WATER FLOWRATES | CALCULATION METHODS PAPER ACCEPTED Technicalm Symposiu posium .org/sym www.cibse il 25-26 Apr 9 1 20 STEMMING THE FLOW It is recognised that using the traditional loading units method overestimates the design flow for hot and cold water systems. Heriot-Watt Universitys Achala Wickramasinghe explains how the Luna project aims to address the issue and calls for data to help validate a new method F or many years, there has been concern that water-supply systems for buildings in the UK are being over-designed as a result of using an outdated calculation method. The traditional loading units (LU) method is used to estimate the design water flowrate of hot and cold systems, which include pipework, booster sets and hot-water generation systems. There is evidence that using this method overestimates the design load, which leads to over-designed watersupply systems and results in larger capital costs, as well as higher energy consumption. In addition, oversized pipework can cause lower-than-anticipated water velocities, resulting in unnecessary heat gain in cold-water pipes and heat losses in hot-water pipes. Low water velocities can also create hygiene concerns because of the risk of biofilm growth in supply systems. While this is an issue across many building types and sizes, the impact is particularly significant in medium to large residential buildings. A research initiative to address the problem widely known within the building services industry as Loading Units Normalisation Assessment (Luna) is now in its second phase as an Innovate UK Knowledge Transfer Programme (KTP) project between CIBSE and Heriot-Watt University. Its goal is to develop a new method for estimating the design flow in residential buildings. Water demand for a property depends, in part, on the number of users and their water-usage patterns, which in turn relates to many factors, such as age, occupation, cultural background and so on. Demand is complicated by the growth in the retrofitting of waterefficient products, such as low-flow showerheads and taps, and waterefficient toilets, which require less water. It can be seen that peak water flowrates for different apartments are unlikely to coincide because of the diversity in water usage. As a result, this has to be factored in when estimating the design water flow of a residential building with multiple apartments. In addition, the design flow of a building differs according to the arrangement of its hot- and cold-water supply system. For example, at a given instant, the total hot- and cold-water demand at the main intake point to a building with instantaneous heat interface units (HIUs) installed in each apartment to serve its domestic hot water demand is different from that value if the domestic hot water demand of each apartment is served directly from a central plant room. It is not economical to design the watersupply system for a possible maximum simultaneous flow, as the probability of its occurrence will be very small. So it is necessary to define a design condition with an acceptable level of system failure calculation methods are often necessary for this task. In the UK, the recognised standard for estimating the design flowrate in water supply systems is BS EN 806-3:2006, the UK version of the harmonised European Standard. BS 6700, which has now been replaced by BS 8558, is also a widely recognised standard within the industry. www.cibsejournal.com April 2019 49 CIBSE Apr19 pp49-50 Luna.indd 49 22/03/2019 16:59