From coconuts to dehumidifiers – the subtle art of staying dry this winter

When one of my neighbours on the marina first mentioned the word desiccant, like a word association game, my immediate first thought was coconut – desiccated coconut?? In the context of a conversation about dehumidifiers, the imagery of a coconut seemed exotically out of place. But I wasn’t a million miles off.

While desiccated (adjective) describes the current state of my imagined coconut (i.e., dried out, without moisture), desiccant (noun) refers to the drying agent (i.e., a [hygroscopic] substance or material used to induce or sustain a state of dryness by attracting and holding water molecules) used for the desiccation process, which typically takes place in a desiccator (i.e., the apparatus that contains the desiccant and does the desiccating – e.g., a dehumidifier).

From desiccated coconut to desiccant dehumidifiers, like this 10L ProBreeze model

Now we’ve made the semantic leap (at least in my mind!) from coconut to dehumidifier, we can look at why desiccation, and desiccating humidifiers in particular, are worth paying attention to as boaters. Moisture, condensation, damp mattresses, mouldy outcrops behind cupboards etc., are all factors that affect quality of life when living in such close proximity to water, and can become a battle – especially in the colder, darker, damper winter days.

The challenge compounds when most common dehumidifiers (i.e., compressor dehumidifiers) are essentially ineffective in temperatures below 15°C. They work by squeezing warm damp air over a cold surface, condensing and capturing any moisture in the air. When the ambient temperature drops to (or below) the temperature of the cold surface in the dehumidifier, condensation ceases to occur, and what comes out is the same as what went in – more cold damp air. Writing this post from a frozen pond with outside temperatures at -3°C, this problem becomes incredibly pertinent.

Schematic showing the different way compressor and desiccator humidifiers work

With a desiccant humidifier, cold damp air passes over a hydroscopic (absorbent) substance (as opposed to a cold surface) effectively trapping water molecules like a sponge. The simplest example of this is the humble disposable dehumidifier (around £1 a pop) that literally captures moisture in a silicone gel substance (i.e. through absorption) that you simply throw away. A modern electric desiccant humidifier does much the same, but relies on a process of adsorption, where water molecules adhere to the drying surface before condensing off into a collection tank to be emptied.  

Simply put. If you want a shot at keeping your boat moisture free through the long winter months, your best bet is a desiccant humidifier. With an effective operating temperature range of ~1°C to ~40°C, and capable of extracting as much as 10 litres per day, they massively outperform comparably priced compressor dehumidifiers, with running costs on a par. And to top that, they also heat the air that passes through them as a bi-product of the desiccation process – so cold damp air in, warm dry air out – for as little as 18p per hour. Not a silver bullet, but an essential bit of winter kit!!  

Testing conditions as the temperatures plummet below zero for another night running