Sizing up to the job: What to consider when sizing a DHW system

Sizing up to the job: What to consider when sizing a DHW system

By Kevin Potter, National Sales Manager – Commercial, Hamworthy Heating

Sizing up to the job: What to consider when sizing a DHW system

When specifying a commercial hot water system, sizing should be based on the anticipated demand of the building.  Within Part L of the Building Regulations (Conservation of fuel and power) for England & Wales is the demand that systems are not “significantly oversized”. However, despite this guidance, oversizing still occurs mainly from a lack of understanding of the different types of systems and how peak demand for hot water impacts these systems.

To correctly size a domestic hot water (DHW) system, it’s important to ensure that a building’s demand for hot water is adequately met but it’s a careful balancing act to juggle. Undersize and you could end up with frustrated building occupants.  Oversize and you could be faced with a reduction in operational efficiency, increased energy waste with longer heat-up periods due to extra storage cylinders being heated, and safety issues, including the risk of Legionella.

Sizing up for the job

To be forewarned is to be forearmed so the best starting point is to gather as much information about the building and its usage as possible. For new build, knowing what the client requirements of the building is will ensure you account for everything such as quality and location of DHW outlets, intended usage, number of users, flow and challenges to be identified. Just as importantly, for refurbishment projects, a site survey of the existing building will highlight any issues with current arrangements such as non-compliance issues or why certain solutions would fit better than others.

Hot water sizing is not an exact science and often requires some guesswork based on assumptions as well as historical and projected usage figures. Asking all the right questions and understanding the priorities of the client from the outset can help get the most accurate picture to work from. Once a demand profile is established you can review this against the type of hot water heaters that best suit the project, taking into account building and budgetary constraints.

Simplifying product selection

First and foremost, when it comes to specifying a hot water system, the chosen system will vary according to the individual application; for instance, where the DHW peak demand must be met in a short period of time, a larger storage capacity is the primary concern.  In comparison, for applications where DHW demand is relatively constant with little storage requirement required, recovery would be the primary consideration.  However, when it comes to balanced loads, where DHW can vary, storage and recovery are equally as important, with sizing focused on demand per hour while making an allowance for simultaneous demands.

Size matters

When it comes to accurately sizing a commercial hot water system, it’s less about determining the size of the actual property itself and more about understanding its peak demand and continuous hot water flow requirements.

For instance, a relatively simple example would be an industrial process requiring a specific amount of hot water, in a specified time, at a specified temperature. All that is required is the lowest cold water supply temperature and then the heater(s) output can be directly related to the amount of hot water required.  If the load is continuous, the heater or heaters, must be sized to cope with the full amount.  However, if the load is intermittent, consideration can be given to a smaller heater installed in conjunction with a suitable sized storage tank.

In comparison, a slightly more demanding example would be a medium-sized hotel, where a known number of people will use baths or showers at a known time, typically over a two hour peak load time, say between 7 am-9 am. Basing the average occupancy of one and a half people per room (unless specified as single rooms), for medium-sized hotels holding between 100-200 guests, it would be advisable to allow 25-35 litres of hot water per person over this two-hour peak period. An example of this would be Drayton Manor Hotel in Staffordshire which features 150 bedrooms. Accommodating a large number of guests at any one time, the installation required an efficient system with both good recovery and storage.  To meet this need, Hamworthy Dorchester water heaters selected as part of the system used to effectively manage the heavy demands placed by guests on the hotel’s hot water and heating system.

Sizing up to the job: What to consider when sizing a DHW system
Dorchester DR-SG

Direct fired water heaters, such as the newly acclaimed Dorchester DR-SG provide an efficient and cost-effective means of delivering large volumes of hygienic water around buildings that have peak demands.  Suitable for both renovation and new build projects, for commercial applications the Dorchester DR-SG meets all the latest Building Regulations and offers significantly improved efficiency and performance, particularly under challenging water conditions.

For applications where hot water demand can be difficult to predict, a balance may need to be struck between what is the maximum expected demand that the system must satisfy and for how long it must be sustained at a given temperature. This will ensure that the water heater can cope with the peak demand and the remaining demand will be adequately catered for.  However, it is worth remembering that the water heater would not normally be sized on all outlet appliances all running at the same time and at their maximum continuous flow rates for a time longer than their peak period duration, as this would result in gross oversizing of heaters.

Having established the number of appliances, usage and quantity of water required during an initial site visit, the output of the heaters must be related to the hot water storage temperature. Any decrease in the cold-water supply or increase in hot water storage temperatures will result in decreased output from the heater. However, various factors need to be considered to determine appropriate storage capacity requirements for the application, and how much, if any, additional storage may be required. These include general consumption throughout the day, recovery times, over how many hours the peak load is spread and whether a larger buffer than the water’s own storage is required to guard against the possibility of high flow rates at peak times.

Reducing operational costs

In terms of cost, sizing true to demand helps to ensure a system meets demand in the most cost -effective means.  By only providing the volume of hot water that is needed, building users can avoid unnecessary high energy costs resulting from heating water that is not used.

Factoring in future needs

Is also worth bearing in mind that existing behavioural patterns change. Similarly, a new building may not be operating at full capacity from the outset. As a result, future needs are an important consideration and, where practical, a system should be sized to provide for anticipated growth and the resulting increase in water demands.

A fitting solution!

Last but not least, let’s not forget the most obvious sizing consideration – will the system actually fit? Water heating systems can be heavy and large which can impact transportation, installation or access.  However, this can be easily avoided by closely checking product specifications for instance, the Dorchester DR-SG stainless steel condensing boiler from Hamworthy, which features a narrow diameter of <80cm, has been specifically designed to fit through a standard 800mm doorway.  Incorporating a lightweight and slim design, the Dorchester DR-SG also offers easy access for service and maintenance intervals.


Regardless of whether it’s a hotel, sports centre or small school, any building that depends on hot water needs the correct water heating sizing and setup to satisfy the envisaged demand requirements safely and efficiently.

Furthermore, with heating (including hot water generation) in buildings responsible for 23% of total carbon emissions in the UK, correct sizing should be a core aim and a prime opportunity to address costly oversized systems that unnecessarily contribute to these emissions, as we journey towards Net Zero.

Hamworthy Heating is a trusted British commercial heating manufacturer.

Tel: 01202 662 552