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Technical Bulletin 01: Domestic water demand assessment for pipe sizing
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Technical Bulletin 01: Domestic water demand assessment for pipe sizing

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In terms of domestic water pipe sizing, the industry is in a period of transition. Reliability questions are being raised about the traditional demand assessment system based on the LU (loading unit) method that was introduced to the UK in 1965. This was prior to the advent of water efficiency considerations, when washing habits were very different, and when mixer taps and automatic white goods where uncommon in dwellings.

Based on empirical data from live buildings, and also considering alternative design methods used in similar European countries, the growing consensus is that the UK LU system will significantly over–estimate the peak simultaneous design flow in multi-residential buildings. This TB provides interim best practice guidance for assessing domestic water demand for pipe sizing in various applications, that can be used until such time as further research is completed (e.g. LUNA1) and/or new design standards have been published.

This technical bulletin has been written by Brian Whorlow BA (Hons) IEng FCIPHE, who is honorary Chair of the LUNA group.


About the author

Summary of key items for consideration


What is the origin of UK loading units?

Inherent loading unit problems

Over-sizing problems

Design Standards

Design methods for multiple residential buildings

Design methods within single dwellings

Design methods for commercial buildings

Design methods for congested applications



Author: Brian Whorlow (Honorary Chair of the LUNA group)

See also the Loading Unit Normalisation Assessment research report An assessment of the validity of the loading units method for sizing domestic hot and cold water services, published jointly by the Chartered Institution of Building Services Engineers, the Chartered Institute of Plumbing and Heating Engineering and Heriot Watt University, which is available here.

First edition: December 2016

Second edition: April 2020

The second edition contains no substantial change to text or technical content.

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