Why dehumidifier capacities are unrealistic

Separate from tank size, rated capacity is the claimed maximum amount of water a dehumidifier can remove each day – usually ranging from 10L (a smaller model) up to 50L (a whopper).

Dehumidifier labelling typically shows only those maximums. But these extraction rates are achieved in conditions far removed from what you’d get over a Kiwi winter.

Realistic test conditions

Manufacturers test their dehumidifiers in optimal conditions – usually about 30°C at 80% relative humidity (RH). However, it’s unlikely you’ll find any flats in Otago with these balmy conditions in July!

That’s why we test dehumidifiers in conditions you’d expect in Kiwi homes over winter – ranging from 8°C and 90% RH to 16°C and 65% RH.

We got very different results.

In our testing, on average, the amount of water extracted after running the dehumidifiers for 24 hours (emptying tanks whenever they were full) was just 25% of their claimed maximum capacities. The results were worse in the lower temperature test (8°C), even with humidity at 90%.

The Goldair GD350 (desiccant type) got closest to its stated daily extraction rate of 8L, extracting an average of 5.7L a day (71%). It does a good job of removing moisture even in colder temperatures when most dehumidifiers struggle. But it costs nearly three times as much to run than the average refrigerant model.

The dehumidifier with the highest claimed extraction rate – the Mitsubishi Electric MJ-EV38HR-A (refrigerant type) – extracted a daily average of 6.8L of moisture, being only 18% of its claimed 38L maximum. This model is one of the most expensive in our database, costing nearly $1,500.

While maximum capacity claims can be overly optimistic, a dehumidifier taking several litres of moisture out of the air will certainly improve the health of a damp home.

Water tank woes

Not only are manufacturers’ rated capacities a little optimistic, their stated tank sizes are usually slightly off the mark too.

We measure tank capacity as being up to the point the dehumidifier switches itself off after detecting it’s full – not just the tank’s total capacity. Usually, this difference is several hundred millilitres.

In addition, tank capacity is always much less than the amount of water a dehumidifier can extract each day. If one of the 30L models really did extract 30L of water per day, you’d be emptying the tank (5L on average) every four hours.

Our view

We’d like to see dehumidifier manufacturers be more realistic about their water extraction rates. The best outcome would be for them to measure the daily capacity of their dehumidifiers at several different temperatures and humidities, labelling them with their average capacities.

To find out how well the dehumidifier you’ve got your eye on performs in Kiwi conditions, check out our database of 23 tested models.