by Chris Woodford. Last updated: September 5, 2016.
Is damp climbing up your walls... and
driving you up the wall? Few things make a home quite so
unpleasant. Whether you have a damp problem with your building (rising or penetrating damp)
or your moisture comes from cooking or drying laundry inside (condensation),
the result can be a horrible musty
smell, mold growing on your walls (and on your clothes), and
a greater risk of respiratory illness. It can take time to sort
out a major damp problem so what do you do in the meantime? One
solution is to invest in a dehumidifier: an electric gadget that
removes moisture from the air. Let's take a closer look at how they
Photo: A typical home dehumidifier.
A machine like this costs about $200 (£100) and uses about 190 watts of electricity
(slightly more than three ordinary 60-watt lamps burning at once), so it's reasonably
economical to run. This one is a DEM10 made by DeLonghi, though many other makes and models
What is humidity?
Most of the time we don't give a moment's thought to the atmosphere in our homes (or outside);
why should we, it's invisible! If we think about it at all, we tend to think of it as
a gas. Look up at the sky and you see a different
point of view. Clouds whizzing over your head are a sure sign that the air contains
water, either as a vapor (if it's dry) or liquid (if it's actually raining). But the
same is true inside your home. You might think the air is dry—and if you have
central heating, it might even feel that way—but there's a huge amount of moisture
around you too. If you dry laundry inside your home or do a lot of stove-top cooking
without proper ventilation, the humidity levels can be surprisingly high.
Condensation on the windows (or, even worse, water dripping down the walls—as it
sometimes does in my kitchen) is a sure sign of a humidity problem.
Why does indoor humidity matter?
We're walking water bags—our bodies are typically 60 percent water—but that doesn't mean our homes should be like fish tanks! High humidity can cause all kinds of
problems. It can make clothing go moldy in your cupboard, it's bad for computers
(it can cause rusting or short circuits inside their cases) and optical equipment (that's why things like
cameras and binoculars are sold
with water-absorbing sachets of silica gel)—and it's bad for your health too. According to a scientific review of
the health effects of humidity published in 1986 by Arundel et al,
high levels of indoor humidity can encourage a flourishing of bacteria, viruses, mites and fungi, and more respiratory infections and sicknesses:
"The majority of adverse health effects caused by relative humidity would be minimized by maintaining indoor levels between 40 and 60%."
Photo: Water-absorbing silica gel, often packaged in little paper bags inside camera and
binocular cases, effectively tackles small amounts of humidity. But it's no help if you have a major excess of moisture in your home.
For that, you need a proper dehumidifier.
What does a dehumidifier do?
A dehumidifier is a bit like a vacuum cleaner: it sucks in air from
your room at one end, takes the moisture out of it, and then blows it
back out into the room again. The moisture drips through into a
collection tank that you have to empty, from time to time. How is the
moisture removed? That's where a dehumidifier is more like an
air conditioning unit (sometimes called an air-con or HVAC,
which stands for heating ventilating air conditioning unit), which, itself, works a bit like a
refrigerator! Confused by all these appliances? Let's look inside a
dehumidifier and find out what all the bits do.
How a dehumidifier works
Dehumidifiers work in one of two ways—by refrigeration (cooling air to remove moisture using similar technology to a refrigerator) or by absorption/adsorption (where moisture is absorbed into or adsorbed onto a drying material and then removed). We'll look at each of these in turn.
- Warm, moist air is sucked in through a grille on one side of the machine.
- An electric fan draws the air inward.
- The warm air passes over freezing cold pipes through which a coolant circulates. (Note: We've simplified this part of the machine quite a lot. It's like a mini air-conditioner or refrigerator endlessly circulating coolant with a
pump and compressor.) As the air cools, the moisture it contains turns back into liquid water and drips downward off the pipes.
- Now free of moisture, the air passes over a heating element
(similar to the one in a fan heater) and warms back up to its original temperature.
- Warm, dry air blows back into the room through another grille.
- The moisture that was in the air originally drips down into a collecting tray (or bucket) at the bottom of the machine.
- A plastic float in the machine rises upward as the collecting tray fills up.
- When the tray is full, the float trips an electric switch that turns off the fan and switches on an indicator light telling you the machine needs emptying.
Not all dehumidifiers work by refrigeration. Some work by "mopping" the water out of the air
with a water-holding material and then "squeezing" the water away to remove it. The mopping action involves either absorption (where the water soaks right into something) or adsorption (where the water is picked up on the surface of something).
I haven't been able to discover if there was an original
inventor of the dehumidifier—someone who first came up with the idea of removing water from air with a machine—or
if the idea evolved gradually over time. Willis Carrier, pioneer of the air conditioner, is certainly one
strong candidate; his machines were largely based on refrigeration technology and, although
described as "air conditioners," could remove moisture from air too. Another early contender is James
Locke, who produced this ingenious absorption dehumidifier and air conditioner for Honeywell in 1939/1940. I've colored it in and picked out a few
of the key components to illustrate the basic principle, but I'm going to gloss over the details.
How does it work?
- Moist air is drawn in from the room through a duct.
- The air moves past a large rotating wheel made of water-absorbing material, which removes the humidity.
- The air is drawn by a fan operated by an electric motor.
- Dry air is blown back out into the room.
- An air duct underneath is kept hot by an electric heating element (yellow).
- The moisture-absorbing wheel rotates through the heated air space and has hot air blown past it to dry it out.
- The air is sucked past by a fan and electric motor similar to the one up above.
- The hot, wet air is blown out through an exhaust duct.
As you'd expect from Honeywell, the whole thing is controlled by thermostats and humidity sensors so you can make the room as hot and dry as you wish. The black lines you can see on the right and bottom of the picture mostly show
electric circuits controlling the machine. The main room thermostat is at the top, shown in gray.
If you're interested, you can find more about how it works by reading Locke's
US patent #2219547: Air Conditioning System.
Artwork: Diagram courtesy of US Patent and Trademark Office.
How do you compare dehumidifiers?
According to The Code of Federal Regulations of the United States of America, there are two key measurements you can use. The simplest one is how much water a dehumidifier removes, measured in liters (or pints) collected in 24 hours of continuous operation: bigger machines will obviously draw out more water than smaller ones. If you have a lot of water to shift very quickly, perhaps after a river flood, this will be the best measurement to go by. If you're using a dehumidifier for much longer (perhaps on a permanent basis, because you live in a damp basement), you'll also need to study how much power your machine consumes. The best measurement in this case is the efficiency of the machine (sometimes called its energy factor), which is how much water it removes divided by how much power it uses, measured in liters (or pints) per kilowatt hour (L/kWh).
Find out more
On this website
- Air conditioning: A step up from basic dehumidifying! See photos and a diagram of the inside of a typical air conditioner.
- Clothes drying: Here's the science behind getting your clothes dry. If you dry clothes indoors, you might as well hurl buckets full of water into your home. But what are the alternatives? Does outdoor drying actually work? And what do you do in winter?
- Heat-recovery ventilation (HRV): How can you ventilate your home without letting all the valuable warmth escape? HRV uses heat exchangers to bring in fresh air without wasting heat.
- Does using a dehumidifier help to reduce heating costs?: by Leo Hickman, The Guardian, 20 December 2011. A reader debate about the merits of using dehumidifiers.
Squeezing Humidity Out of the Air by Matthew L. Wald. The New York Times, 12 August 2011. Describes the dessicant-based air-conditioning and humidity control systems produced by Advantix Systems.
- Damp homes 'could cause asthma': BBC News, 4 March 2007. There are links between damp homes and asthma, although it's uncertain whether damp is a cause or a trigger for asthma caused by other factors. (House mites tend to thrive in damp homes.)
- How to dry out a flooded house by Jay Romano, New York Times, 9 July 2006. Some simple tactics for reclaiming and restoring your flooded home, including dehumidifiers.
- Dehumidify to control moisture laden air/build your own dehumidifier: Popular Science, August 1961. This an old article from Pop Sci, but the advice is still good today.
For those of you who like the real technical nitty-gritty, patents are a good place to start. Here's a small selection: