The Consumer Council on Wednesday called on dehumidifier manufacturers to provide more transparent and precise information about the performance of their products, after finding some don't work as well as expected.

The watchdog tested 14 dehumidifiers and found that the dehumidifying capacity of six of them was lower than claimed, by between 0.6 and 5.3 percent.

Meanwhile, two of them, made by Whirlpool and Zanussi, were significantly less able to remove moisture after the air filters that came with them were fitted.

While both machines have grade-one energy labels – indicating they are among the most energy efficient models available – they fail to live up to their rating with an air filter on, the watchdog said.

In response to the council, Zanussi said the filter is complimentary and people can choose not to install it.

But Nora Tam, who chairs the council's research and testing committee, said more clarity is needed, as ordinary people would likely use an air filter if it comes with their dehumidifier.

"If you give them something, which will affect the energy efficiency and the dehumidifying capacity, it should be mentioned to the consumers somewhere in your leaflet or in your label," Tam said.

"Particularly, if the energy efficiency label will drop from [grade] one to two or even to four, that's very important information, because that will cost you more money when you want to make your room drier."