The Which? Dehumidifier Report Explained

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The latest Which? test report is misleading dehumidifier buyers

The test takes little account of the very high running costs of desiccant dehumidifiers

Desiccant dehumidifiers have very high running costs – up to £500 per year more than compressor dehumidifiers.

John Elliott discusses Which? Report with Mark Page

1. Test conditions

Which? undertakes the dehumidifier tests under conditions that rarely occur in the UK climate and are therefore misleading. One test is at a temperature of 21ºC with 59% Relative Humidity and a second test at 10ºC and 59% Relative Humidity. These conditions rarely occur in a UK home where a dehumidifier is used to deal with condensation and dampness.

We understand Which? chose these conditions because they believe it to be a “tough test” presumably because a lot of water is extracted. The reality is the complete opposite. A great deal of water is removed at these conditions because almost any dehumidifier will be able to extract at lot of water when there is a lot of water in the air.

The performance at these conditions is therefore not a reliable guide to a good general performance. It doesn’t follow that a dehumidifier with a good performance at 21ºC with 59% Relative Humidity will perform well in typical British home conditions.

Domestic dehumidifiers are not like any other appliance because there are big differences between different products and there aren’t any international standards that reflect UK operating conditions.

Dehumidifiers are mainly used in the UK to control condensation and dampness during the winter months. In the USA, 27ºC and 60% Relative Humidity are common testing conditions. These conditions will never occur in the UK. Dehumidifiers in the UK are most commonly used to control condensation and dampness in unheated rooms during the winter months in occupied homes.

Dehumidifiers are normally used in a living area, which is heated at least for some of the day. The relative humidity will be lower where the dehumidifier is placed compared to windows and external walls - especially in unheated rooms. Moisture still needs to be removed - even though the adjacent air to the dehumidifiers is relatively dry.

However water still needs to be removed at this low Relative Humidity - which could often be below 40%. Condensation and dampness is caused by the outside temperature dropping and cooling the inside surfaces of external walls and windows especially in unheated rooms or spaces. It takes a totally different approach to design a dehumidifier to perform well at 22ºC 40% Relative Humidity rather than 27ºC 60% Relative Humidity. It therefore follows that a dehumidifier that is designed for the maximum performance at 27ºC 60% will not give the maximum performance at 22ºC 40%.

2. Water extraction capacity

Which? prioritise the total amount of water that can be extracted as the biggest effect on their dehumidifier overall rating. Ebac’s opinion, based on selling over 1.25million dehumidifiers over 30 years is that all but the most extreme damp and condensation issues can be solved by using a domestic dehumidifier rated between 12 litres and 25 litres extraction. Therefore a higher extraction is of no value and can in fact result in wasted electricity from removing more water than required.

Extraction capacity: 2 litres per day

3. Running Costs & Energy consumption

Which? don’t prioritise energy efficiency in their test - even though the difference in running costs at their test conditions of 21ºC with 59% RH between the best and the worst is a factor of 300%. This is estimated to increase the running costs of the least efficient dehumidifiers by £100 per annum. Some of Which’s best buys will cost between £50 to £150 more per year to run than other dehumidifiers with much lower ratings. These lower ratings being due to the fact that they have a lower extraction capacity, which is more than sufficient to take out all the excess water anyway.

4. Control Systems

Which? take no account of the dehumidifier control system even though there are a range of options which have a large effect on running costs. Manually adjusted humidistats need regular adjustment since the best setting changes especially when the outside temperature changes. A lower outside temperature means that a lower Relative Humidity setting is required. Not getting the setting right means that either the condensation/damp problem is not solved or the unit over runs - which can be a big problem with the very high extraction units such as desiccants. A further problem with manual humidistats is that some can only be adjusted in 5% steps again meaning under or over running.

How people use a dehumidifier

Ebac has over 44 years of experience of talking to customers who use dehumidifiers on a regular basis. There is no other organization in the UK with more experience or knowledge. We have surveyed in excess of 13,000 customers over the past 10 years. They have told us that 75% of dehumidifier users leave their dehumidifier on all day or all day and night during the winter.

This is absolutely the correct thing to do - as moisture levels vary throughout the day and night, as does Relative Humidity and the external air temperature. They also told us they use it to solve damp and condensation problems – not to maintain a constant relative humidity. Hence the focus of performance should be how the dehumidifier solved the problem in the most efficient way. Not as the Which? test promotes - which is how much water did the dehumidifier take out.

Despite our repeated attempts to help Which? re-engineer their tests to reflect the way customers really use their dehumidifiers and to help them reassess how customers value different aspects of a dehumidifier brand, they have declined. We therefore have published this article to ensure the British consumers are properly informed of the real costs and benefits of running a dehumidifier.

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