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Hot water is required for both HVAC and domestic needs in health-care settings. On the HVAC side, providing hot water systems for critical health-care and comfort functions is required per such guidelines as “Facilities Guidelines Institutes (FGI) Guidelines for Design and Construction for Hospitals and Outpatient Facilities.” FGI currently references ASHRAE Standard 170: Ventilation of Health-Care Facilities, which mandates certain temperature and humidity ranges based on space type.
Whether the design team is looking to satisfy temperature requirements by providing hot water to a variable volume box for space heating or providing heat for dehumidification needs in an operating room, heating water plays a pivotal role as part of the overall HVAC system. In addition, on the plumbing side, domestic hot water (DHW) is required for such functions as plumbing fixtures throughout the facility as well as the kitchen area.
Due to the critical nature of hot water systems serving HVAC and plumbing needs, it is essential to provide water heating systems that are energy-efficient, sustainable and safe for the patients throughout the facility’s life.
In the quest for energy-efficient and sustainable solutions on the HVAC side, condensing boilers have emerged as powerful contenders. This technology has gained significant popularity in commercial buildings where heating demands are high and energy consumption must be optimized.
Whether the design team uses a variable primary or primary secondary system approach with condensing boilers providing hot water to the facility’s HVAC system, it is advantageous to have this system also provide heating requirements to the plumbing system. This would be via such equipment as plate-frame heat exchangers in lieu of the standard use of tank-style water heaters.
By combining the advantages of HVAC system via condensing boilers and plate-and-frame heat exchangers providing heat to the plumbing system, commercial buildings can enjoy enhanced efficiency and reduced energy costs, while also providing a safer environment for all patients by mitigating bacteria due to limiting or killing Legionella concern.
System Equipment: Condensing Boilers
Condensing boilers have transformed the heating industry by efficiently recovering heat from flue gases that would otherwise be wasted. Unlike traditional boilers that release hot gases into the atmosphere, condensing boilers use a secondary heat exchanger to extract and harness this valuable heat. This innovative configuration enables condensing boilers to achieve exceptional efficiency levels of up to 98%, resulting in significant energy savings and a reduced environmental impact (see Figure 1).
Moreover, condensing boilers stand out not only for their efficiency but for their compactness and modularity, making them an ideal choice for health-care buildings. Compared to traditional boilers, condensing boilers occupy less space and can be installed as multiple units, providing adaptability to varying heating requirements. This modular design optimizes space usage and allows for easy system adjustment or expansion, enabling health-care facilities to meet evolving demands without extensive modifications.
Additionally, their compact design addresses the space constraints commonly encountered in health-care buildings, optimizing the use of valuable real estate. With their outstanding performance and versatility, condensing boilers offer a reliable and efficient heating solution aligning with the sustainability goals of modern health-care facilities.
System Equipment: Plate-and-Frame Heat Exchangers
Plate-and-frame heat exchangers are widely employed in commercial buildings to facilitate efficient heat transfer between fluids, such as water or refrigerants. These heat exchangers boast a large surface area, significantly enhancing heat transfer efficiency. The intricate patterns and corrugations of the plates further increase the available surface area, and promote turbulent flow, disrupting the boundary layer and ensuring effective heat transfer even with lower flows and temperature differences (see Figure 2).
The net result is better thermal contact and further enhances heat transfer efficiency. Additionally, the modular nature of plate-and-frame heat exchangers allows for easy customization and adaptation to specific heating requirements. Depending on the configuration, additional plates can be added or removed to adjust the heat transfer surface area, accommodating varying heating demands with ease (see Figure 3).
The Synergy
The integration of condensing boilers and plate-and-frame heat exchangers creates a formidable synergy, maximizing energy efficiency while reducing equipment costs. This powerful combination offers significant first cost reduction, reduced maintenance and improved heating performance for continuously operating facilities such as hospitals.
Within this integrated setup, the condensing boiler functions as the primary heat source for the mechanical hot water loop, while the plate-and-frame heat exchanger facilitates heat transfer between the mechanical hot water loop and the DHW loop. This innovative arrangement ensures efficient heat transfer without mixing the two water streams. As a result, the need for a separate DHW heater or boiler can be eliminated, leading to substantial cost savings in the project budget.
Proper system design is crucial to ensure optimal heat transfer to the DHW system and prevent potential issues such as low-temperature domestic water and the growth of Legionella bacteria. Legionella growth is a critical concern as it directly impacts the health of both facility staff and patients.
Legionella thrives in temperatures between 77 F and 113 F, stagnant water and the absence of direct sunlight. Given potential hot water to become aerosolized in showers, Legionella growth poses serious health risks to the building occupants.
Mitigating the risk of Legionella entails implementing regular maintenance, conducting a thorough cleaning of piping systems, periodic flushing, and occasional long-term building shutdowns. These measures effectively eliminate biofilm, organic debris and materials prone to corrosion from the water system. Traditional DHW systems with storage tanks can provide an optimal breeding ground for Legionella bacteria if not properly operated and maintained.
However, the synergy between condensing boilers and plate-and-frame heat exchangers operating at the right temperatures eliminates the need for a DHW storage tank. This integration ensures a continuous flow within the domestic piping system, preventing stagnant water and significantly reducing the risk of Legionella contamination.
The design temperature of the mechanical water heating system also plays a critical role in Legionella prevention. Scientific studies indicate that maintaining a hot water supply temperature above 140 F can inhibit the growth of Legionella, while temperatures exceeding 158 F can effectively eradicate the bacteria.
Therefore, it is highly recommended to set the water supply temperature of the condensing boiler within the range of 150 F to 180 F in the HVAC heating system. This ensures that the temperature of the DHW system reaches the desired levels to control or eliminate Legionella growth effectively.
By harnessing the synergy between condensing boilers and plate-and-frame heat exchangers, commercial buildings can achieve optimal energy efficiency, lower equipment and operating costs and mitigate the risks associated with Legionella growth. This integrated solution not only enhances heating performance but also ensures a safe and healthy environment for building occupants.
Huiyuan Zhang, PE, is a mechanical engineer in SmithGroup’s Phoenix office. With more than nine years of experience, he specializes in designing HVAC systems for a wide range of building types.
Eric Martin, a mechanical engineer in SmithGroup’s Phoenix office, brings more than 15 years of experience in HVAC system design for various building types and plays a prominent role as a health-care engineering leader within SmithGroup.