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A comparison of UK domestic water services, sizing methods with each other and with empirical. data, JESS TINDALL BENG HONS , Faculty of Engineering and Environment Northumbria University. jess tindall northumbria ac uk, JAMIE PENDLE MENG, Graduate Mechanical Engineer Arup. Jamie Pendle arup com, Abstract, Evidence suggests that DCWS sizing methods in many countries around the world tend to. overestimate the actual peak water volume flow rate 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 This. paper compares the three commonly used UK DCWS sizing methods and reveals variance in the. methods the design flow rates calculated and the amount of diversity applied by each method BS. EN 8061 returns the lowest design flow rates of the three methods and also applies significantly. greater diversity , Empirical DCWS volume flow rate data from two case study buildings revealed that all three UK.
sizing methods significantly overestimated the peak water volume flow rates but that BS EN 8061. Page 1 of 26, was the closest Additional empirical data from seven more buildings has been used to validate the. data from this study and add confidence to the findings This research provides useful evidence to. help engineers select the most appropriate UK DCWS sizing method and to anticipate the likely. range and fluctuation of DCWS flow rates , Keywords DWS domestic water service DCWS domestic cold water service DHWS . domestic hot water service water loading unit volume flow rate probability oversizing. Engineers tend to be conservative by nature and generally err on the side of caution to ensure that. there is never any cause for customer complaint about their designs This is understandable but. there can be negative consequences if the full implications of such decisions are not fully. understood This paper reveals significant oversizing compared to empirical data from each of the. three UK DWS sizing methods and highlights the reasons for this The practical application of this. paper lies in the presented results data and analysis which will help engineers make this important. choice between the available sizing methods , 1 0 Introduction. There are three commonly used sizing methods for domestic cold water services DCWS for. United Kingdom UK buildings at the time of publication BS EN 8061 is the current European sizing. standard but in part 3 Annex C it grants engineers discretion to use the UK method as defined in. BS 85582 when deemed appropriate Some engineers have preferred to use the guidance issued. by the Institution of Plumbing3 believing that this results in more accurate sizing than the UK. method BS 85582 also offers complementary guidance to BS EN 8061 and confirms that an. investigation is underway into the loading units LUs used in the UK s traditional sizing guidance. Page 2 of 26, i e BS 85582 Until the publication of new recommendations designers may use BS EN 8061 for. residential applications and BS 85582 elsewhere , CIBSE AM124 states that experience from continental schemes indicates that BS 67005 factors.
now published as BS 85582 are too conservative for predicting design Domestic Hot Water. Service DHWS volume flow rates Instead it directs readers to use the Danish Standard DS 4396 . Agudelo Vera C7 states that oversizing pipework reduces water velocities which mean that the. water remains in the distribution pipework longer than is ideal for health and hygiene reasons This. problem is most extreme in tall buildings where the domestic cold and hot water pipework may run. within the same riser space resulting in undesired heating of the cold water . There have been some reports that current sizing methods can lead to an overestimation of water. demand Wong L8 calculated a theoretical design water flow rate using a fixture unit method . probably following the ASHRAE9 guidance although not specifically stated in the article This. prediction was compared with theoretical data from a model developed using measured data from. 1300 households in 14 typical Hong Kong high rise buildings The data revealed that the water. demand predicted from analysis of the measured data method was around 50 60 of that. calculated using the fixture unit estimation method which was said to be the current design. practice adopted for high rise residential buildings in Hong Kong A Brazilian team10 found similar. results with measured data being 23 lower than the design flow rate calculated using the Brazilian. standards , Researchers from The Netherlands7 analysed the Dutch guidance on drinking water supply. systems which had been developed using data measured between 1976 and 1980 The authors. state that the old guidelines overestimated peak demand values due to an increased range of. available appliances and changes in the behaviour of building occupants The importance of. accurately estimating peak demand values is stressed as poorly designed and oversized systems. Page 3 of 26, are less efficient thus more expensive but can also cause stagnant water By using data gathered. from a range of buildings of different water usages the team constructed a stochastic model called. SIMDEUM standing for Simulation of Water Demand an End Use Model Subsequent research11. highlights that in designing a DCWS distribution system the peak value of the total water demand . referred to in their report as the MMFcold or maximum momentary flow of cold water is of great. importance The research uses a procedure developed in 2010 to derive design demand equations. for the peak demand values of DHWS and DCWS in both residential and non residential buildings . The study found a good correlation between their demand equations and measured patterns of. use which was much more accurate than the current Dutch guidance indicating that their. calculations were reputable , Using these demand equations the Dutch study7 found that the results of simulations matched. measured values of peak water demand and that the pipe diameters in the systems they studied. were considerably larger than necessary They hypothesise that the issue of oversizing may be. present in other countries and state that their SIMDEUM model could be easily adapted for use in. other countries when specific information of users and appliances is available The authors confirm. that a 2013 revision of the Dutch guidelines incorporated the design demand equations presented. in the paper , Based on this review it is likely that the UK DWS sizing methods may be overestimating the peak. flow rates for buildings The questions were how significant is the margin of oversizing and which. of the three UK sizing methods would be most appropriate for multi unit residential buildings This. research has been designed to answer these two questions The next section compares the three. sizing methods and draws out the distinctive features of each The latter sections set out the method. by which measured data was obtained and compared to the design flow rates predicted by each of. the three sizing methods Additional datasets gathered from the same type of building are used to. Page 4 of 26, validate the data from this study The results published in this paper enable engineers to be.
confident regarding which DWS sizing method to apply to avoid oversizing their DCWS systems . 2 0 Sizing method review, This section introduces the three most commonly used UK DWS sizing methods and compares. them with each other All three methods utilise loading units LUs generated from a probability. analysis developed from the original work of Hunter12 These methods set the chance that the. design volume flow rate will be exceeded for 1 of the time as discussed in CIBSE Guide G13 All. three methods require the summation of the downstream loading units for each pipe section and. some form of conversion chart is used to determine the design volume flow rate These charts. apply varying levels of diversity The differences in approach taken by each method are discussed. and compared in the following three sections , 2 1 BS 8558 20152 Guide to the design installation testing and maintenance. of services supplying water for domestic use within buildings and their. curtilages, The UK method as defined initially in BS 67005 which was first published in 1987 revised in 1997. and 2006 before being withdrawn and superseded by BS 8558 in 2011 which itself has been revised. in 2015 The harmonised European standard BS EN 806 3 explains in Appendix C that designers. are free to use their nationally approved sizing method BS 67005 for the UK now BS 85582 if. they deem it appropriate BS 8558 20152 suggests that BS EN 806 31 may be used for residential. buildings and that traditional UK loading units BS 85582 may be used for commercial and public. buildings , Page 5 of 26, In the UK method there is a single LU value given for each outlet type with the exception being. wash hand basins where a higher value is applicable in buildings with periods of peak usage such. as schools or theatres and a lower value for non peak uses . A chart is provided to convert LUs to a design volume flow rate The chart is formatted with a. logarithmic scale allowing ample detail to be provided at LUs between 10 and 200 while enabling. the chart to extend up to a total of 8000 loading units which equates to a flow rate more than 30. litres per second , 2 2 BS EN 806 3 20061 Specifications for installations inside buildings.
conveying water for human consumption, In 2000 Britain began the process of standardising the guidance for DWS with the European Union. EU In 2006 BS EN 806 31 which presented new guidance for the sizing and design of DWS was. published and superseded BS 67005 , The use of LUs in BS EN 806 31 is the same as BS 85582 apart from there are no ranges of loading. units given for any outlet type to account for peak or non peak use . Figure 1 is the LU to volume flow rate conversion chart for BS EN 806 31 Again the axes use. logarithmic scales but this time the largest LU value is 5000 loading units which equate to 9 litres. per second significantly less than the two other methods A notable feature of this conversion chart. is that for loading units below 300 the resultant design flow rate depends upon the highest single. value LU fed from that pipe section , Page 6 of 26. Figure 1 BS EN 806 31 Fig B 1 Loading units to design volume flow rate. 2 3 The Institute of Plumbing3 Plumbing Engineering Services Design Guide . The Institute of Plumbing IoP guidance3 is preferred by some UK engineers because it provides. ample data and a much more significant discussion regarding the generation of the LU values and. also because it appears to offer the designer more flexibility The choice between low medium and. high LUs depends upon the frequency of use Low use is applicable where outlets have 20 minutes. between uses which it suggests is appropriate for dwellings It recommends Medium use for. applications where there are 10 minutes between uses as predicted in public buildings with no. peak use Applications where there are just 5 minutes between uses such as in concert halls or. theatres i e peak use should use the high use values . The charts presented to convert LUs to flow rate use logarithmic scales and peak at 8000 loading. units that equates to approximately 30 litres per second In these respects it is similar to the chart. presented in BS 85582 Where it differs is that it also enables pipe sizes to be directly selected. Page 7 of 26, based on the pressure head loss per metre run of pipe and the resultant water velocity In this. way many useful features are combined into the one chart . 2 4 Comparison of Loading Units between the three sizing methods. BS EN 806 31 states that one loading unit is equivalent to a draw off rate of 0 1 litres per second . In contrast the LUs presented in the Institute of Plumbing Guide and BS 85582 are not simply. linked to flow rate as BS EN 806 31 but also take account of the length of time which the outlet will. be in use and the outlet s patterns of usage Therefore the basis for the value of LUs is significantly. different for BS EN 806 31 compared to the other two sizing methods . Table 1 displays the LUs for a range of common outlet types for each of the three sizing methods . The number of LUs for the BS EN 806 31 and Institute of Plumbing3 low use columns are identical . where values are displayed BS 85582 LUs are similar to those provided by the Institute of. Plumbring3 for medium use and therefore appear broadly comparable In contrast the Institute of. Plumbing high use loading units are significantly higher than either alternative method . Table 1 Loading Units comparison, IoP LUs, BS 8558 LUs BS EN 806 3 LUs.
Low Med High, WC flushing cistern 2 1 1 2 5, Wash basin 15mm 1 5 3 1 1 2 4. Sink tap 15mm 3 2 2 5 10, Bath tap domestic 4 4 8 16. Bath tap 20mm 10 8 11 , Bath tap 25mm 22 , Shower 3 2 2 3 6. Page 8 of 26, 2 5 Comparison between conversion charts. All three sizing methods provide a conversion chart to determine a design flow rate for a known. number of loading units Data from these charts have been used to produce Figure 2 which. illustrates the relationship between flow rate and LUs for each method The data. Instead it directs readers to use the Danish Standard DS 4396 Agudelo Vera C7 states that oversizing pipework reduces water velocities which mean that the water remains in the distribution pipework longer than is ideal for health and hygiene reasons This problem is most extreme in tall buildings where the domestic cold and hot water pipework may run within the same riser space resulting in

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