Cistern Displacement Devices and Water-Saving Toilet Fixes for London Homes

What Are Cistern Displacement Devices?

A cistern displacement device is any object placed inside a toilet cistern that occupies volume and thereby reduces the amount of water that fills the cistern after each flush. By reducing the cistern capacity, the device reduces the volume of water used in each flush without requiring any modification to the flush mechanism or the installation of new plumbing components. The devices are designed to fit inside the cistern without interfering with the float valve, flushing mechanism, or overflow pipe.

The most widely known cistern displacement device in the UK is the Hippo the Water Saver, a plastic bag-shaped device that is filled with water and placed in the cistern. When the cistern refills after flushing, the space occupied by the Hippo means that less water enters the cistern, and the subsequent flush uses correspondingly less water. The saving per flush is typically between 1 and 3 litres depending on the size of the device and the capacity of the cistern.

The Save-a-Flush Bag: Thames Water Free Provision

Thames Water has periodically offered save-a-flush bags free of charge to metered customers in London as part of its water efficiency programmes. The save-a-flush bag is a polymer bag that absorbs water and swells to a fixed volume when placed in the cistern, displacing approximately 1 litre of water per flush. Thames Water has distributed millions of these devices to London households since the programme began, and they remain available through the Thames Water water efficiency team.

To obtain free devices from Thames Water, London households should check the current availability on the Thames Water website under the water efficiency or save water sections. Free devices are typically available to metered customers and may also be available to unmetered customers as an incentive to register for a meter. In addition to save-a-flush bags, Thames Water has offered free tap aerators, cistern retrofit kits, and water-saving showerheads at various points through its conservation programmes.

How Much Water Do Cistern Displacement Devices Save?

A cistern displacement device saving 1 litre per flush in a household where the toilet is flushed thirty times per day saves 30 litres per day, or approximately 10,950 litres per year. At Thames Water metered rates, the annual cash saving from a single displacement device is approximately thirty to forty pounds, which is a meaningful return given that the device costs nothing if obtained free from Thames Water or a few pounds if purchased from a hardware store.

In a rental property where the landlord pays the water bill, fitting displacement devices in all toilets across a portfolio of ten London properties saves approximately three hundred to four hundred pounds per year in water bills at a total cost of zero if the devices are obtained from Thames Water. Even purchased devices at five to ten pounds each represent a payback period of only a few months.

Limitations of Cistern Displacement Devices

Cistern displacement devices have one significant limitation: they are only effective in older toilets with cisterns that are larger than necessary for the installed flushing mechanism. Modern low-flush toilets with 6-litre cisterns have been designed to use the minimum water volume needed for effective flushing of the bowl and trap, and fitting a displacement device in such a cistern may reduce the flush volume below the minimum needed for effective performance, leading to incomplete flushing and the need for double-flushing, which would negate the saving.

Displacement devices should only be fitted in toilets with cisterns of 9 litres or more, and the device should be sized to leave at least 6 litres of water in the cistern after displacement. In older London properties with large Victorian or Edwardian high-level cisterns, or post-war close-coupled toilets with 9 to 13-litre cisterns, displacement devices can be fitted safely and produce worthwhile savings.

Fixing Running Toilets: A Higher Priority Than Displacement Devices

While cistern displacement devices save water on every flush, fixing a running toilet saves far more water because a running toilet wastes water continuously rather than just at flush time. A toilet where the flap valve or siphon washer is worn may allow water to seep continuously from the cistern into the bowl, wasting between 200 and 400 litres per day. This continuous leak is often not immediately visible but can be detected by placing a piece of dry toilet paper against the back of the bowl; if it becomes wet without being flushed, the toilet is running.

Repairing a running toilet by replacing the flap valve, float valve, or siphon washer is a job for a plumber and typically costs between sixty and one hundred pounds including parts and labour. The annual saving in water costs from fixing a running toilet at current Thames Water metered rates is between twenty and fifty pounds per month, making it one of the highest-priority plumbing repairs for metered London households. Prestige Engineers carry out toilet repairs and cistern mechanism replacements across all London areas and can combine this work with the fitting of displacement devices or dual-flush mechanism upgrades in the same visit.