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It is well documented that the built environment is responsible for 25% of the UK鈥檚 carbon emissions and that 80% of buildings that will be occupied in 2050 already exist. This demonstrates the need for carbon emissions to be tackled in both new and existing buildings if the UK government鈥檚 target of net zero carbon by 2050 is to be met.

While there are a variety of heating, ventilation and air-conditioning (HVAC) solutions for the commercial market, this module is focused on fan-coil units (FCUs), which are frequently specified to meet the challenges of heating and cooling modern commercial buildings. FCUs are most commonly specified to improve indoor air quality, regulate temperature and provide some dehumidification.

FCUs offer tenants the flexibility to control the internal temperature of the commercial space throughout the year and help create a comfortable working environment. However, without the careful design of FCUs, fit-outs can increase carbon emissions and construction waste, and can cost extra time and money.

Learning objectives:

• Awareness of the issues that can arise when fit-outs do not include careful consideration of FCUs
• Knowledge of the functionality and types of FCUs that are available for specification
• Understanding of the considerations needed to specify the correct FCU for development fit-outs

Mismatched requirements

Once a new building is constructed, the interior needs to be completed to offer basic functional requirements. Known as category A (cat A), the first fit-out is undertaken by the building owner and includes items such as raised access flooring, finished perimeter wall coverings, suspended ceilings and toilets. From a mechanical and electrical perspective, air-conditioning is provided as FCUs.

Once the space is rented, the tenant will carry out the second fit-out, category B (cat B). This fit-out tailors the space to meet the tenant鈥檚 business needs and will include dividing the interior space to create offices, reception areas, kitchen areas and meeting rooms, along with installing all the supporting IT, audiovisual equipment and lighting infrastructure.

Following the cat B fit out, it is common for the original position of the FCUs to be misplaced. The problem occurs because of changes in layout design that impact heating and cooling requirements, creating a potential mismatch between the requirement of providing air-conditioning to an original open-plan cat A interior space and the more complex airflow requirements created after the cat B fit out. The situation requires reassessment of the M&E design, including a recalculation of the heating and cooling loads for each of the spaces (affected by the number of people in the room and any heat generated by equipment and appliances). This can lead to the installation of additional FCUs and the removal of FCUs that are no longer correctly positioned and/or oversized.

The removal of the original fan coil units takes time, costs money, generates embodied carbon and creates waste. In addition, at the end of the lease period, the new tenants may require alterations to the layout that result in the repositioning of existing or the purchasing of new FCUs. Cue more waste, more cost and more carbon emissions in a process that can repeat itself each time a lease is changed.

The drive to reuse, reduce or eliminate waste sent to landfill and the growing call to take a circular economy approach to construction will only intensify interest in finding new ways to eliminate waste. The cat A to cat B fit-out process presents an opportunity to do just that, by carefully considering the most appropriate FCUs.

Functionality

To design effective FCUs, designers must be aware of the functionality and types of FCUs that are available for specification.

A FCU can condition the air within a building by using a water-to-air heat exchanger and a fan. The coil of the heat exchanger can have low-temperature hot water or chilled water flowing through it. This enables the air drawn through the heat exchanger by the fan to either heat or cool a room. Each FCU will require ductwork to supply air. The size of the space that an FCU can condition will depend on its size, with larger units being able to cover around a 6m or 9m bay and smaller ones supplying a 3m or 4m bay.

FCUs are available as either a two-pipe or a four-pipe system:

• A two-pipe fan-coil system contains a single water coil unit connected to two pipes and takes its supply from a single source. One pipe supplies water to the coil and returns the water once it has passed through the coil. The source of the water determines the mode of the FCU. It can be either cold water supplied by a chiller unit or warm water from a boiler/renewable technology. The two-pipe system can also contain a changeover sensor to determine the mode of the system 鈥� either heating or cooling. It cannot heat and cool simultaneously. Two-pipe systems are usually more widely used in the residential market, although they can also be specified for some commercial applications. The requirement to be in one mode or the other, however, can be challenging for the UK climate.
• The four-pipe system has two separate cooling and heating water coils. Each coil has its own set of supply and return pipes, meaning that no changeover sensor is required as the system has both warm and chilled water available. It can therefore provide either heating or cooling, according to demand, and is suited for spaces where there is likely to be a requirement for both within the building.

When specifying FCUs the size of the bay they need to condition, and the heat loss or gain required, will play a part. At the cat A fit-out stage, extensive open-plan areas are common and require the specification of larger, more powerful FCUs to be able to supply the volume of conditioned air required.

Around the perimeter of the building, FCUs are sized to meet heat loss and heat gains through the glazing as well as small gains from powered equipment, lighting and people. In central areas where there is minimal heat loss, FCUs can be specified as cooling only.

Configuration

There are two types of configuration for FCUs: airside and waterside.

Airside FCUs use dampers on the back of the control unit to direct the air through a cooling or heating coil or a bypass section. There is no water control as the water runs freely through the coil, but the air is diverted. Frequently used between 1990 and 2000 (particularly in London), these FCUs are still on the market but, because of energy efficiency issues, are rarely specified.

Waterside FCUs use modulating water flow control valves to control the water flow to the cooling coil, the heating coil or neither. This type of unit has a constant air flow. If the hot and chilled water supply is off, the unit still circulates air. Waterside units are highly energy efficient due to their EC modulating fans, 2 port valves and variable flow water system. They are used on the vast majority of FCU systems.

Specifying for the space

The design of FCUs will be informed by the physical space available, alongside aesthetics, project limitations, design requirements and the purpose of the building. The behaviour of the tenants and the FCU control strategy adopted will also need to be considered.

The four main FCU subtypes available for specification have similar performance levels and can supply the air-conditioning required as either heating and cooling or cooling only. There is also a fifth type, comprising small hybrid FCUs known as cassette units.

Design styles and their benefits for specific building types include:

• Horizontal concealed: These FCUs are compact and are hidden from view in the void behind suspended ceilings. The ceiling and insulation provide sound buffering, allowing these units to work slightly harder without compromising acoustic comfort. They are commonly specified in commercial projects.
• Horizontal exposed: These FCUs are left in open view, with all parts visible including controls and ducting. They tend to be specified when a client is seeking an industrial look for its building. Due their exposed nature, both aesthetics and acoustic performance of the FCUs become more important.
• Underfloor: Many commercial buildings have raised floors, so the void underneath can be used for services and the installation of underfloor FCUs. This helps create additional space and promotes build efficiency. A perimeter underfloor solution, along with displacement ventilation, can help combat heat gain in buildings with large, glazed areas.
• Vertical concealed or exposed: Often used for building refurbishment projects where low ceiling heights can restrict the space available, these FCUs can be positioned high up or low down on a wall and are usually relatively quick to install. However, they do limit future changes in the buildings. For example, if an open-plan floor was to be segmented into cellular offices, not all office spaces would have heating or cooling service.
• Cassette units: These are smaller hybrid fan-coil units. To overcome the issue of original cat A FCUs being unsuitable for conditioning smaller spaces, more cassette-style units can be specified instead. However, each unit requires pipework and electrical connections, increasing the connection time and resources required as well as the carbon footprint of the fit-out. FCUs generally run at 50% of their maximum output, whereas cassette units run closer to the output limit during normal operation. This makes FCUs better placed to cope with fluctuations in demand and have spare capacity should additional spaces require conditioning. Typically, one horizontal concealed FCU can provide the same service as three cassette units.

Considerations

Four main considerations will inform the specification of the most suitable distribution system. The design should be discussed with a specialist, including reasons for system selection, size and integration with other components. This is particularly important for projects that combine energy sources or emitters.

Designers, specifiers and contractors should consider the following when designing effective FCUs:

• HVAC requirement. Will the FCU deliver space heating, comfort cooling or both?
• Application. Spatial limitations, the function of the building or design objectives can be important drivers for FCU selection, and here the flexibility of FCUs is pivotal.
• 黑洞社区 type. This is more pertinent in refurbishments and retrofits, but some buildings will be better suited to accommodate new pipework or to adapt to the current pipework.
• Aesthetic and space design. As mentioned previously, for best results, the system design and the choice and integration of FCUs should be holistically integrated with the building and its services.

It is clear that selecting the most appropriate FCUs for a project depends on the space available for their installation and the conditioning they need to deliver. Once the unit is sized appropriately and any requirement to integrate controls is considered, a suitable cat A specification can be created. However, the key for M&E design is factoring in what happens when the building is leased.

Reducing waste

There are four options that could reduce the waste associated with the cat A to cat B cycle.

1. Go straight to cat B design
   This requires additional co-operation at the design stage and is only viable if the final client for the building is known. It may be possible where a large corporation decides to construct, own and use a building, or guarantees they will move in on a long-term lease. Although this happens, it is rare and not without problems, as the cat B specification needs to be set in stone early in the project.
2. Use smaller FCUs
   The use of more, smaller FCUs, instead of fewer, larger models is an option that would reduce waste. However, this increases the cost to the building owner as every extra FCU needs a controller, a valve set, pipework and an electrical connection on top of the cost of the unit itself.
3. Use adaptable larger FCUs
   There are larger FCUs designed to be compartmentalised to service the requirements of up to four separate spaces. This is facilitated by ducting running from the adaptable FCU into the spaces to be conditioned, with each room having separate controls to ensure that the FCU meets the individual needs of each space. This arrangement can satisfy the needs of the open-plan cat A fit-out and be adapted to meet the more complex requirements of a cat B fit-out.
   
   The extra ducting required for adaptable FCUs is sometimes anticipated to be an acoustic issue, with the potential for talking from one room to be heard in another. However, there is little risk of this happening, as demonstrated by acoustic testing of the systems. The only other additional design element that may need to be considered when using adaptable FCUs is fitting a separate small duct to supply fresh air. This is required if the central FCU turns off the fan in an area when the required condition has been reached to ensure that a supply of fresh air is always available.
   
   This design option should not be confused with a larger FCU that has more than one duct running into separate rooms but provides only one control. All the rooms get the same conditioning regardless of their requirements. This can lead to either overheating or over-cooling rooms that have no control over the air temperature from the FCU.
4. Adoption of a 鈥渉alfway house鈥� strategy
   On large-scale projects, one option that could be considered is to fit out and market only a limited number of cat A floors. The idea is that other floors could then go straight to cat B once tenants are secured. This reduces the potential for waste generation; however, it does sacrifice the economies of scale that are possible when fitting out all the floors to cat A at one time.

Embodied and operational

Reducing waste will have a significant impact on the sustainability of a development fit-out. However, it is important to remember that the whole-life carbon footprint of a project is made up of embodied carbon and operational carbon (which starts from the sourcing of a product鈥檚 materials to the end of the product鈥檚 life).

While specifying FCUs that remain in place throughout both fit-outs prevents additional embodied carbon being added to the building鈥檚 carbon footprint, the sourcing and manufacturing process of those products should also be considered.

A method of measuring the contribution of building services to embodied carbon is provided by the Chartered Institution of 黑洞社区 Services Engineers (CIBSE) in its TM65 methodology. Manufacturers of FCUs have the option of providing key information requested by TM65 to ensure that the embodied carbon contribution of their product, including any recycled content, is accounted for within the overall building services calculation.

Additionally, specifying FCUs with improved efficiencies and lower power consumption can have a positive effect on reducing the operational carbon of the building. Choosing product manufacturers that have achieved an environmental management system certification such as ISO 14001 can help realise additional sustainability benefits in schemes such as BREEAM, LEED and NABERS, when measuring the responsible sourcing of materials.

Control valves

The design of an FCU鈥檚 control valve can also lower operational carbon. The heating and cooling loads supplied by the FCUs are controlled by varying the water flow rate. Controlling the flow rates at each FCU will affect the total flow rate of the system. The better the valves can control flow at the FCUs, the lower the pressure needed to keep the system operating.

Mechanical pressure-independent valves, or PICVs, are commonly used within the industry and comprise an automatic flow regulator in the form of a flow cartridge that maintains a certain flow rate based on the cooling or heating load required. However, the PICV can suffer from an effect known as hysteresis. This is where the accuracy at which the flow rate setting is maintained depends on whether the pressure differential across the valve is rising or falling and can lead to two different flow rate readings being obtained.

However, an electronic pressure-independent control valve, or ePIV, can enhance the control and efficiency of fan-coil unit systems by encompassing an ultrasonic flow rate meter that calculates the flow travelling through the pipe and a two-port control valve with an actuator. This allows the valve to continuously modulate to provide the correct water flow rate, and, unlike the PICV, it does not suffer from hysteresis. The EPIV can also operate at much lower pressures than the PICV 鈥� typically from 15kPa to 1kPa, depending on system load. The accurate flow meter output can be fed into the building control system, reducing the system pressure requirement and, in turn, reducing the load and energy consumption of the pump.

Designing fan-coil solutions using Ability鈥檚 Matrix/Multiroom approach allows control of the conditioning of several rooms from one single FCU. Designed to reduce the cost and time of on-site installation, each fan in the FCU can be preconfigured to its own duct and set to specific air volume and water flow rates using an ePIV valve. Two versions of Matrix and Multiroom are available: with one being ideal for residential developments with restricted ceiling height, and the other suitable for residential, commercial and mixed-use developments.

Final thoughts

The cat A to cat B fit-out cycle can significantly alter heating and cooling requirements. This results in a wasteful process of removing the originally specified FCUs to replace them with different FCUs better suited to the changed layout. For a cat A fit-out to reduce long-term wastage, it is necessary as far as possible to build in flexibility when choosing FCUs to overcome the changes that will occur in the building. Early engagement with your chosen supplier during the initial design stages is strongly advised.

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