Dual-flush toilets in the U.K., designed to save water with a two-stage flushing mechanism, typically come with flush valves that are so leaky that the toilets waste more water than they save, a water company there claims.

The toilets give homeowners two flushing options, one for solid waste and one for liquid waste, offering potential water savings every time the liquid-only button is pressed. But the type of valve installed in these toilets is prone to leaks, which collectively add up to 400 million liters (about 106 million U.S. gallons) a day, according to a water efficiency advocacy group there called Waterwise.

Thames Water, the U.K.’s largest water and wastewater company, says losses outweigh the benefits. “Because there’s so many loos that continuously flow all through the day and night, collectively that water loss is now exceeding the amount of water the dual-flush design should be saving,” the company’s water efficiency manager, Andrew Tucker, told The Guardian.

The report puts the blame on something called a drop valve, which sits at the bottom of the toilet tank and releases water when activated. The mechanism can fail to close completely because of debris or other problems, which allows water to flow into the toilet bowl continuously. Older siphon systems work differently and are not prone to leaks, the report said.

Jason Parker, the managing director of Thomas Dudley, one of the country’s largest plumbing manufacturers, said the faulty valves could begin leaking within a week of installation, The Guardian reported. “It could be two years, but they will leak,” Parker said.

Waterwise estimated that a single leaking toilet wastes between 215 and 400 liters of water per day and that as many as 8% of all toilets leak. Most of those use newer dual-flush valves.

Does the problem extend to the U.S.?

Dual-flush toilets are available in the U.S. from a number of manufacturers, including Kohler, American Standard, Toto, and Niagra, all of which list them on their websites. The toilets have special levers or buttons that allow the user to choose how much water to release. One Kohler model, for example, releases 1.1 gallons on the low setting and 1.6 gallons on the solids setting.

Do the dual-flush toilets available in the U.S. use the same type of valve found on problem toilets in the U.K.? That’s hard to say. Niagra, which makes Glacier Bay toilets sold at Home Depot, did not respond to an email. Kohler didn’t answer questions about the potential problem.

American Standard initially said it would check on whether its toilets used the same type of valve, and whether there have  been reports of widespread leaks. Later, however, a spokesperson said in an email that no one at the company was available to discuss the issue.

Toto did respond. Depash Patel, of the company’s development engineering department, said in an email that he had never heard of a “drop valve.” Overall, Patel said, Toto isn’t aware of any widespread leaking issues with its in-wall tank drain valves. In the few cases the company is aware of, the problem was the “non circularity of the pipe and O-ring combination” or an installation error. Patel suggested that the type of material used to make the seals could be the root of the problem in the U.K.

“Usually what we see with these overseas market products is the seals aren’t as robust as for the products that are made for the USA market,” Patel wrote. “They use NBR rubber materials there, which will deteriorate and cause leaks. We use EPDM and silicone-based rubber materials for our seals, which last much longer.”

While not responding to requests for information, Kohler posts instructions at its website on how to repair a toilet that leaks occasionally or runs when it’s not supposed to. Included is a diagram showing a conventional flapper valve at the bottom of the tank as well as a canister-style valve that resemble photos of drop valves posted online. The problem with the second type of valve is with a gasket between the bottom of the canister and the floor of the tank. Replacing the gasket should fix the problem.

GBA also contacted the U.S. Environmental Protection Agency, which runs a voluntary water conservation program called WaterSense. Toilets get a WaterSense rating when they use no more than 1.28 gallons of water per flush (this is 20% less than the federal minimum efficiency standard of 1.6 gallons per flush). Some but not all dual-flush toilets meet the WaterSense standard.

The EPA didn’t reply when asked whether the agency was aware of problems with dual-flush toilets in the U.S.

If you’re wondering whether your toilet valve leaks even if you can’t hear or see anything that would suggest it, the EPA recommends this test: place a few drops food coloring in the tank and wait 10 minutes. If the coloring shows up in the toilet bowl, you have a leak. The agency posts instructions on repairing leaks here.

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-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.