HEATING TECHNOLOGY Figure 3: A typical
machine-specific published pressure enthalpy diagram.
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Simon Witts
With over 37 years’ experience, Simon Witts leads masterplanning, sustainable design, engineering design, and project management, for complex building services. His diverse systems design background includes ESD, heating, ventilation, medical gases, air- conditioning, steam and HTHW systems, domestic services, and above ground drainage. He also brings expertise in healthcare, laboratory, and education design, plus energy transition masterplanning for large healthcare facilities. As co-author and editor of the Victorian Design Guidelines for Hospitals and Day Procedure Centres for the Department of Health, he serves as a peer reviewer for the Australian Institute of Refrigeration, Air- conditioning and Heating (AIRAH) handbook. A Chartered Professional Engineer and Chartered Engineer, he is a Fellow of the Institute of Engineers Australia (FIEAust), and a Member of the Chartered Institute of Building Services Engineers (MCIBSE). He is also on the National Engineers Register (NER), and recognised nationally.
-40 -20 0 20 40 60 80 10 12 14 16 180 2000000 50,000 100,000 150,000 200,000 250,000 300,000 350,000 400,000 450,000 500,000 550,000 Enthalpy (J/kg)
temperatures at the hot side. The DHW system must be arranged to heat the coldest water last, and produce the lowest temperature after the ‘condenser’. This is illustrated in the outer loop on the diagram in Figure 3.
What does the solution look like now? Manufacturers are using cascading heat pumps with different refrigerants to achieve a low-grade heat source, with an air-to-water heat pump feeding into a water- to-water heat exchanger to get the standard boiler temperatures. This solution is practical, and provides an improvement against resistive heating, even with a low night-time temperature. Typically, as the ambient rises, the CoP of the first stage increases vastly, thus providing a real alternative to resistive heating. We can achieve the 80/60 and thus re-use the
existing secondary systems and pipework. The CoPs will not be that great, as this would most probably be part of a cascading system, and each system would have a compressor. The total kW input would be higher than if a single stage system was practical, but this will only ever be a Band-Aid for ageing buildings. New buildings will be built around the most efficient, lower temperature heat pumps. Who needs to solve the 80/60 boiler replacement heat pump?
Potential options Looking at the available refrigerants, there are quite a few capable of operation at sub-critical cycles with a hot side at 80 °C. The number of stages and the resulting CoP aside, it is possible to do, with either a single machine or a cascade of two heat pumps. The question becomes what is the incentive for manufacturers to concentrate on this solution? There is no requirement for such a solution in new buildings, as the more efficient, lower temperature machines that can provide cooling as well would be the obvious choice. The other building components would be designed around these. So, such systems would only be for existing building stock, and indeed many such buildings would continue to use gas-fired plant for some considerable time. For many, a shutdown for a complete refurbishment – including the necessary pipework and coil upgrades in order to work
34 Health Estate Journal February 2025
with the new temperatures, is a viable option, especially if the refurbishment is undertaken alongside other substantial changes in the building, such as change of use. In addition, in the northern hemisphere many heating systems have been designed using condensing boilers operating in the 40 to 50 °C range, and such systems are well suited to a boiler / heat pump swap without much in the way of alteration. This leaves only buildings (or sites) that have a specific need to move away from gas, that equally, cannot go offline for an extended period. So, the question will be whether a large enough sector exists for manufacturers to specifically focus on? Our feeling is no.
In summary Physics means that it is not practical to remove a boiler and plug in a heat pump in its place. There is a practicality gap, and as you increase in size from systems that are measured in kW, to those measured in MW, that practicality gap becomes exponentially larger the bigger the system becomes. In a complex health environment, hot water is a key element in critical healthcare, and underperforming heat pumps can easily introduce unforeseen risks to hot water systems such as increased incidence of Legionella; this is something healthcare designers must be cognisant of when retrofitting heat pumps. In Australia, building services engineers will be working with heat pumps that are available now, towards solutions that involve multiple machines with different strengths and weaknesses, and each site will have a bespoke solution driven by the understanding of its history and the skill of the individual designer.
Acknowledgment n This article – based on a paper given at the IHEA’s
Conference in June 2024 in Melbourne, and titled ‘So, you want to install a heat pump?’, first appeared in the Spring 2024 issue of Healthcare Facilities, the magazine of the Institute of Healthcare Engineering, Australia. HEJ acknowledges the help of the author, the event organiser, Iceberg Events, and the publisher of Healthcare Facilities, Adbourne Publishing, in allowing its publication here in slightly edited form.
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Courtesy of Danfoss
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