Chilled water (CHW) cassettes are increasingly being seen as the go to solution for comfort cooling thanks to their simplicity, flexibility and ease of installation. However, these benefits come with some trade-offs, so Troy Chambers, Head of National Sales at Ability by Dimplex, discusses what you need to know before specifying a chilled water cassette.
CHW cassettes are, in essence, a type of fan coil unit (FCU) designed specifically for ceiling mounting. They are available with a two-pipe or four-pipe configuration, so can be used for a choice of heating, cooling, or heating and cooling, in some cases with the option of cooling and an electric heating element. Where they differ from a standard FCU is that a CHW cassette is a single self-contained unit. There’s no ducting, pipework or air terminal devices. This provides some obvious advantages, but also some limitations worth considering.
Which applications are CHW cassettes best suited to?
The major advantage of a CHW cassette is its ‘all in one’, compact design. They offer quiet and effective heating and cooling for projects where space is limited, ductwork cannot be installed or where smaller heating or cooling loads are required.
They are ideally suited for use in commercial buildings, such as reception areas and cellular offices; leisure and retail units including shop floors, gyms, restaurants and back of house areas; medical facilities including hospital waiting rooms and laboratories; and residential back of house and amenity spaces.
Chilled water cassettes are usually mounted concealed, above a floating ceiling, although some manufacturers offer models designed for architectural integration that can be exposed to view.
As well as being an advantage, the lack of ducting can become a design limitation. CHW cassettes must be positioned centrally within the room. This compromises aesthetics and, because a floating ceiling is usually required, affects head height, so CHW cassettes are unsuitable for most residential applications within dwellings themselves.
Another limitation is the position of the air diffusion grille within the unit, which can result in cold spots because air movement is limited when compared to a traditional FCU. The type of grille can also further exacerbate cold spots. A central swirl diffuser blows air downwards, which can be quite uncomfortable for anyone directly underneath. A better option is four adjustable louvres positioned along each side of the unit. These louvres can be adjusted to ensure the air blows out horizontally above people’s heads to help reduce cold spots. Despite this, CHW cassettes are not generally considered the best solution for open plan offices due to the uneven, ‘breezy’ distribution of chilled air.
How chilled water cassettes contribute to sustainability goals
Currently, CHW cassettes need careful consideration when it comes to projects that prioritise sustainability. The Future Homes and Buildings Standard effectively mandates that all new non-domestic buildings are built using low carbon heating systems, such as heat pumps. This can present a problem because CHWCs are currently designed for high-temperature systems and cannot be adapted to accommodate low temperatures. As a result, they may be less efficient when working with heat pump systems or very low temperature hot water systems.
When it comes to embodied carbon, as a rule of thumb, the greater the weight of the unit and the more components required, the higher the levels of embodied carbon. CHW cassettes are small, feature one fan coil and have no ducting, so if just one unit is required, it is likely to have lower levels of embodied carbon than a comparable FCU. However, if more than one unit is required, a multi-room FCU may be a better choice because it only requires one fan to cool or heat upto four or five spaces.
It’s also important to consider where the units are made. Most CHW cassettes are manufactured in Europe, which dramatically increases transport emissions when compared to UK manufactured FCUs. Location also has an impact on lead times. Many Spanish and Italian manufacturers operate plant shutdowns for two to three weeks in late July or August to allow for maintenance and employee holidays. This can significantly impact lead times and should be factored in during the specification process to ensure that shutdowns do not impact the build schedule.
Chilled water cassettes remain an effective and practical solution for many commercial comfort cooling applications, but they are not a one-size-fits-all option. Achieving the best results requires specifiers to look beyond the headline benefits of compactness and ease of installation, and carefully consider factors such as air distribution, occupant comfort, compatibility with low-temperature heating systems and embodied carbon.
By taking a holistic approach to specification and balancing performance, sustainability, and installation requirements, specifiers can determine whether a chilled water cassette is the right solution for the project or whether a fan coil unit system or a combination of the two would be more appropriate. Ultimately, successful specification is about selecting the right product for the application, rather than the most convenient one.