Siphon vs valve flush - Information Hub Green Building Store

Siphon vs valve flush

Most of the low flush toilets currently available on the UK market are dual-flush valves (which either use a button-press or sensor system). These can save water until the valves become faulty, when, unless rectified, they leak water continually down through the pan. It is estimated that in the US about 20% of WCs leak at a rate of 20,000 US gallons per year per WC (76m3/year).

Green Building Store would argue that siphon flush mechanisms offer a number of advantages over valve flush mechanisms, including:

  • Consistent, reliable water efficiency over the long-term because siphons, unlike valves, are inherently leak-free.
  • More familiar to British plumbers and easier to repair

The following resources explore the siphon vs valve flush debate further:

‘Water-efficient’ toilets leak millions of litres every day

An article by Ben Webster, Environment Editor at the Times (20/06/17) highlighting water leakages occurring from dual-flush valve flush toilets. About 4 per cent of such toilets leak from the cistern with the average leaky valve toilet losing 215 litres a day, according to Waterwise, a not-for-profit body promoting water efficiency. Read the full article

Save our toilets!

Short film by journalist John-Paul Flintoff making the case for siphon flush WCs.

Conserving water in buildings

The following is extracted from the Environment Agency’s ‘Conserving water in buildings’ publication (Section 1: Toilets):

“Before January 2011, all domestic toilets in the UK had to use a siphon flush, which was originally developed to prevent water wastage. When we pull the handle a piston lifts water to start a siphon, which empties the cistern into the toilet bowl. When the cistern is empty the siphon is broken and the cistern refills ready for the next flush.

Since January 2001 approved drop valves and flap valves have been permitted. These allow a button operated flush and more obvious two button dual flush operation. But sooner or later, unlike the siphon, they will leak. The Water Regulations require endurance testing of 200,000 flushes, under laboratory conditions but mechanisms can be incorrectly installed and debris can enter the cistern during installation, causing immediate leaks. Evidence from water companies shows that leakage from valves is already becoming a serious problem.

From a sample of over 500 properties that were investigated due to unusually high water bills. Bournemouth & West Hampshire Water identified 31 properties where the cause of the high water use was leaking drop valve operated toilets. On average, six month water bills for these properties were over £370 more than the bill for the six month period before the leak”.

Bournemouth & West Hampshire Water research

Graph showing water wastage through leaking valves – download pdf

AECB Water Standards

AECB Water Standards

The following is extracted from the AECB Water Standards

WC best practice:

Less than or equal to 4.5 litre maximum flush with water supply connected. Flush mechanism to utilise a leak-free siphon or to be fitted with a suitable leak detection warning device. Full flush volumes less than 4 litres are considered to be experimental.

Article on best practice

‘Focus – water closets: best practice since the Water Fittings Regulations 1999’ – Article by Nick Grant & Mark Moodie in ‘Building for a Future’ magazine, Autumn 2002 – download PDF

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