How do you compare two dehumidifiers, or even worse, how do you compare 102?

One of the measures used is the capacity. Every dehumidifier is an x pint or a y pint unit. That means it will take out x pints or y pints in a day, right? No, wrong.

Most home dehumidifiers have a fan, a compressor and condense water vapour on to cool coils. These "refrigerative" dehumidifiers are the type we are discussing here.

Dehumidifiers are tested for many things including the "water extraction rate", measured in pints per day. The problem is the water extraction rate varies according to two factors; the air temperature and the relative humidity of that air.

Imagine a sponge, that's the "air". You soak the sponge completely in water. You take out the sponge and you squeeze it once in your hand. That squeeze is one "day". How much water would you get out in one "day"? Enough to soak your shoes?

Next, take an identical sponge, completely dry. Fill a thimble with water and carefully pour the water onto the sponge. Repeat the single squeeze and see how much water falls from the sponge; a few drops at most?

Imagine a sponge that grows as it gets warmer. Let us say that at 80F degrees it is twice the size it was at 60F degrees. It seems fair to say that one squeeze would get twice as much water out of a "full" sponge at 80F than it would at 60F.

Air is rather like that weird sponge. It can "hold" twice as much water at 80F degrees as it can at 60F, and twice as much at 60F as it can at 40F, and so on. To put it another way, if the relative humidity at 60F degrees is 80%, and the temperature rises to 80F degrees and the absolute amount of water stays the same, the relative humidity is halved, to 40%.

Now this little article does not pretend to be the last word on the subject of relative humidity. If you would like a qualified scientific explanation you could start with this page at Wikipedia - http://en.wikipedia.org/wiki/Relative_humidity. All that's needed here is to understand that this is how the air and moisture thing works, not why.

Because this is how it works the Association of Home Appliance Manufacturers (AHAM) specifies a standard set of conditions for testing the water extraction capacity of dehumidifiers. This standard is used by all dehumidifier manufacturers who have signed up to the AHAM conditions.

The conditions specify a temperature of 80F degrees and relative humidity of 60%. That way anyone comparing two dehumidifiers from two manufacturers, both signed up to AHAM, knows Manufacturer A's pints were collected under the same conditions as Manufacturer B's pints and can make a fair comparison.

So, what about Manufacturer C? He didn't sign up so how were his pints collected? Well he may use the AHAM standard, and if he does he'll probably want you to know. If he doesn't use that standard he should tell you the standard he does use.

Let us take a real case. Manufacturer A makes a 50 pint dehumidifier. He is signed up to AHAM and his unit was tested at 80F degrees and 60%RH. Manufacturer C makes a 50 pint dehumidifier. He is not signed up to AHAM and his dehumidifier was tested at 90F degrees and 80%RH. Manufacturer C is an honest man and also publishes test results at 80F degrees and 60%RH, at which his "50 pint" dehumidifier becomes a 25 pint unit.

I could name the manufacturers but I won't. The point of this discussion is to make it easier for you to compare dehumidifiers on a "like for like" basis.

Next time you are looking for a dehumidifier you will probably want to know the conditions under which it was tested.

If the information is "not available", or the seller will not tell you, my suggestion is that you halve the stated amount and then compare it with all the others.

After all, who really wants a 50 pint dehumidifier that only extracts 25 pints, or less?