Dual flush toilet

A dual flush toilet; note the two buttons at the top of the cistern.

A dual flush toilet is a variation of the flush toilet that uses two buttons or a handle mechanism to flush different amounts of water.

The purpose of this mechanism is to reduce the volume of water used to flush different types of waste. The design takes advantage of the fact that liquid waste requires a lesser amount of water to flush than solid waste; the smaller button is used to dispose of liquid and the larger button for solids.^[1]

Development

The system was developed by Japanese sanitary product manufacturer TOTO in 1960.^[2] It was equipped with two levers and built-in hand-washer, and also notable in that it reused the water in the hand-washer for flushing. However, it was not very commercially successful either in Japan or internationally. In 1976, American industrial designer Victor Papanek proposed the dual flush system in his book Design for the real world,^[3] but the first practical implementation was designed in 1980, by staff at the Australian sanitary-ware company Caroma, with flush volumes of 11 and 5.5 litres.^[4] The design caught on, and a redesign in 1994 cut water usage to 6 and 3 litres.^[4]

The dual-flush toilet has become almost universally adopted in Australia, New Zealand, Singapore, Sweden, Israel and many other countries, with its use in new buildings often mandated by legislation in those countries.^[5] The more complex dual-flush mechanism is more expensive than many other types of low-flush toilets.^[6]

Mechanism

Push-button

Due to being a development of the traditional Australian flush toilet, the dual-flush toilet differs from siphon-flush toilets in that it relies on gravity to remove waste from the toilet. The lack of siphoning also means that the toilet requires less water to operate.^[6] Due to this, the waterline is considerably lower than that in siphon-flush toilets.

The toilet has two buttons on the cistern rather than the single-flush one; one button delivers a lesser amount of water (eg. 3 litres) and the other a greater amount (eg. 6 litres).^[7] It also uses a larger 10 cm trapway in the bowl, allowing for water to come out faster and clear the bowl efficiently.^[7]

Lever

There are also dual-flush toilets that use a siphon valve operated by a lever rather than buttons, with a ≤ 6L full flush, and a ≤ 3L half-flush if the flush handle is held down^[8] or released immediately after flushing.^[9]

Tipping bucket

The tipping bucket cistern can operate in a dual flush mode when the lever is rotated halfway 2.5/5 litre.^[10]

Advantages

The dual-flush toilet typically uses less water, resulting in lower running costs and less environmental impact. It was promoted by the Australian Government under its "Target 155" campaign.^[11] The first dual-flush toilets had a 4.5 litre (half) and 9 litre (full) flush, but innovations by Caroma brought that down to 3 litres and 4.5 litres respectively, achieving a WELS rating of 4 and 5 stars in Australia.^{[citation needed]}

Australian governments have used rebates to encourage the replacement of old single-flush toilets with more water-efficient ones.^[12] For dual-flush toilets, with a star rating of 4 or higher, owners may be able to qualify to claim a rebate from the State Government in Victoria,^[13] New South Wales, The ACT or South Australia.

In Britain, the cost of a dual-flush mechanism which can be retro-fitted to an existing toilet was estimated to be from about £15 in 2007.^[14]

Disadvantages

While dual flush reduces running cost, the initial purchase price is higher, and replacing a toilet entails installation cost. In many cases, it is possible to replace the flushing mechanism of an existing installation; such retrofitting can cost about US$30.^[6][15]

In the United States, the Energy Policy Act was signed into law in 1992 and took effect in 1994, requiring that toilets sold use no more than 6 litres (1.6 US gal) per flush. Fixtures that use a maximum of 20% less than the federally mandated maximum of 6 litres (1.6 US gal) receive the WaterSense label, a US Environmental Protection Agency (EPA) program designed to encourage water efficiency in the United States.^[16]

For dual-flush toilets to receive this label, the average flushing volume of two reduced flushes and one full flush must be below 4.8 litres (1.28 US gal).^[17]

While all dual-flush toilets are commonly seen as water-saving, this does not apply to all designs. In the US, some dual-flush toilets have flushes of 1.6 and 1.28 US gallons (6.1 and 4.8 L), which do not fulfill criteria for the WaterSense label and thus cannot be classified as high-efficiency toilets.^[18] Based on the WaterSense averaging rule over two reduced flushes and one full flush, a dual-flush toilet with a full flush at the US legal maximum of 1.6 US gallons (6.1 L) must have a reduced flush of 1.12 US gallons (4.2 L) or less to meet the WaterSense standard of 1.28 US gallons (4.8 L) on average.^[17] A common combination for dual-flush toilets meeting the WaterSense standard is a reduced flush of 1.1 US gallons (4.2 L) and a full flush of 1.6 US gallons (6.1 L).^[19][20]

Dual flush mechanisms are also more likely to develop leaks than a traditional siphon; the UK supplier Thames Water claimed in 2020 that dual flush toilets were likely to be wasting more water than they save due to a combination of leaks and confusion over which button to press.^{[21][failed verification]}

References

1. "Here's why toilet flush has one large and one small button". timesofindia.indiatimes.com. The Times of India / Bennett, Coleman & Co. Ltd. 1 February 2019. Retrieved 4 December 2024.
2. "【歴史】手洗付隅付ロータンク". Japan Toilet Association.
3. Papanek, Victor (January 1976). Design for the real world:Human ecology and social change (PDF). Chicago Review Press. ISBN 978-0897331531. Retrieved 25 July 2014. Because what one does while sitting on a toilet differs in both quantity and quality, it seemed simple to redesign the apparatus so that one could select whether a great deal or only a minimal amount of water was needed for flushing. This concept again was rejected by my client - a man who makes his living manufacturing toilet bowls - as being 'in bad taste'
4. "Dual Flush Technology". Archived from the original on 29 August 2007. Retrieved 4 January 2019.
5. Household and clothing: Dual flush toilet, Powerhouse Museum. Accessed: 15 February 2015.
6. "How the Dual Flush Toilet Handles Waste". How Stuff Works. 11 November 2008. p. 2. Retrieved 8 July 2009.
7. "The Lowdown on Low-Flow Toilets". HGTV. Retrieved 8 July 2009.
8. Allan Hanson (8 September 2017). "How a dual flush syphon works". Https. Retrieved 4 January 2019.
9. "Macdee Motion dual flush syphon". YouTube. 8 September 2017. Archived from the original on 20 December 2021. Retrieved 25 October 2020.
10. "Water Flushdual flush". 8 September 2017. Retrieved 25 October 2020.
11. "Our Water, Our Future". State Government of Victoria. Archived from the original on 19 May 2009. Retrieved 8 July 2009.
12. Toilet Rebates Archived April 18, 2010, at the Wayback Machine
13. "Our Water, Our Future - Eligible Products". State Government of Victoria. Archived from the original on 21 July 2008. Retrieved 8 July 2009.
14. Callard, Sarah (29 September 2007). "Is it worth it? Installing dual flush". Daily Telegraph (UK). Retrieved 31 May 2019.
15. "Dual Flush Toilet for $30". Archived from the original on 11 February 2009. Retrieved 8 July 2009.
16. "20 Years of the Energy Policy Act: 18 Trillion Gallons Saved Through More Efficient Toilets". Alliance for Water Efficiency. Retrieved 25 December 2023.
17. Environmental Protection Agency (2 June 2014). "WaterSense Specification for Tank-Type Toilets, Version 1.2" (PDF).
18. "Dual flush toilet. Not water saving". Archived from the original on 22 December 2019. Retrieved 11 January 2020.
19. "WaterSense Product Search - Toilets". lookforwatersense.epa.gov. Retrieved 22 March 2023.
20. Martin, Megabek (23 November 2021). "Best Dual Flush Toilets With Maximum Water Savings In 2022". Retrieved 22 February 2022.
21. Heap, Tom (28 September 2020). "The water-saving device wasting billions of litres every week". BBC News. Retrieved 22 February 2022.