Note that on some of these units, you can remove the water collection tray for cleaning by just pulling the water collection drawer out and using a stubby cross-head driver to remove its retaining screws without opening the whole unit. Use something soft to clean out the three ports that lead down into the collection tray, and make sure they're completely free of slime and gunk.
This video was mainly intended to show people how to open the cases on this common style of desiccant drum dehumidifer. There are a few hidden screws. This model is an Ecoair DD122 Simple.
This is a fairly recent style of home dehumidifier, based on an industrial technology. I regard these units as still being in development, and they have a few weaknesses that result in shorter working life than traditional compressor based dehumidifiers.
The main advantages of desiccant drum units is that they are very quiet and can operate in much lower temperatures than classic compressor units.
In operation, air is pulled through the unit by a powerful fan. It passes over a plastic condenser to cool it to room temperature, and then goes through a rotating drum of corrugated desiccant material based on a mineral called zeolite - a microporous mineral that absorbs moisture readily.
Most of the rotating drum is exposed to the airflow, but a section like a pizza-slice has a heater in it, and a fan that circulates hot air through the desiccant material (driving moisture out of it) and then through the condenser, which provides a cool surface for the vapour to deposit on. The air then continues in the loop to get heated up again and drive out more moisture.
As the desiccant drum rotates, the dried area then enters the main airflow again to absorb more moisture.
The moisture that forms on the cool (room temperature) surfaces of the condenser then pools together and drips down into the collection tray.
The biggest problem with early units was failure of the heater if used at the high setting a lot. Modern units seem to be more reliable in that regard.
The water path is very prone to gunking up with hair or one of the many classic slimes that inhabit air conditioning units. If the performance drops, the unit starts leaking water or it keeps cutting out then it may need the area where the condenser goes into the water collector cleared. This video shows how to do that.
There's huge similarity of construction between different brands, hinting at a common factory making the internal modules. The biggest difference is usually the controller, which can be a simple mechanical humidistat (works with a hygroscopic polymer film that changes length based on humidity) or a sophisticated electronic controller with resistive/capacitive sensor and loads more to go wrong.
I recommend the simplest units with minimal electronics and running them at their low setting - which is more efficient anyway.
All the units I've seen have had two levels of thermal protection. One that simply shuts the unit down and displays a warning, and a series of one-shot thermal fuses that will render the unit unusable if all else fails. In the early days the one-shot fuses were prone to tripping too easily. They are usually located on the back of the heater "pizza-slice" and on the other side of the desiccant drum from the heater.
I live in a very humid area (a very stormy island with the sea nearby), so I use two types of dehumidifier. Compressor in summer for efficiency, and desiccant drum in winter when they have the benefits of working in a cool house and also put out a stream of warm dry air.
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