CPD PROGRAMME | FAN COIL UNITS Filter Heating coil operation at very low speeds (to meet the smaller load) they will not operate with optimum performance. As a consequence, some of the larger FCUs may be removed during a Cat B fit-out and possibly replaced by smaller FCUs that are better suited to cater to the requirements of the updated layout. These larger FCUs could potentially find use elsewhere in the project but, in many cases, they are no longer needed. The manufacturers typically do not accept these units for return, since they have been previously installed, commissioned, and have already been in operation during the Cat A phase essentially rendering them second-hand. Contractors might choose to retain the large FCUs and store them for potential use on other projects; however, it is likely that a significant proportion end up as scrap for metal recycling. The process of removing the original FCUs not only consumes time and money but also generates additional embodied carbon and waste. At the conclusion of the lease period, new tenants may require modifications to the layout, necessitating the repositioning of existing FCUs or the acquisition of new ones to effectively manage the heating or cooling demands of the space. In certain cases, particularly when disputes between landlords and tenants lead to lease terminations, landlords might insist on restoring the building to its original condition. This typically involves the removal of the FCUs and the installation of new, larger FCUs according to the building owners original specifications. However, these larger FCUs might be considered as somewhat temporary and may be replaced when the space is leased again. This cycle further exacerbates carbon emissions and contributes to construction waste. There are several options that could help prevent this wasteful and likely repeated cycle of FCU removal and replacement, as discussed below. The first option is that the project could bypass the Cat A stage and proceed directly to the Cat B phase. However, this approach demands a high level of collaboration during the design phase, and is only feasible when the ultimate client for the building has been identified. This scenario becomes possible when, for instance, a large corporation makes the decision to both construct and occupy a building or commits to a long-term lease of the property. While this does occur, it remains uncommon and not without its challenges. The Cat B specification must be fixed relatively early in the project and, consequently, any design modifications Chilled water EPIV LTHW EPIV Cooling coil Fan 3 Fan 2 Fan 1 Spigot 3 Controller Spigot 2 Spigot 1 Figure 1: An example of a commercially available multizone FCU that is capable of supplying one, two or three different zones necessitated by shifts in the companys strategy, structure or size can introduce their own set of issues if not carefully managed. Alternatively, an option is to utilise a greater number of smaller FCUs, offering enhanced inherent flexibility and potentially decreasing the waste generated during removal and replacement that might otherwise be needed. Nevertheless, while this approach could diminish the likelihood of having to revisit the FCU strategy during the Cat B phase, it would lead to increased costs for the building owner while reducing the tenants Cat B expenses. Each smaller FCU would also involve the installation of a controller, valve set, pipework and electrical connections. Increased numbers of FCUs will also add to the complexity of structural coordination when setting out a reduced size zone (or bay). While the use of smaller FCUs can enhance the propertys appeal and make it more attractive to prospective tenants, the initial cost linked to procuring and installing additional FCUs may pose challenges when justifying this expenditure to the building owner. A more radical option, for larger multi-floor projects, is to fit-out only some MULTIZONE FCU Advances in the design of compact digitally controlled electronically commutated (EC) motordriven fans enabled the development of a single FCU housing that incorporates multiple, partitioned, independently controlled direct-drive EC fans, which supply separate zones, drawing from a common plenum of treated (cooled/heated) air (as illustrated in Figure 2). The units typically contain a multi-row cooling coil and a heating coil. The speed of the fans may be individually controlled by the integrated controller that interrogates all the separate zone temperature sensors and assigns a priority zone. This will be the zone that is furthest from its set-point and, for example, could require cooling. With the multizone FCU unit in cooling mode, the priority space receives the full design air volume owrate, with other cooling zones receiving proportionately the air owrate they need to satisfy their lower cooling requirements. Any space that requires heating at that time has its fan stopped and no air is delivered. Once the cooling demands have been satised, and if there is still a need for heating, then the cooling coil valve closes and after a purge period the cooling zones fans stop. The heating coil valve is opened and the heating zone(s) fans activate. Once the heating requirement is settled, the unit will revert to cooling mode and the cycle may then continue. The multizone unit would typically be located to serve similar thermal zones. The FCU integrated controller optimises the cooling and heating water owrates and the individual fan speeds to deliver the required zone temperature control in the most energy-efcient way. In the unit illustrated in Figure 1, the heating and cooling control valves are electronic pressure independent valves (EPIV). The EPIV provides the same function as a pressure independent control valve (PICV), as discussed in CIBSE Journal CPD Module 140; however, the EPIV achieves this with a close-coupled temperature compensated, inline ultrasonic owrate meter that provides a signal of the water owrate to the valve actuator controller. The required owrate is determined and sent to the actuator by the FCU integrated controller. This allows the valve to continuously modulate to provide the correct water owrate. An EPIV will operate at lower pressures than that required by a PICV typically from 1 to 15kPa, depending on the system peak load (compared with 20-30kPa for a typical PICV). The signals from the EPIVs and the fan speed controllers can, through the integrated FCU controller, provide information to the building energy management systems (through protocols such as BACNet) for monitoring, recording, optimisation and preventative fault diagnosis. 58 December 2023 www.cibsejournal.com CIBSE Dec 23 pp57-60 CPD Module 227.indd 58 24/11/2023 16:13