Down to the shed
Maintaining a comfortable environment for shoppers in a retail ‘shed’ can be challenging. Ian Dagley explains how a flexible approach helps to address these challenges
Compared to traditional retail buildings, ‘shed’ type outlets clearly require a different approach to the design of HVAC systems. Ensuring thermal comfort with good indoor air quality (IAQ) is a common factor for all types of retail premises, for the sake of the customers and, very often, for keeping the merchandise in good condition.
Moreover, retail environments tend to change on a regular basis, so that ventilation requirements in certain spaces also change as aisles and merchandise are modified. A level of flexibility in the design of the system to make re-configuration straightforward with minimal disruption is therefore also desirable.
Of course, these same issues also apply to other types of ‘shed’ such as warehouses and factories. The activities in the space may be different but the high air volumes and high ceilings pose similar challenges – as does the tendency for the layout of the space to change, though perhaps not as frequently as in a retail outlet.
Also, the owners of the building may want to keep their options open, in terms of how the building may be used in the future. Today’s cash & carry may be tomorrow’s exhibition space or art gallery and landlords don’t want to be forking out for a new HVAC system every time the building’s usage changes.
Thus, when designing these systems building service engineers need to include some inherent adaptability by choosing a system that meets all of these requirements. This may be in the form of centralised air handling plant serving a network of ducts and diffusers, decentralised ventilation units operating independently of each other or an integrated system that combines several units, each configured to the space it is serving. The most appropriate choice will depend on the nature and layout of the space.
Centralised systems tend to be quite inflexible and typically lack adaptability without considerable re-configuration of ductwork and diffuser locations. This is disruptive and adds to cost, to the extent that building owners may choose not to alter their ventilation, so that the system is then no longer optimised for the layout of the space.
As a result, decentralised ventilation systems, or ‘hybrid’ systems that combine the characteristics of both centralised and decentralised systems, are increasingly popular because of their versatility in meeting a wide range of requirements.
Unlike central air handling units, decentralised systems normally work without supply and extract ductwork. This makes design easier and reduces installation costs. Nowadays, installation is very simple, using pre-wired units that are easy to change at a later date.
With decentralised ventilation systems it is easy to provide each space – or each zone within a large space - with its own single ventilation unit. Each unit can then be controlled independently of other units in other spaces/zones to address different activities, variable occupancy patterns and changes in usage. The ability to adjust ventilation rates to the actual demand within each space (demand-controlled ventilation) minimises running costs without compromising on comfort.
The latest designs of decentralised ventilation units also incorporate air injectors that distribute supply air at different temperatures to avoid draughts and ensure that the ventilation for each space within the building is optimised. The air distribution pattern can be changed automatically and is infinitely variably between vertical and horizontal, thereby delivering the desired adaptability mentioned earlier. Also, the air injectors give very effective air distribution, allowing a lower air volume to be used, resulting in a further reduction in investment.
A further benefit of air injectors is their ability to minimise thermal stratification, which can be very wasteful in high-ceilinged buildings. Well-designed vortex air distributors enable easy adjustment of the air stream range from 4m to 25m and regulate the scatter angle (induction) of the air stream as a function of the mounting height. Temperature stratification is thereby limited to 0.15K per mounting height metre (K/m) – compared with up to 1.0 K/m in conventional systems.
Additionally, this decentralised ‘island solutions’ approach ensures that there is no contamination of one zone by another, which can be an issue with central plant serving ductwork distribution systems – albeit more in industrial applications than retail. For large installations this also facilitates phased investment to spread the capital costs.
Lifecycle costs are also an issue and one key element in lowering running costs is heat recovery, where much of the heat energy in the extract air is recovered and transferred to the incoming supply air via a plate or rotary heat exchanger. In such cases, it is important that the system works automatically and is ‘user-friendly’.
We have seen paybacks as low as two years for these kinds of installation, due to lower running costs – which then contribute to increase profits through the remaining life of the plant.
As noted earlier, each space will be different in various ways and require a tailored HVAC solution. Another option is a modular, scalable approach that brings together the features of more traditional designs to provide both space heating and ventilation.
This ‘system’ approach is based on a three-module concept, comprising a heat source (e.g. condensing boiler, heat pump), ventilation units providing recirculation or air changes, and a control system. If heat pumps are the approach on the project instead of gas boilers, then decentralised units can be combined with reverse cycle air source heat pumps to provide heating or cooling. In some projects it may also be useful to interlink with existing heating or cooling solutions from different manufacturers.
The system approach may also include zone control which, in conjunction with temperature- and time-based zone control, can optimally adapt the heating times and space temperatures to logistical and energy-related requirements.
In all cases, it is particularly important to take account of installation and maintenance factors at the design stage of the project. This is another reason for the growing popularity of roof-mounted decentralised units, as they offer several advantages in this respect. The fact that they are installed at high level, or on the roof, means that installation has minimal impact on the activities below and the units are easily accessible from the roof for servicing and filter replacement.
A further benefit is that they do not occupy any valuable floor space, so they can be positioned optimally without conflicting with the layout of the area below.
Whether for retail, events, manufacturing or logistics, the different uses and requirements of shed type buildings are most easily met with a system based on decentralised units, operating on a standalone basis or as part of an integrated system. The adaptability of this approach not only makes it straightforward to meet current requirements but also helps the building to adapt to changing usage in the future.
Ian Dagley is general manager of Hoval