentiate between shallow horizontal ground source heat pumps and the deep vertical borehole versions.

The DHEG also takes for granted that all systems will put into use weather compensation energy saving flow temperature control. Whilst every MCS approved piece of equipment has to be capable of this level of control, there is no requirement on the user’s part to actually use it. This is more of a surveillance measure than a DHEG issue.

These issues are at the moment being developed by the heat pump certification body of the MCS, the working group of which is currently taking ownership of the DHEG.

The trade associations are also building a case to let systems be voluntarily metered so as to determine the renewable heat quantity in cases where it could reach more than 2.5 SPF threshold despite being able to work at a peak flow of over 53 degrees C (air source) or 65 degrees C (ground source). This would mean electrical input also having to be metered.

The remaining issue.

The question that remains is what happens if the system, for whatever reason, does not give an SPF of over 2.5 in which case the owner does not qualify for RHI payments? It has to be determined who is responsible, and whether it was down to poor weather conditions. Degree day correction could be utilised if local data is available, although this makes it difficult for the layman to comprehend and predict performance.

Some exceptions ought to be noted. Namely, for second homes and bi-valent or hybrid systems, for which the renewable heat output is measured and therefore directly rewarded up to a set maximum kWh. This is essentially to prevent more production of heat than is needed in order to get payments greater than the cost of generating fuel (within years 1-7). In these cases, it would be necessary to meter energy input so as to determine that the lowest 2.5 threshold has also been met.

Image by Nana B Agyei

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