Commercial Hot Water Sizing

24/7 domestic hot water (DHW) supply is, without a doubt, a business-critical service for many commercial projects. Unfortunately, oversizing of these commercial hot water systems is a surprisingly common occurrence, leading to higher capital costs, demands for more space, more complex system builds, longer installs, and higher fuel bills for the life of the system.

As we push towards Net Zero, large scale commercial renovation of properties to address emissions across the UK is a given. Faced with inherently more complex replacement systems, correct sizing should be a core aim and a prime opportunity to address costly oversized systems that unnecessarily contribute to building emissions.

Oversized systems can typically be attributed to the use of online sizing programmes, which are often treated as a simple DIY option. The problem is that for commercial projects faced with DHW systems that have many variables and decisions on diversity, sizing programmes will typically oversize to prevent perceived hot water problems. When specifying a commercial hot water system, sizing should be based on the anticipated demand of the building (based on BS EN 12831-3). Within Part L of the Building Regulations (Conservation of fuel and power) for England & Wales is the demand that systems are not “significantly oversized,” but we would argue any oversizing will have a negative impact on the efficiency and operational costs of a DHW system. So accurate sizing is critical in terms of delivering an optimal thermal efficiency assessment.

Under Part L, the assessment of a commercial hot water system is deemed to include the heat generator and any integral storage vessel but will exclude all secondary pipework, fans, pumps, diverter valves, solenoids, actuator and supplementary storage vessels from the calculations. Despite this simplification, oversizing still occurs and this inherently comes from a lack of understanding of different types of the hot water system, how they fit in the design software and the way that fluctuating demand for hot water impacts these systems at peak.

Of course, the flip side of the problem is under-sizing a project. A system is going to be undersized if the water temperature drops to less than 45°C. That happens when there is a failure to correctly assess factors including maximum occupancy; colder incoming water during winter months; and all people wanting to use water in the peak hour. Such a failure of a hot water application it is fair to say is more unusual today as it typically only occurred with older model calorifiers.

Dynamic and Static Storage in Commercial Hot Water

When sizing a water system, the first thing to understand is the difference between a dynamic direct water heater system (with 20 minutes reheat) versus static storage (with a two hour heat up). Difficult to undersize, a dynamic water heater with high heat input and low storage will provide a 20 to 30 minute heat-up time and will not be designed to go cold. Static storage, with a calorifier, can be undersized. Designed to dump then reheat, these systems will have a small heat input, but offer a large volume store, meaning it can take up to two hours to reheat. Anytime the system draws hot water at a faster rate than can be heated to 44°C complaints are going to occur once the initial store is gone. At the opposite end of the scale, with a dynamic system, over design of the flow rate (by as much as 45%) is unlikely to cause complaints. At least not from shower using occupants!

The simplest assumptions, such as the use of pillar taps rather than mixers or designing for a high percentage of baths rather than showers can lead to oversizing. It is also important to recognize that a gas-fired water heater is not a storage vessel. Under the EN89 seasonal efficiency test an indirect tank has storage losses that should be input into SBEM calculation. However, standing losses of the water heater are already included, if this figure is entered the losses are doubled up, which will cause the hot water system to fail analysis. This in turn commonly leads to systems being unnecessarily oversized to address the “failure”.

Understanding Commercial Hot Water Peak Periods

The second core requirement for correct sizing is understanding occupancy. Determined by the number of people and the type of building, the peak period represents the amount of hot water used in a period of time. “Peak Hour” as it is often referred to can, in reality, be any length of time, from just 15 minutes to continuous for eight hours, and range from normal to intensive use. For example, a hotel might reflect a normal usage curve, with peaks of demand in the morning for occupants showering, then over lunch and dinner from the restaurant. Offices will show a lower, but continuous demand, cinemas intensive spikes.

So, sizing needs to be based on occupancy to accurately determine peak volume and peak length. This understanding and how it influences the commercial hot water system is critical when sizing and why it is so important that sizing be carried out based on experience, test data, and supported by IOP/CIBSE G regulation. Modern dynamic systems will supply demand through a combination of storage and burner power. If the peak hour has been correctly identified, then a system will supply all other demand periods without needing to be oversized.

Safely sized, and reducing unnecessary costs

Simply put, oversizing is down to a lack of good design and a tendency to err on the side of caution by including additional factors of safety. The drive to integrate greater sustainability into commercial hot water systems in the form of Solar Thermal and lower temperature Air Source Heat Pumps increases the complexity of systems and by default the chances of oversizing when using sizing programmes. From our perspective, all commercial sizing should be carried out with a 60°C supply and 10°C designed incoming cold mains temperature. These are temperatures optimised for commercial supply, storage and cleaning, as opposed to personal use (showering and bathing) which requires temperatures of 43°C.

Talk to your contractor/supplier to ensure they use or have access to specialist sizing guides which include sufficient factors of safety to not need to ever oversize. At Adveco, we have created our own in-house sizing guides and our application team can design, recommend and supply systems for all your commercial water heating projects. It is only when corners are cut, and reliance is placed on automated sizing programmes that we see projects suffer with what should be avoidable build and operating costs.