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Damp toolbox part 5: Dehumidifier practicalities & expert recommendations


The headlines

The previous articles in this series on damp have:

  • Discussed the issues caused by damp, from health impacts, to cost and comfort.

  • Explored the practical steps that we can all take to reduce moisture levels.

  • Looked at the vital role ventilation plays (good ole fresh air!).

This article focuses primarily on dehumidifiers and:

  • Their role in reducing humidity levels.

  • Evidence on their effectiveness, practicalities, limitations, and costs.


Expert views

Using them for keeping humidity levels down in a home

They are an option for getting rid of damp air. All independent experts agree though that dehumidifiers are only a partial solution to managing damp and condensation and that ventilation is a much healthier and also cheaper way to deal with problems.


Dehumidifiers don't address other indoor air pollution and air quality issues that are becoming an increasing problem in UK homes. Only ventilation can do that.

They can actually increase other indoor air pollution issues by encouraging users to keep homes sealed up to minimise dehumidifier running costs (such as by keeping windows and trickle vents closed).

The evidence is very clear: Ventilation is by far the best cure for damp problems. We shouldn't be afraid of fresh air.

If you have issues with damp, before committing to a dehumidifier, try to address problems by doing the two things in previous articles:


If you are certain you want to use a dehumidifier to deal with general moisture issues, the section further down describes the practicalities and how to get the best out of them for the lowest running cost.


Using them specifically for laundry drying

In contrast to the above, dehumidifiers are VERY effective and relatively low-cost way at helping dry laundry if used in the right way. This other article on the site has ESSENTIAL do's and DON'Ts on how to use a dehumidifier to help with laundry.

 

The need

Human activities in a home can add between 2 and 4 litres of water to the air person per day.

Something that can suck all that moisture back out of the air sounds like an ideal solution. This is what dehumidifiers do of course. They the silver bullet answer though.


Quick links to sections below:

 

The practicalities of using a dehumidifier

Dehumidifier size

A dehumidifier which is powerful enough to extract a large amount of water from high levels of damp air is an expensive commitment, both to buy and to run.

It is possible to buy small low power dehumidifiers (around 40 Watts). I have seen many comments online from people who have tried these devices. They consistently say that they are not effective. I am not surprised by these comments. This short video supports these views.


Running costs

A dehumidifier needs to be carefully managed to avoid unnecessary running costs. If you dehumidify more than you need to you are just wasting money.

From my research, small dehumidifiers won’t help much with damp problems. If you want to manage damp in a smaller sized house or flat (or help in a specific room) aim to use a dehumidifier with a minimum power of around 150 Watts.

A dehumidifier of this power would cost around 4p an hour to run. A higher power one (say 250 Watts) will cost around 7p an hour to run (assuming 27p/kWh energy cost).

A 'few pence per hour' may not sound a lot but can actually mount up to a significant amount over time.

Depending on the type of dehumidifier and how it is being used they do give off a little heat. If you use one in a centrally heated room, it will reduce the amount of gas you use. BUT, gas is a much cheaper source of energy than electricity. If you have acceptable humidity levels in a room, then it is cheaper to turn the dehumidifier off than use it as a source of extra heat.


Dehumidifier running cost discussions on social media

I've seen a range of comments on social media about the running costs of dehumidifiers used to control damp. The costs they quote range from around £8 to over £30 a month. Some people say they spend less than £8.

It's really important when looking at social media comments to bear in mind that every home and situation is different.

  • the type of house other people live in.

  • The number of occupants in a home.

  • how draughty or insulated they are.

  • lifestyle differences.

  • typical indoor temperatures

  • how much moisture householders are generating.

Given all these differences, I would strongly advise against taking decisions about dehumidifiers based on information on social media.


Running time

Unfortunately, there are no guides or information on how long dehumidifiers need to be run. I have seen anecdotes on Facebook from people who clearly run theirs most or all day. Some people run them 24 hours.

Ebac are one of the manufacturers of dehumidifiers. Their website implies that a suitably sized dehumidifier might need to run between 12 and 18 hours a day to manage indoor damp. If dehumidifiers have smart controls this will help minimise the amount of run-time, but the general messaging is that they might need to run many hours a day.


Tips on minimising costs

If you use a humidity meter to decide when to use a dehumidifier, never leave it on a windowsill. This will be the coldest part of the room and it will show a higher relative humidity than if it was away from the window. It may therefore fool you into thinking the humidity in the room is higher than the room really is and you may run the machine unnecessarily.

If you set the target humidity on a dehumidifier lower than you actually really need, run time will be longer and it will cost you more. See this other article explaining relative humidity and a very useful chart at the bottom with a link to further info.


Where to locate them

They are most effective when located near the source of moisture. This might mean kitchens, where you’re drying laundry, or near bathrooms (though not inside for safety reasons).

A dehumidifier of any useful size weighs at least 10kg, the weight of a large full bucket of water. This is something to bear in mind if you think you will need to move it between rooms. Some people choose to have more than one, with one upstairs and one downstairs.


Other important practicalities

Dehumidifiers work most efficiently in airtight rooms or buildings. If you have significant draughts ANYWHERE in your home, a dehumidifier is likely to use energy unnecessarily. This is because once it has dried the nearby air, it will then start to suck water from draughty the air which has been drawn in from outside. It may collect lots of water but much of this could be from outside.

For the same reason, care should be taken not to leave windows and exterior doors open when running a dehumidifier. It has concerned me recently to see comments on social media of people saying they use a dehumidifier constantly and will keep their home sealed until the spring. It is highly likely this will aggravate other indoor pollution issues.

To ensure the dehumidifier is not consuming energy unnecessarily, it's important to understand its features and controls. The amount of water they extract and the settings you use normally depend on the situation and weather.

Using a dehumidifier means regular emptying of the water collection tank.

I found many websites recommend that dehumidifiers are thoroughly cleaned at least once every three weeks. One such example is here. A dirty dehumidifier can actually release mould spores into your home, which is the opposite of what you want it to do.

Filters also need cleaning regularly.


Experiences of dehumidifier owners on Facebook

Many dehumidifier owners post enthusiastically on social media about their machine. Posts generally provide some information e.g.

  • how much water is collected by the dehumidifier.

  • how low their indoor humidity levels are.

  • the number of hours the dehumidifier runs for.

BUT without more information it’s impossible to know how well their dehumidifier would work in a different setting, i.e. in your home. Other factors include: The nature of the moisture generating activities, home use patterns, standard of insulation, draughtiness of the home etc.


Independent test evidence on how well they work to reduce damp issues

I can find no evidence on-line of any thorough independent testing of the costs and effectiveness of using a dehumidifier to help with damp in a home.


I have only been able to find TWO credible and useful real-world experiments by members of the public. They are here:


Fix my Roof

This video shows testing of a compressor dehumidifier in a lounge with the door shut. In a closed room like this it is clearly very effective, with running costs in this case around 11p an hour.


Charlie DIYte

As part of his very good YouTube video a dehumidifier is tested in Charlie's family home.


The Dehumidifier is measured and monitored thoroughly over the course of two weeks.


The home had a decent amount of ventilation, and the family had a good focus on minimising damp going INTO the air. The results were surprising.


Humidity levels in the house were low during the period when the dehumidifier was used. Charlie then had a surprise....that the humidity levels were ALSO low for two weeks after the dehumidifier was turned off.


Charlie concluded that the dehumidifier extracted a lot of water over the two weeks, but that it was water that was being sucked in from outdoors. This is entirely plausible.


He felt that the results showed that the dehumidifier didn't actually make much difference.

 

What the experts say about dehumidifers

Toronto University

The view of an, Jeffrey Siegal, an expert from Toronto University is:


“Dehumidifiers are fine in general,” Siegel says. “The challenge is that they’re energy-intensive and they don't fix the root of the problem. They’re a stopgap. They have their role, but people shouldn't use a dehumidifier and think ‘problem solved.’


Richardson and Starling - UK experts in building preservation

Quotes from their web article here


'We understand why these products can look appealing as they promise quick and most importantly cheap results. The important thing is to understand that using a dehumidifier is more like plucking leaves when you need to cut the whole branch to solve the problem completely'


'...domestic dehumidifiers can only make a moderate impact on relative humidity and can only do so in rooms where the dehumidifier is present'


Douglas Kent - UK Chartered Building Surveyor

From this blog article about dealing with damp issues in UK homes

'The permanent use of dehumidifiers is a poor substitute for efficient heating and adequate ventilation when it comes to dealing with damp'

 

Alternatives to dehumidifiers to take moisture out of the air

There are many products on the market that are essentially bags or containers of chemicals that absorb moisture from the air. These are usually types of salt crystals.

These have several big weaknesses:

  1. As mentioned before, several litres of water goes into the air in a typical home each day. Products like these can only extract a small amount of water in localised spots, such as on a windowsill. They cannot deal with severe or 'house scale' issues of damp air.

  2. Once these products are 'full', they either need to be thrown away or replaced. Some can be dried out and reused, but drying uses energy and puts that captured moisture back into the air.

  3. For these to work effectively you need to keep the window and door closed in the problem room. Some people leave the window or trickle vent open near these products. In these cases, if the damp issue improves, it is far more likely to be because of the ventilation than because of the use of one of these products.

  4. They are not particularly cheap to buy or use. Ventilating, and living with a small amount of heat loss would probably be cheaper.

  5. There are very mixed opinions on them both online and social media. Some people are passionate about how well they work. Others say they don't have much impact.

All costs above assume unit cost of energy of 27p/kWh from Oct 2023

 

Quick links to the other main articles on this subject.

Other supporting articles on damp:

 
 

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Thanks.


Mark Thompson


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