And, of course, the main weakness


for ASHPs is that as the temperature of the outdoor air reduces (in winter), the COP will reduce – just when there is the greatest need for heating. So, any meaningful statement of performance should consider the operation over the whole working ‘season’. The seasonal performance factor (SPF) (see CIBSE Journal October 2011, p51, for definition), which considers the ratio of annual useful heat produced compared to the energy supplied, allows for the inevitable changes in COP as the evaporator and condenser temperatures vary due to altering external temperatures and internal load requirements. It will also account for other de-rating factors, such as the de-icing cycles required to keep the evaporator clear of ice at low outdoor temperatures, as well as accounting for parasitic power used in fans and control systems. For ASHPs supplying heating and hot water systems, Part L requires the SPF to be a minimum of 2.7 for new build and 2.5 for retrofit installations (this is in line with the requirements of the very useful BS EN 15450:2007 – Heating systems in buildings – Design of heat pump heating systems). In 2010, the Energy Saving Trust5


(EST) reported relatively poor seasonal performance of retrofitted heat pumps in domestic applications, using a slightly different measure called ‘System Performance’ that also takes account of energy used in controlling and applying the heat to the building load. Half of the installations surveyed had a system performance of 2.2 or less – although there was no specific quality assurance in place when these heat pumps were installed. Future COPs (considering the ideal ‘Carnot cycle’ that determines the thermodynamic upper limit of ‘heat engines’) may theoretically reach towards seven and, indeed, European experience has shown SPFs in their more mature, experienced markets to be significantly higher than the majority of those in the EST report.


Microgeneration Installation Standard: MIS 3005 As part of the Microgeneration Certification Scheme (MCS), and in an attempt to improve standards, the Microgeneration Installation Standard MIS 30056


sets out


the required procedure for contractors for the design, selection and installation of heat pumps up to 45kW heat output. This will range in application from domestic to small commercial premises. The products


52 CIBSE Journal January 2012


Figure 2: An air source heat pump with external evaporator unit, internal condenser unit and matched thermal store


Supplementary heating required


Heating completely by heat pump


electric heating – supplied as part of an integrated ASHP package. This will lead to larger (and probably more expensive) units being installed, but should ensure that there is less use of direct electric heating, which has been a concern in installations to date. If the heat pump is sized to operate at


Design temperature Base temperature Outdoor air temperature


Figure 1: Monovalent heat pump installation sized at 100% of heating design load requires supplementary heating when outdoor temperatures are below design conditions


themselves are also certificated by the MCS as meeting appropriate standards. The objective of the government’s microgeneration strategy is an attempt to create conditions under which technologies, such as ASHPs, become a more realistic and acceptable alternative energy solution. As well as being evidence of good practice, MCS compliance is required if the installation is to take advantage of the recently introduced permitted development rights in England, or is being used to gain any government funding such as a Renewable Heat Premium Payment7


or,


potentially, the Renewable Heat Initiative payments that could be commencing for ASHPs later this year. The most recent revision to MIS 3005


(version 3, which comes into effect from 1 February 2012) requires that if the heat pump is the only source of heating (a ‘monovalent’ system) then it be selected so that it provides at least 100% of the calculated design building heat loss (determined using standard methods such as those given in BS EN 128318


or CIBSE


Guide A). The key impact of this new requirement is that this is the heat pump rating – excluding any supplementary


100% output at the design heat loss, then at times when the external temperatures fall below design conditions, additional heating will be required (as in Figure 1). Using MCS-recommended design conditions, this could be in the order of 90 hours per year and, in a monovalent system, this heating is likely to be provided by an associated electric heater. An alternative is to ‘oversize’ the ASHP to satisfy the extreme conditions, but because the COP drops as the outdoor temperature reduces, there is likely to be a disproportionate increase in heat pump size for the relative short period during the year that the additional load will be used. Oversizing is likely to increase on/off cycling at moderate loads, which may reduce the SPF. For the majority of traditional installations, there is a supplementary heat source that will also be required to satisfy top-up loads for the domestic hot water requirements. For bivalent systems (those that have an


alternative heat source, such as a gas or oil boiler) the combined installed design load should satisfy the full design heat loss – again, without including the output of an electric heater. To maximise the SPF, and reduce cycling,


there should be some storage element within the system, as is frequently offered in a package such as that shown in Figure 2. ASHPs are also regularly used


to provide cooling in commercial applications using ‘reversible’ systems (the function of the evaporator and condenser being reversed), which frequently uses variable refrigerant flow and building zoning to allow controlled concurrent heating and cooling across the building. Where there is a consistent diverse heating and cooling load, there can be a strong case for using cooling by effectively moving the heat from the overheated areas to the areas with a heating need, using the heat pump technology. However, the use of cooling on its own, in both commercial and domestic premises, can normally be – and, for environmental and energy use reasons, should be – avoided in all parts of the UK. However, if cooling is used, the removed heat should – wherever possible – be applied to domestic hot water storage.


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Building heat loss Air source heat pump output


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