The risks of oversizing domestic water piping - Measurement of peak flows in multistory residential buildings

Over the past few decades, domestic water consumption in residential areas has declined significantly. This reduction can be attributed to several factors, including the widespread implementation of individual metering, rising water prices, development of more water-efficient sanitary fixtures, and an increased public awareness of water scarcity. However, there’s a growing concern that plumbing designers may be inadvertently oversizing pipe installations for domestic water due to reliance on outdated design manuals and standards that promote overly conservative methods. Traditionally, the primary standard for dimensioning domestic water pipes in Denmark has been to ensure that the installation can deliver the required water flow rate in 99.9% of outlet usage instances. To investigate this issue further, a recent project conducted by the Department of the Built Environment at Aalborg University, on behalf of The Danish Authority of Social Services and Housing, involved measuring the domestic water flow in eight multi-story apartment buildings. The findings revealed that, on average, the investigated pipe installations were oversized by a factor of 2-3. This general oversizing results in an extended retention time for domestic water within the pipes, which poses risks such as the growth of Legionella bacteria—an increasingly prevalent issue not only in Denmark. Practical application: The study contributes to determine whether there was indeed an oversizing of the water pipe installation, as this could potentially lead to various adverse consequences. The conclusion based on the performed flow measurements was that the guideline for designing domestic water distribution pipes in general significantly oversized both hot and cold pipes. Two proposals that reduces the design flow rate by 25% and 50% was therefore developed to update the national Danish standard DS 439 and during 2023 it was finally agreed in the standardization committee to implement the proposal for a reduction of 25%.