Water heating


Size matters in commercial systems


What causes oversizing and how can we improve accuracy in sizing domestic hot water systems. Jason Hartigan, senior business development manager ACV UK investigates


T


he sizing of domestic commercial water systems (DCWS) is a key responsibility for designers, yet research has shown that


common standard sizing guides can be misleading and can result in systems being oversized. Engineers use the current published sizing


standards from official bodies. But these guides – including the CIBSE Guide G:2014 Public Health Engineering, the IOP Plumbing Services Design Guide, British Standards BS8558:2015, and BS EN 806- 3:2006 – each give different information based on demand or load units (LUs), and each steer towards a static system design. Research has shown that this static approach ultimately leads to overestimating demand and the inefficiencies that accompany it. It’s vital to size hot water systems correctly to meet demand in the most effective way. We have to be able to respond to shifting demand – as population levels rise, and behaviour changes along with advances in technology – and the increasing scarcity of water and other precious resources. Correctly sized systems will run more efficiently, resulting in reduced heat losses, reduced energy usages and a lower carbon footprint.


No flexibility


The most commonly used guides cover sizing in a number of ways and rely on loading units (LUs). This assigns a discharge value to plumbing fixture outlets compared to a base appliance of 1 LU. Using tables of data, design guides - such as the IOP and BS8558:2015 – provide base data for simultaneous demand for a base appliance (e.g. a hand basin with a frequency of 1.2L/ second) with all other appliances having a higher value. After gauging frequency of use, and taking into consideration pressure at the inlet to the building to ensure velocity and to guard against pressure loss, the optimum pipe size can be calculated.


Oversizing


Firstly, the method doesn’t take into consideration the significant improvements in building design, insulation, and heat efficiency that have been made since creation of the model in the 1960s. The model is ignores seasonal changes in building usage, for example a UK hotel experiencing high capacity in the summer and low off-peak occupancy. Finally, these sizing guides don’t account for advances and developments in technology and the increasing prevalence of green tech.


16 April 2022


shared data with the Northumbria paper and looked at probability models (such as the widely used guides which look at likely outcomes based on a set of factors), stochastic models (structurally complex models that can yield a range of different outcomes and are hard to interpret) and empirical models (which depend on high quality data inputs). In its conclusion it stated that its findings


The most commonly used guides for sizing are: CIBSE Guide G:2014 Public Health Engineering IOP Plumbing Services Design Guide British Standards BS 8558:2015 BS EN 806-3:2006


Two empirical studies have carried out deep dives


into three of the key standards for sizing DHW and their findings can help explain why system/pipe oversizing is so commonplace. Independent technical papers by Northumbria


University and Heriot Watt University (the LUNA study) look at hot water system sizing and especially pipe sizing, alongside the relationship and accuracy of loading units. The studies compare calculation results from the guides with actual results in real world scenarios. They talk about how oversizing occurs when using loading units and explain why oversizing pipework is inefficient. The Northumbria report says ‘Oversizing DWS


systems does waste materials and money, but it also increases the length of time that it takes for water to pass through the system which can increase water temperatures with the associated risks that presents’. The report looks at the three most commonly used guides; BS EN 806, BS 8558, and the guidance issued by the Institution of Plumbing. The Northumbria paper concludes that although ‘BS EN 806 [is] suitable for use for very large projects without excessively overestimating the design flow rate… all three sizing methods overestimated the design flow rates … and this knowledge should influence decisions such as the selection and specification of booster sets.’ The LUNA (Loading Unit Normalisation Assessment) project was initiated by Heriot Watt University to meet the growing concern that the traditional loading unit method used in the UK was causing an over-estimation of the design flow for domestic hot and cold water systems. The study


‘strongly suggest that an empirical model is best suited to the sizing of pipework and pump systems for mid-large size residential buildings.’ Both studies reveal that the current DHW sizing methods overestimate peak water volume flow rates which can result in: • A far larger footprint than is necessary to meet demand, accommodating oversized pipes, pumps and ancillaries


• Decreased efficiency across the system • Stagnant water due to increased length of time for water to flow through the system, leading to higher risk of legionella


• Higher cost of larger components


How can we more accurately size DHW? DHW sizing is never going to be an exact science. However, all estimates need to be based on a selection criterion from the outset. To make an accurate assessment designers should


always consider: • Application (school, hotel, nursing home, apartment block, etc)


• The number, type and flow rates of the fittings being served


• The number of people (because this is where the demand comes from)


• Time (the peak period(s) of use) • Time (the potential simultaneous flow rates) Using these criteria to design a dynamic system to meet base, fluctuations and peak requirements will deliver a DHW system that is difficult to undersize, and is fit for purpose, efficient and scalable. The dynamic system is designed with:


• Relatively low storage • A preference for ‘always hot’ • Faster reheat time (10 – 30 minute) These dynamic, flexible systems respond far better to variations of demand, use less energy and waste less water, making them a more responsible, sustainable design. Equally, dynamic systems have a far smaller footprint, using less gas and pipework, and smaller, more efficient cylinders.


www.heatingandventilating.net


DOWNLOAD THE HVR APP NOW


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44