Report No DWI0735

Oct 1995


The water economy of 9 litre, 7 litre and 6 litre WCs was investigated by BSRIA on behalf of the Drinking Water Inspectorate. The purpose of the research was to evaluate the contribution that low flush WCs make to water conservation and to assist consideration of the maximum flush volumes which may be prescribed in the regulatory framework which will replace the Water Byelaws in 1997.

The trial consisted of the detailed weekly monitoring of twelve new 7 litre, twelve new 6 litre and fourteen existing 9 litre WCs over a period of two months in a commercial environment followed by a user survey.

The results indicate that low flush WCs save significant volumes of water relative to older 9 litre full flush WCs. However, the existing dual flush 9 litre WCs were also effective in saving water and performed slightly better overall than the new 7 litre WCs.

The actual flush volume of WCs depends on water supply pressure but also on manufacturing and installation variables. However, the long term water economy of WCs depends both on flush volume and flushing efficiency since failure to effectively clear the contents of the pan will result in double flushing.

While the overall benefits of changing from 9 litre full flush to 7 litre WCs were very clear (34% water saving) the additional benefits of changing to 6 litre WCs (were moderated by reduced flushing performance and consequently increased double flushing depending on the sex of the users.

Some aspects of WC performance were influenced by the difficult installation conditions at Marsham Street and the detailed findings would not necessarily be duplicated at other sites.

Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.