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Getting heat pump design right in commercial buildings


Getting the design of heat pump systems right is increasingly important to keep energy consumption and operating costs under control. But according to BSS ASHP technical manager Matthew Henderson, one of the biggest challenges is that some contractors are still approaching heat pump installations in the same way they have traditionally designed gas boiler systems


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hat can result in systems being significantly oversized for the building they are serving. Installers have traditionally selected


boilers with plenty of spare capacity, so the system can quickly respond to demand. That approach made sense when boilers and their fuel were relatively inexpensive, but heat pumps need to be designed around how the building actually operates. This becomes particularly important in


commercial properties, where heating demand can vary significantly throughout the day. An office, hotel, leisure centre or public building


may have very different requirements during busy and quiet periods. A system that has been selected simply by matching the output of an existing boiler may therefore be considerably larger than necessary.


The starting point for any heat pump installation should be an accurate assessment of the building’s heat demand. Insulation, glazing, ventilation, room sizes,


occupancy and the building’s design temperature all need to be considered when determining the required heat output. The existing heating system can provide useful information, but its capacity should not automatically dictate the size of the new heat pump. A building with a 100kW gas boiler, for example,


will not necessarily require a 100kW heat pump. This is partly because heat pumps operate


very differently from conventional boilers. Their efficiency is commonly described using the coefficient of performance (COP). A COP of 3, for example, means that for every unit of electrical energy consumed, the heat pump can deliver around three units of heat. That makes operating conditions and system design particularly important. An oversized heat pump can cycle more


frequently, potentially reducing efficiency and increasing capital costs. It can also make it more difficult to operate the heating system consistently at the lower flow temperatures at which heat pumps perform most effectively. For larger buildings, thermal storage can be an important part of the overall system design. A stratified thermal store, for example, allows


water at different temperatures to be held within the tank. Hotter water can be drawn from the upper


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Above: Matthew Henderson BSS ASHP technical manager for BSS ASHP


section when demand increases, while cooler water remains available to be reheated. This can help a heat pump system respond to fluctuations in demand without requiring the heat pump itself to be dramatically oversized. That can be particularly useful in buildings such as hotels, where there may be significant peaks in hot water demand at certain times of the day. A secondary heat source can also be incorporated


where appropriate. An electric or gas boiler, for example, can provide additional capacity during short periods of exceptionally high demand, allowing the main heat pump system to be designed around the building’s normal requirements. The result is a system that is designed around


how the property actually operates rather than simply replicating the capacity of its previous heating plant. There are several other factors that need to be


considered during the design process, including flow rates, emitter sizing and Delta T (∆T). These details can have a significant impact on


the performance of the finished system, particularly where existing commercial heating infrastructure is being adapted for lower-temperature operation. For contractors working on complex projects, specialist design advice can therefore be valuable. BSS provides an advisory service for customers designing heat pump installations, with the technical team able to review plans and drawings and provide


recommendations intended to maximise system performance and improve COP. Heat pump systems require more consideration at the design stage than a traditional combi boiler. Getting the fundamentals right can make a significant difference to the running costs and performance of the system over its lifetime. Commercial heat pump designers and installers also need to consider the industry’s increasing use of natural refrigerants such as propane. Propane can provide high energy efficiency and


has a relatively low global warming potential, but its flammability means installations need to take appropriate safety considerations into account. This includes factors such as the positioning of the outdoor unit, available space and drainage arrangements, particularly where there is a possibility of refrigerant pooling in the event of a leak. For commercial property owners, the transition to heat pumps is therefore about more than simply replacing an existing boiler with a lower-carbon alternative. Accurate heat-loss calculations, appropriate


system sizing, thermal storage, emitter selection, flow temperatures and refrigerant considerations all need to form part of the design process. Getting those decisions right at the outset can


help commercial buildings achieve the efficiency, reliability and lower operating costs expected from their investment in heat pump technology.


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