Features June 2013 Issue

Dehumidifier Field Tests

Dehumidifers, desiccants, heaters: What’s best on a boat?

Dampness and boating seem to go together like picnics and ants— not dangerous but certainly irksome. We recently tested desiccant driers (PS, November 2012) and contrasted those to ventilation and gentle heating as a means of preventing condensation on board. Each method has limitations. Desiccants are slow, have limited capacity, and they need to be replaced every three months or so, which adds to the cost. Ventilation works in dry-to-moderate climates, but often fails in very humid areas, cool and damp areas, or when the surrounding water is much colder than the air. Heating doesn’t remove humidity, just prevents it from condensing in the warmed portions of the boat, so the condensation often finds its way into the back of a locker or under a mattress. Sometimes, we need something a bit more robust.

covering boat
Photo courtesy of Mahina Expeditions, www.mahina.com

Once boats are buttoned up in humid climates, the battle against mildew begins. Keeping humidity below 65 percent is adequate.

Most of us are familiar with compressor-type dehumidifiers. Common in basements and storage areas, they are big, loud, and usually have rollers to move them about—not at all suitable for the average cruising sailboat, where anything that is big and not built in eventually gets in the way.

This fall, one of our testers experimented with a very compact and inexpensive thermo-electric dehumidifier, the Eva-Dry 2000. Small, light, quiet, and with only one moving part, it seemed like a good fit for a 32-foot catamaran that spends every winter in cold Chesapeake waters.

While the tropics have more severe humidity in the summer, humidity can be a serious problem during the change of seasons in cooler climates. Within a few days, the weather can swing drastically—from sunny 85-degree daytime temperatures to frost warnings at night. The boat is saturated with 85-degree air and then, as the temperature drops to 40 degrees or less, the water has nowhere to go but into the upholstery or dripping down the portholes. And in the depths of winter, when you need to button up the boat, the dampness has nowhere to go.

Dehumidifier Choices

When selecting a dehumidifier, a boater has two choices: compressor dehumidifiers and thermo-electric dehumidifiers. A compressor dehumidifier operates like an air conditioner; air is cooled over the expansion coils and water condenses. However, instead of exhausting the heat outside, it is used to re-warm the air, and the condensation drips into a collection bucket.

DampRid desiccant
Photo courtesy of manufacturer and Drew Frye

Desiccants like DampRid and the Absortech bag don’t require power, but long-term cost efficiency lags far behind that of powered dehumidifiers.

Air conditioners and dehumidifiers have other notable differences. To reduce dripping, window air conditioners are generally designed to evaporate much of the condensate back into the warm air. Air conditioners lack a defrost cycle because they are not operated in cold temperatures, and they are controlled by a thermostat instead of a humidistat. Compressor dehumidifiers can operate at all temperatures; manufacturers often state a minimum of 40 degrees, and the units don’t remove significant water below that temperature anyway. Cold air has very little humidity, and whatever exists can immediately frost the coils. Compressor dehumidifiers are available in large capacities for heavy-duty drying.

There are also marine-rated compressor dehumidifiers, such as the Mermaid Dry-Pal, that are simpler, ruggedized versions of home dehumidifiers. Although we did not test these, two owners that we interviewed reported few problems and good performance.

Thermo-electric dehumidifiers operate without moving parts, other than a small fan to move air past the cold plate. Like the thermo-electric refrigerators we’ve tested (PS, April 2007) these dehumidifiers operate by the Peltier Effect.

The Peltier Effect is named after Jean Charles Peltier, a wealthy French tinkerer, who in 1834  discovered that when an electrical current is passed between two different metals, heat transfers from one metal to the other because electrons pass through different metals at different speeds. In 1838, Russian Emil Lenz advanced the concept by passing a current through a bismuth-antimony joint and froze a drop of water. More recently, astronauts have benefited from the Peltier Effect, which is employed for the cooling and heating of food.

Peltier dehumidifiers are very simple and do not contain a humidistat, which isn’t needed because of its limited size; nor does it have a defrost cycle, one of the system’s potential limitations. For cruising sailors, one of the most appealing factors is their simplicity. In the adjacent table, we offer a generic comparison of the two types of dehumidifiers—Peltier effect and compressor-type. 

Some makers advertise small electric heaters like the Goldenrod as dehumidifiers. Raising the temperature reduces the relative humidity, so these devices can work in small spaces, but they don’t actually remove any water. Desiccant driers, which typically contain calcium chloride, are also advertised as dehumidifiers. Desiccant driers have such limited capacity that they would be more honestly marketed as passive chemical absorbents.

Absorbag from Absortech

Absorbag from Absortech

What We Tested

Our principle aim for this test was to compare two “active” systems for removing moisture—compressor dehumidifiers and thermo-electric dehumidifiers—with each other, and then compare them with the passive desiccants we looked at previously. Our compressor dehumidifier was a 40-pint Sears model (Kenmore/Hampton Bay Model BHDH4000ASO) that was unfortunately discontinued after our test had begun. We found several comparable dehumidifiers with similar specifications. The Danby Premiere 45 Pint Dehumidifier (model DDR45P), available from Home Depot (www.homedepot.com) for about $250, roughly matches the specs and appearance of our test unit. Sears and Amazon.com sell the EdgeStar 40 (model DEP400EW) for about $142.

Representing thermo-electric technology was the EDV2200 Electric Peltier Dehumidifier from Eva-Dry, which sells for only $70 online. Judging by the product marketing, users can save a substantial amount on upfront costs.

We also included two desiccants, a small hanging Absorbag from Absortech, which uses calcium chloride, and a high-capacity tub of DampRid, another calcium chloride desiccant. Both of these products were also evaluated in our November 2012 comparison.

How We Tested

We had two test beds: The garden shed used for desiccant testing in the November 2012 article was used again for side-by-side comparison of the compressor dehumidifier, thermo-electric dehumidifier, and the desiccants. We compared performance at warm, average, and low temperatures.

On the water, we used two identical staterooms on a catamaran to test the desiccant and thermo-electric unit through a Chesapeake Bay winter afloat. We measured water removal under relatively controlled conditions in the shed, and then again during on-board use. The test boat has a dry bilge and was sealed up tight except for a slight gap in the companionway that is protected from rain by a hard top. The boat was sailed about every two weeks, including a few two- to four-day cruises. Our tests did not simulate a situation with extensive onboard cooking.


Among the compression humidifiers, the Sears 30-pint unit was a workhorse, removing large amounts of water and operating well at all temperatures. Moisture removal dropped dramatically as temperatures fell, but that was primarily a consequence of the low humidity of cold air. For families living aboard and with access to shorepower or a genset, this may be the most practical solution.

In warm conditions, thermo-electric units can be expected to perform according to manufacturer specifications, but none we researched have de-icing cycles, so none are rated for use below 65 degrees. Below that temperature, frost begins to form on the cold plate, creating an ice block that remains until warmer temperatures allow melting. When ice covers the fins, the unit doesn’t work and is vulnerable to damage.

We combated this icing by providing a defrost cycle. To do this, we plugged the unit into a simple lamp timer and set it to run eight hours each day, allowing it to thaw the remaining 16 hours. We also tried controlling operating hours with a photocell lamp switch, so that the dehumidifier would go on at night, when temperatures are lower and relative humidity is higher. Eventually, the Eva-Dry would frost up, shut off (automatically controlled by the photocell switch), and then defrost during the day when temperatures rose. This setup reduced the operating hours, but it allowed the unit to function every day, except a few, without frosting up. The exceptions were particularly cold days, when the unit remained frozen through the day. On those days, there was so little moisture in the air that the frost build-up was minimal, and there was no damage to the unit.

While it is possible to build a 12-volt DC adapter cord as we did to power these units, we didn’t find this practical. Generally, the draw is too great for a boat not on shore power, and we needed to use a 110-volt timer to prevent icing. There are 12-volt timers as well, but it is simpler to use these as 110-volt appliances.

We also realized that the Eva-Dry would need a drain in order to operate continuously and to prevent standing water from bursting the tank in subfreezing weather. A drain was easily fitted using a common compression fitting and a length of vinyl hose. We were concerned that the water in the hose would freeze, but because the unit removes very little water in sub-freezing conditions, there was not enough accumulation to cause damage. The unit operated from October though April without interruption.

Eva-Dry 2200

The Eva-Dry 2200 (above) is smaller and lighter than most conventional compressor-type dehumidifiers, such as the Danby Premiere. It also uses significantly less power.

For use in moderate temperatures, there are calcium-chloride desiccants, which were well covered in the December 2012 issue. Calcium chloride is available in a wide range of packaging styles—gels, beads, crystals, powders, etc.—but they all work on the same principle. We have learned that calcium chloride, like all dehumidifiers, virtually shuts down when the temperature drops below 45 degrees. In part, this is due to lower absolute humidity, but there are also changes in crystal/solution equilibrium, and that makes the reaction rate 100 times slower in the winter than the summer. Don’t expect calcium chloride to do much for you in the cold season, if there are any moisture leaks. It can sit there for a month, doing nothing.

Sears Dehumidifier

Our Sears 30-pint dehumidifier was a 1995-vintage compressor-type unit. It has been used to keep a storage shed dry for 17 years. In our warm-weather testing under conditions very close to those used for rating dehumidifiers (80-degrees and 70-percent relative humidity), it still performed very near to its maker specification. We would consider this to be a legitimate representative for all dehumidifiers of this type.

It was a bit loud to have in a small space, and it also produced a significant amount of heat; this was only a problem as outside temperatures rose in the spring. This specific model is no longer in production, but we found several others that matched its specifications in the same price range.

Bottom line: This technology is excellent for larger boats in very wet environments and for live-aboards. We Recommend it.

Mermaid Dry-Pal A marine-rated compressor dehumidifier, the Mermaid Dry-Pal is basically a more simple and rugged version of a home dehumidifier. We did not test this unit, but the owners we spoke with were happy with theirs. One unit had been in service for four years, the other six years, both without trouble. It has no defrost cycle, but since the owners are  live-aboards, this was not an issue.

Bottom line: At about $500, it is twice as expensive as a home dehumidifier, hard for most sailors to justify. But for those needing higher capacity for long-term use, this is a product worth looking into. We will try to test it in the future.

Eva-dry 2200

In warm conditions, the Eva-Dry 2200 performed according to manufacturer specifications, removing one to two pints of water each day. However, just as the manufacturer stated, icing began at 60 degrees and became severe at 55 degrees. We successfully controlled icing by restricting operation to eight hours per day (as described above), and it still removed about seven times more water than the calcium-chloride desiccant drier we had hanging in the adjacent hull.

Danby Premiere

Danby Premiere

Although there is no humidistat in the unit, we found the Eva-Dry to be effectively self-regulating. As temperatures drop, there is less water in the air, and drying slows. As humidity becomes available and temperatures rise, capacity increases. The relative humidity stayed between 62 percent and 68 percent at the storage temperature—if the heat was turned on in mid-winter, relative humidity would drop as low as 30 percent—at all times, low enough to prevent condensation and mildew. A timer is also another way of providing some regulation.

To improve fire safety—always a concern with unattended operation—we placed the unit on the stove, and the power cord and 100-volt/12-volt adapter in the stainless sink next to it. To prevent standing water from bursting the collection tank in subfreezing weather, we added a drain hose. This was easily fitted using a common compression fitting and a length of vinyl hose that drained into a container for measurement; in actual use, this drain would go to the sink.

Did the hose freeze-up? Because the unit removes very little water in sub-freezing conditions, there was not enough accumulation to cause damage. (No dehumidifier or desiccant will remove much when it’s below 45 degrees as there is very little water vapor in the air.) What is the northern limit of this technology and defrost method? We believe it would work in any location where the water does not freeze for more than a few weeks; beyond that, we are uncertain.

Bottom line: Recommended for cruising boats 30 to 40 feet that are kept in the water.

Absortech Absorbag

The Absortech Absorbag functioned in much the same way it did in the December 2012 testing, passively removing water at a very slow rate. As temperatures dropped into the 40s, absorption practically stopped. It removed only a few ounces in January. This allows the unit to last through a season, but it cannot keep up with humidity generated by air leakage or even water evaporating from the hull.

Instead of removing gallons through the course of a season, it removed only about one pint, and most of that in the warmer months, never reaching saturation even after five months. Thus, its usefulness is limited to boats stored on land where electricity is not available, or for compartments that are kept closed. If more capacity is needed, consider four to eight of the more economical Damp-Rid High-Capacity desiccant tubs reviewed in the December 2012 article. (DampRid also sells a range of gel and hanging products that are less prone to spilling.) Adding a heating element like the Goldenrod, or even a simple light bulb, to confined areas or near a vent to promote air circulation can also help fight mildew.

Testers have also changed our opinion of the spill potential of the Absortech bags. The bag has lived at the foot of the most heavily used bunk through a five-month winter season, been bumped by sailors and duffels, and sailed in violent Force 7 conditions (we forgot it was up) while nearly full, without spilling a drop.

Bottom line: Recommended for cruising boats 30 to 40 feet that are sailed in the winter, though larger boats will certainly need multiple units.


Within four weeks of the start of the test, the faint but tell-tale mustiness we had lived with since we began boating 25 years ago vanished from the pillows, bedding, and stored clothing. That is the real bottom line. (The tester’s wife loves it.) All of the units we reviewed are useful tools, each with a proper application. Even though thermo-electric dehumidifiers are not intended for operation in cold climates, we’ve proven that with simple modifications, they are well suited to use aboard moderate-size boats that are stored in the water. We think there is an opportunity for some enterprising manufacturer to develop a marine-specific product.

What about summer? Desiccants become useless, unable to cope with the volume of moisture, but our warm-weather testing suggests the Eva-Dry 2200 will have 20 times the capacity of the Damp-Rid High Capacity Absorber and 70 times the capacity of the small Absortech bag. Most sailors rely on ventilation, but we’re going to button up tight and continue running the Eva-Dry 2200 on a timer. By adjusting the timer, we’re hoping to keep the relative humidity near 65 percent, a practical maximum for reducing mustiness. We’ll report back in the fall.

Comments (7)

Interesting Article. I like your comment suggesting that a marine friendly product should be developed. Two questions. 1. Could you make a table perhaps with specific reference to the energy and power used by these products. As a function of the amount of water removed? 2. As the resultant water is really a pure condensate - could a system, be rigged up to feed the water to your tanks. ie. A mini water maker? Or is the water amount so little not not make sense? Thanks. Peter

Posted by: peteh007 | June 24, 2017 12:35 PM    Report this comment

Thanks for the informative article. I want to suggest that PS incorporate some of the useful body of consumer feedback found on Amazon. When researching the Eva-Dry 2200 on Amazon I found a number of customers reporting that the transformer was overheating and even "smoldering". That seriously scares me and immediately disqualifies that item for consideration for my 38' fiberglass boat. Is PS really comfortable recommending this device to sailors when stories like that are floating around the public domain?

Posted by: Brice D | May 21, 2017 12:21 PM    Report this comment

Why not include some photos of how you modified the Eva Air 2200 for continuous drainage. A picture is worth a hundred words. Perhaps its time to start a YouTube channel as well.

Posted by: MDT | September 5, 2016 7:18 PM    Report this comment

As I was doing research, I was looking at potentially installing a household Honeywell dehumidifier in a 58' wooden boat (vent it outside), while connected to shore power, for use in NE where temps this weekend dropped to the 20s. The water outside obviously is colder than the air and the outside air is freezing. However, the boat is well designed with good ventilation with two stoves to keep the cabin warm. The builder a wise sea captain told me he would not install a dehumidifier in a wooden boat. In a fiberglass boat, yes, because they sweat and start to smell of unpleasant things. Moisture condenses on the glass structure and tends to pond in inconvenient places. But, this is a wooden boat planked in Oak but I still have a minor concern a bit because ventilation with the cold extreme may not be as effective?

On the other hand, keeping a wooden boat from drying out in a heated shop is always a problem, especially a boat planked with oak. I think the concerns are to keep them from drying out and opening up. The real keys, in normal operation, in interior maintenance are cleanliness and ventilation but if there are often wide temp differences like a warm cabin and freezing weather outside, that make me have some mild concerns. I do not want to dry the boat out but I also am concerned the boat could possibly create moisture in the air but am confident it remains well vented. So, should I be concerned or just relax and keep the boat clean and vented. If the boat was in San Francisco, CA Bay as example, I would not be as concerned. Its the temp extremes that concern me. Any suggestions?

Posted by: Terry Beaver | October 19, 2015 5:05 PM    Report this comment

Where did you drill to attach a ftting for the hose? What size fitting did you use and can you post a pic of the hose install?

Posted by: Heritage | October 7, 2015 3:41 PM    Report this comment

The devil is in the details. Here in Nova Scotia we have about 6 months of "shoulder season" with large amounts of rain, drizzle and fog and 32-to-55 degree (F) temperatures. I don't know whether to seal the boat up tight and run a dehumidifier or desiccant, or make sure that it is extremely well ventilated with outside air. Suggestions, please?

Posted by: PeterGregson | June 2, 2015 9:40 AM    Report this comment

OK, maybe more options: 1-For ~30' boat/RV in temperate winter, the Sunpentown SD-014V Desiccant dehumidifer uses a desiccant wheel and no compressor (like some large-scale, industrial systems). Portions of wheel that's near water saturation rotates through hot air stream to be re-dried. At Amazon or eBay, it retails about $252 (with shipping). Its 110V demand ~615 Watts, so to run system probably need a full-time power cord, unless large solar recharger. 2-For our 30' sailboat wintering on Ches. Bay, cabin has 2 solar-powered, day-night vents (each 4' dia.) pulling out air and 2/3 of boat is covered with loose-fitting tarp, using the boom. Yes, some cases and harsh environments may benefit from a tight-fitting winter cover. But, for decades, the working watermen in our County only install top tarps and say that boats (even non-wood) have to breath. John R. Mathews

Posted by: John R | May 22, 2013 7:57 PM    Report this comment

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