Space Heaters Are Not Dehumidifiers!

by Tim on December 31, 2008

It’s fun to write about cutting-edge technology, but sometimes it’s worthwhile to take a closer look at the basics. Like mildew-prevention.

The easiest way to prevent mildew is to keep things dry, and the easiest way to keep things dry is to keep the interior of the boat a few degrees warmer than the outside air. Warmer air holds more moisture, so a little interior heat can prevent atmospheric moisture from condensing on surfaces in the cabin.

Any source of (dry) heat will work, but some heat sources are better suited for use aboard an unattended boat.


You’ve probably seen these devices at your local chandlery. You might have considered them, seen the price tag, and said “No way! I can buy a nice little ceramic heater for less than that! It’ll keep my boat dry while I’m away, and keep it warm when I’m there.”

OK, maybe you didn’t say that. But I did, and it seemed pretty sensible at the time.

Recently, though, I learned why I should reconsider, and I ended up buying a Davis AirDryr 1000 from my pals at Fisheries Supply.

In my new slip on Lake Union, nestled between old-Seattle houseboats of the Sleepless In Seattle variety, I learned that the number one risk associated with small boats, while moored, is fire.

They’ve had lots of fires on Lake Union over the years. And my new landlord tells me that virtually all of the fires in recent decades were caused by electric heaters aboard boats.

The biggest problem is that these heaters, when operating properly, cycle on and off. Each time the unit cycles there is the possibility of a spark. A tiny spark, to be sure, but if combustible gasoline or propane vapor has collected inside the boat, that tiny spark can ignite it.

A fire aboard a boat is always a bad thing. But when you’re tied up alongside $1m+ houseboats where people live, the stakes are higher. It’s a boat owner’s responsibility to ensure that fuel is stored properly and to eliminate potential ignition sources.

The Davis dehumidifier consists of simple heating coils mounted in an enclosure designed to allow air to flow through it. The coils heat up air within the unit, which rises through convection, without the need for a fan, pulling in colder air through the bottom of the unit.

The exterior never gets too hot to touch, and the unit never cycles on and off. It runs constantly, so there are no sparks. It doesn’t make the cabin comfortable, but it does keep it a few degrees warmer than the outside air, which is enough to keep the boat dry.

{ 3 comments… read them below or add one }

Tony Fassbind December 31, 2008 at 4:00 pm

Another way to keep your boat safely dry is through forced air circulation. I stored a boat on Lake Washington (next to Lake Union) for several years with only a small computer fan (50cfm) in a porthole adapter that sucked in outside air. It is critical that the fan run 24/7. The boat was a 25 foot cuddy cabin, IO, with a snap on canvas top. I never experienced any mildew.

Previously, I almost ruined a boat interior from mildew and mold in a boat stored all winter without a fan or heat.

The trick is avoid condensation, which occurs whenever moist air comes in contact with a surface that is below the dew point of the air. In a sealed boat, this happens every night when the outside air drops below the dew point of the air trapped in the boat. Cold surfaces, like windows and canvas, form condensation rapidly. If you use a fan to constantly replace the air in the boat, condensation is avoided because you push out the moist daytime air before it can condense, and replace it with outside air which is slightly below the dew point.

A fan sized to change the air in boat every 15 minutes or so will do the trick. The fan also kept gas fumes from acumulating in the boat, which happened every time I left the fan off. If you have compartments or lockers you will need to make sure air circulates through them too.

A small fan will cost much less to operate than an electric heater, but it is a little harder to install.


John Williams January 5, 2009 at 3:31 pm

Computer fans work well. I installed one so that it circulates air through the normally inaccessible space between the hull and liner. Keeps lockers and their contents dry.


Dan Corcoran (b393capt) January 7, 2009 at 12:05 pm

How about using an actual dehumidifier.

I use an Eva-dry unit that is small enough to fit in a boat sink. I drill a hole in the collector tray so that it won’t fill up.


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