Sizing Up Hot Water Storage Systems for Commercial Applications

When specifying a hot water storage system there will be several considerations to make, but sizing will be key to ensuring a building’s demand for hot water is met. George Linder, Product Manager of Cylinders at Heatrae Sadia, explains how to size unvented hot water storage systems according to the application and usage of commercial buildings.

Generally, an unvented hot water heating system is fed directly from the cold water mains, delivering pressurised hot water to all outlets. With a wide range of types, hot water capacities and means of heating available, these systems are not restricted to domestic applications and can be suitable for other installations and applications, including those in the commercial space. For example, where there is continuous or substantial demand for instant hot water without loss in performance, such as in hotels, leisure centres, office blocks and shopping centres, unvented hot water heating systems can provide large scale, commercial capacity and fast flow rates to satisfy even the highest demands with fast heat recovery times.

Striking a balance

In a domestic application, the sizing of a hot water cylinder generally comes down to the number of occupants in a property, how many bedrooms and bathrooms there are, and the personal habits and lifestyles of individual occupants. In a commercial building though, the calculation becomes less about the size of the property and more about peak demand and hot water usage patterns, as understanding hot water usage on an individual-by-individual basis can be a near-on impossible task.

There is a careful balancing act to consider – under-sizing the hot water storage system will mean there is not enough hot water for the building’s occupants while oversizing it will mean a vast amount of energy is wasted. If sized correctly, the system should meet the needs of the building without its occupants ever having to wait for it to become operational.

Predicting peak demand

With all schemes, it is important to be able deliver sufficient hot water during a building’s busiest period. These periods of peak demand will differ depending on the building at hand but establishing the natures of the scheme will help to determine likely hot water usage patterns and peak periods. Different sectors have different demand patterns. For example, domestic homes will have a completely different usage pattern to the likes of schools, offices and hotels.

A school, for example, has set opening and closing hours and its pattern of hot water usage is likely to be fairly regular and continuous without noticeable peaks in demand. However, if a school has catering facilities to feed pupils at particular periods within the day and showering facilities for sports, this will likely determine peak demand. Offices, on the other hand, tend to have fairly continual usage so point of use style water heaters are often an effective solution.

Other commercial use buildings have a very high hot water usage, particularly at certain parts of the day. For example, a hotel is open 24 hours a day, seven days a week, and will often experience extremely high hot water usage over a period of around two hours in the morning and in the evening when guests are using showering facilities and on-site kitchen facilities are in use. In these instances, centrally located large capacity hot water stores tend to work well and applications can be met by unvented water heaters with capacities of up to several thousand litres and high recovery heat inputs.

Sizing and recovery

In general, there are some key factors that need to be established to help with the sizing of hot water generators effectively in commercial buildings. These include: the number of sanitary outlets and items such as kitchen appliances; their respective flow rates or cyclic consumption; how many busy peak periods are likely; the duration of each peak period and the time in between each spike; the number of occupants.

Sizing and recovery go hand-in-hand. Only once you have assessed the maximum demand, frequency of delivery and the recovery time necessary, can you determine the size of the vessel required to meet the end-user’s hot water needs.

Recovery requirements will usually dictate the heating power required but this can also be dependent on the fuel type used and the output availability. This is especially significant where electricity is the primary heat source and the maximum power available is often limited.

Final thoughts

While it can be easy to oversize hot water storage systems, particularly for large-scale commercial applications, for commercial buildings which are already faced with large running costs for the building’s services, making savings on hot water usage can go a long way. Therefore, understanding a building’s hot water usage patterns as closely as possible will mean that demand is met effectively and efficiently.

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