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Domestic hot water flow rates have always been challeging challenging to calculate, because we need to take into account the simultaneous usage of hot water tapping points. Not all tapping points (like showers) will be use at the same time, so a so we need to take into account in each part of the system, the maximum probability, or the maximum DHW flow of simultaneous tapping points. These are defined in diversity standards. Most of these methods only account for simultaneous flow rates, and not for simultaneous power, because domestic hot water networks are mostly operated as single pipe / fixed temperature systems.
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