Saturday, December 22, 2012

Dehumidifier vs Desicant

Four years ago, in the 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.

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 the Eva-Dry 2200. The no-compressor  principle is elegant, the only moving part being a small fan. The over simplified explanation is that of a thermocouple, but in reverse. Instead of a temperature difference creating a small electric current, the current is forced in the reverse direction through a whole stack of thermocouples, creating a cold plate. Water condenses on the cold plate and runs into a tank.

The size and weight (just a few pounds) are appropriate; when not in use it's easy to swing it into a cabinet for storage, although for day sailing I just put it in the sink to keep it from falling. They are prone to icing in temperatures below about 45F, but deicing by controlling the run cycle with a light timer is simple; 8 hours on at night and 6 hours off during the day. Even in the depths of the winter, the interior of the boat warms above freezing during the day due to solar heating.

I modified the tank by drilling a hole and tapping in a 1/4 NPT x 3/16" ID hose barb. The hose runs to the sink so that it never fills and to prevent freezing and breaking in the winter.

 So far--four years--the unit has just  purred along, removing 2-8 ounces per day, keeping the relative humidity at 45-65% at temperatures from below freezing to 95F. The boat is always dry, the bedding fresh, and interior corrosion issue non-existent.Just like home.
 Cold plate frosted

Cold Plate Thawed 

The only real alternative is calcium chloride descant. I did an article about these for Practical Sailor  magazine and still had a few left over. I held a race several years ago, the Eva-Dry in the port cabin and a large absorbant bag on starboard. It wasn't even close. It would take ten 5-pound buckets of CaCl2 to do what this unit will do over 3-4 month winter season.

I also tested a cheaper version from Perfect Home. It lasted about 8 months, just like the reviews said it would.


  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!