Dehumidifier Advice

by Mark at a Public Message Board

(found at a public message board) "pelmark" wrote:

What to look for is "AHAM" rating, which is measured at 80 degrees F., and 60% RH, which is far closer to what you are actually going to be dehumidifying. ("AHAM": American Home Appliance Manufacturer's spec of 80 degrees F., or 27 C., at 60% RH, or "relative humidity.")

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AHAM may not be voluntarily provided, as AHAM is a much truer measure of what your dehumidifier will actually remove in the humidity conditions found in normal homes and structures, even when humidity levels are high, and as you obviously do, the homeowner wishes to dehumidify. I live in the subtropics, the southern tip of Florida, and I seriously doubt that "up here" you experience humidity levels we do.

I am doing a major flood right now in which the home's RH is 78%, and is soaked. I am talking water surfacing when you walk on carpets from the cushion (pad), and dry wall soft enough to push holes through with light fingertip pressure. Even at this level of water present inside the structure, my dehumidifiers will not reach their maximum stated extraction; the temperature is too low (76 F) and so is RH at 78%. I have a dehumidifier rated at 24 gallons which is probably pulling 18 gallons every 24 hours. That's a guess. Now, I will get this to increase over time, and for awhile (until significant moisture removal is accomplished), by using air movement to raise RH.

Its rating is established at 90F @ 90% RH, and it has a 12,700 BTU compressor, and is moving 450 CFM. If I get the indoor air to 90F, I will increase RH to probably 90--95% (water will evaporate to the air), but I then run the risk of really activating biological growths, and more damage to the structure and contents.

So I add blowers to get water vapor to the air, which then increases RH levels through evaporation, and my dehumidifier removes the water vapor. Air movement alone, will indeed dry out a structure, but it will take far longer which increase damages, both structural and contents. And without air movement, dehumidifiers will never dehumidify adjacent rooms unless the moisture laden air is *brought* to them.

It would, however, allow the dehumidifier to remove more water in 24 hours. Warmer air carries more water than cooler air. Air movement gets water to water vapor to air to increase RH, and moves this air to the dehumidifier.

Previous message(What I know is that I've seen. For the size of building and approx. humidity,the recommended unit was way too small. IOW, the "recommended" 25 unit would shut off during the night)

Because if at MAX, it removes 25 liters per day, then at the humidity levels in you home which are closer to 80F and 60% RH, it is actually removing approximately 13 litres per day. Why it shuts off, depends on the unit. Tank filled?

Your 50 litre per day dehumidifier is assuredly rated at high temperature (90-95 F) and maximum humidity of 90-95% RH.

Unless your structure is at those levels, it will never extract that much water.It is simple, really. Your advice was correct: get a larger unit, because the rating is deceptive as to capacity. We are differing on the why sort of.

(Ergo, less humidity removed unless I get up to empty the tank or have it hooked to a drain (not yet)).

Perhaps you have the owner's manual? See what it says (if anything) on *how* it is determined the unit removes "50 litres" per day.

You may misunderstand me: I am saying that if you need 25 litres of water per day removed from the structure, you will require at least a 50 litre per day dehumidifier. Not a dehumidifier which is rated at 25 litres per day, maximum water removal, which will only remove 13 litres. And dehumidifiers are sold rated at "maximum water removal", NOT what they will remove in a 75 degree house.

The marketing on dehumidifiers is simple: they state the maximum amount of water a dehumidifier will remove, but it is not stated that this amount requires saturation which is high temperature and high humidity, much higher than what is in your home, unless your home is at 90F and 90% RH.

There is not a home dehumidifier unit, excluding HVAC system units, sold by any retailer which matches the smallest commercial building dehumidifiers, and the smallest of these are mostly rated at 14 gallons per day, or 7.25 gallons at AHAM. They also cost $775 US.

Dehumidifiers NEVER match what they are rated at, per sales claims, UNLESS they are placed in structures where those conditions they are rated at are met. In drying out buildings for eight years, I have never encountered a job in which I walked in and found the air at 90F and 90% RH, and that includes a telephone switching four story building in which a two inch water main broke and ran for several hours before discovery; a very hot, wet building. I measure every job with a digital hygrometer which is accurate to 1% of RH and temperature, and I can tell you *exactly* how much water I have removed from a structure after I compare a few hours of running, and the inside/outside temps/RH's. It is the science of psychometry, and removing water from air is simple physics.

Buying dehumidifiers is simple, also: compare apples to apples, not oranges. What will the dehumidifier remove at your needed point. If your home is kept at 75 degrees F, and the humidity level is 60%, how much water will it remove per day? What is the RH in the home, and what do you want it to be? The cooler the air, the less water is easily removed. Below 33 F (and usually in the forties), refrigerant dehumidifiers don't even work, water will not condense from the air in these types of dehumidifiers at these temperatures.

AHAM is used *precisely* to eliminate these overblown claims of what dehumidifiers will remove. I don't *care* what it removes at some theoretical temp/RH levels it will *never* actually be exposed to; I distinctly care about what it will remove in the building I put it in.

Mark

Editor's Note: See our selection of dehumidifiers at AllergyBuyersClub.com

First Published: late June 2002
Updated: April 2007

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