SPONSOR CPD PROGRAMME Bivalent heat pump systems for heating and hot water Continuing professional development (CPD) is the regular maintenance, improvement and broadening of your knowledge and skills, to maintain professional competence. It is a requirement of CIBSE and other professional bodies. This Journal CPD programme can be used to meet your CPD requirements. Study the module and answer the questions on the final page. Each successfully completed module is equivalent to 1.5 hours of CPD. Modules are also available at www.cibsejournal.com/cpd This module explores some of the options for providing a successful application of bivalent heat pump system to maximise performance and efficiency When designing systems that include both an air source heat pump (ASHP) and, typically, a gas-fired boiler in a bivalent system, the optimal operational conditions for both technologies must be considered so as to maximise heat pump utilisation while maintaining system performance and overall efficiency all while avoiding conflict in the bivalent system. This CPD will present some of the options that have been applied in practice to provide a successful application for bivalent heating and hot water systems. A building fully served with ASHPs may well be able to achieve one of the lowest carbon footprints in new commercial building stock, as discussed in detail in the recently published CIBSE AM17 Heat pump installations for large non-domestic buildings.1 As the ambient air temperature rises, so does the heating efficiency and capacity of an ASHP. Since space heating loads decrease as the external temperature rises, sizing an ASHP for peak load at a design temperature that occurs only rarely will result in a unit selection that is oversized for the majority of its operational life. In any case, it is not always possible to engineer all projects and meet budgets by solely employing ASHPs. This is particularly the situation with refurbishment and system upgrade projects, which often include inaccessible physical space, limited available electrical power, capital expenditure (capex) constraints, poor building fabric, or inadequate system infrastructure. Amalgamating traditional heating with low and zero carbon technologies is a well-established method to reduce the carbon impact of the heat generators, and is likely to include technologies such as combined heat and power (CHP), biomass, ground source heat pumps (GSHPs) and ASHPs integrated with a natural gas-fired boiler. ASHPs and high-efficiency condensing boilers or direct electric appliances in a bivalent system may offer a practicable solution to overcome project limitations while meeting heat demand more sustainably. The challenge, however, is to design the heat pump and boiler into a single, integrated system that successfully maximises the efficiency of both technologies. If a combined boiler and heat pump system known as a bivalent system is considered as being appropriate for a specific building application, then ideally the preliminary sizing should maximise the contribution of the heat pump to the total load (while taking any limiting factors into account). AM17 notes that such a bivalent system will typically employ a primary and secondary heating generator. The primary system provides part of the peak load, with the secondary system supplying either the remainder of the peak load a parallel bivalent system so minimising the contribution by the secondary plant, or able to meet the entire load, under peak conditions an alternate bivalent system. As the primary and secondary generators are likely to be hydraulically connected, careful design is required to ensure that there is no unwanted impact on the efficiency or performance for either one. As noted in CIBSE AM17, the use of an hourly load model can help in understanding the impact of different secondary heat source sizes when considering factors such as: www.cibsejournal.com November 2022 93 CIBSE Nov 22 pp93-96 CPD 205.indd 93 21/10/2022 16:55