Saturday, December 22, 2012

Dehumidifier vs Desicant

 rev. 4-19-2014

This fall I noticed something I had not seen before; the dreaded black spots, this time on cabin carpeting. Clearly, better humidity control was in order for the cooler season.

The water runs into the sink via a small hose;
Stick the hose in a bottle, which over flows 
into the sink, and you will have all the battery
water you will ever need!
A large compressor unit would be a beast on a small boat. Though I have a 30-pint unit at home, the downsides are major:
  • A lot of amps on the shore power cord.
  • A back injury waiting to happen; the unit is huge and awkward.
  • In the way.
  • Loud.
  •  More capacity than I needed.

So I looked around and found this. The principle is elegant, with very few moving parts. The size and weight appropriate. They are prone to icing (found this while testing in the garden shed in cool temperatures) but deicing by running it off a light timer is simple; 10 hours on and 14 hours off. A photo cell lamp switch would work just as well. I needed a drain so that the tank did not fill and potentially freeze, something easily provided with a few fittings, some aquarium tubing, and a gallon jug on the floor (it could go into the sink, but I'm taking measurements). What if it stays below freezing a few days and does not melt? There is no problem with the tank or hose freeing because there is no water flow. There is little bother with excess ice build up, because at those temperatures there is little water in the air. So far--2 months--the units is removing a few ounces per day and keeping the relative humidity at 65% at 40-50F temperatures. I can reasonably project that it will remove 5 gallons (42 pounds) during the November-March season. Bear in mind that all dehumidifiers are very inefficient at low temperatures; this unit would pull 20 ounces per day in a typical basement running 24 hours.
 Cold plate thawed.

Cold plate frosted. If below 55F this will occur in 3-8 hours. Upon melting, roughly the daily water removal capacity. Can the ice grow to a damaging extent? I left it on in icing conditions for 3 weeks once and the ice had not reached the screen. The ice provides so much insulation it simply stops working.

The only real alternative is calcium chloride desiccant. I did an article about these for Practical Sailor  magazine and still had a few left over. I'm holding a race, one on port, one on starboard. The reality is that it would take 10 5-pound buckets of CaCl2 to do what this unit will do over the next 3-4 months. The bag I show will take 3 months to pull about 1-pound of water from the air, based upon prior testing.

Perhaps I'll run it all year. The mattress and bedding are already noticeably drier and more pleasant.


Notes 2-14-2013: So far? In 3 1/2 months the CaCl2 bag has removed about 1 pint of water to the dehumidifier's 16 pints. Not even competition. And the bedding smells fresher than it ever has. The relative humidity has been holding at about 65%.

Notes 9-20-2013: Still running fine. If I set the timer on a shortish cycle the solar panels replace the drain just fine.

Notes 4-19-2014: I've been running it all-year for over 2 years. Still quietly purring along. I started running a Perfect Home 2 pint unit a year ago in another boat. No problems.


  1. I've been doing some research on this topic.
    Have a look at the "Sailing today" dehumidifier review:
    Sailing Today Dehumidifier Review

  2. The "sailing Today" link is

  3. I personally think dehumidifiers are the best machines for moisture removal. They come in different sizes and can fit in most spaces where they are needed. Dehumidifiers are efficient at low temperatures but they work the best between the temperatures of 64-90 F. However, some dehumidifiers do work at low temperatures which makes them perfect for basement use. I installed one in my basement because my children have bad allergies and they worked great. Now it is impossible for molds to grow. I would highly recommend!