Dehumidifiers work in the much the same way as air conditioners. Both contain refrigeration coils that cause moisture to condense out of humid air. In fact, American engineer Willis Carrier was actually searching for a way to control humidity when he invented modern air conditioning. The main difference between a dehumidifier and an air conditioner is that a dehumidifier reheats the air that passes over its refrigeration coils before blowing it back in the room.
When a dehumidifier won't do its job and stops removing moisture from the air, the filters are the first thing to check, because they could be impeding air flow. It's also possible the coils need cleaning or de-icing. These are repairs you can do yourself, but fixing problems with the refrigeration system, such as leaks and compressor malfunction, is a job for a licensed refrigeration technician.
How It Works
If you can understand an air conditioner or a refrigerator, you can understand a dehumidifier. The cooling is provided by the change of state of an inert chemical—called a refrigerant. The condenser compresses this refrigerant in its liquid state and forces it through a tiny aperture into a system of coils. As the refrigerant passes through the aperture, it vaporizes, which is an endothermic reaction that draws heat from the surroundings. The sudden cooling condenses moisture out of the air surrounding the coils. The condensation collects on the coils and drips into a drainage system, reducing the humidity of the air that exits the appliance.
Check the Filters
The efficacy of this process depends on the air flow around the coils. If air can't get into the chamber in which the coils are located, or it can't get out, the humidity around the appliance won't change.
Many dehumidifiers have two filters—one on the inflow side and one on the outflow side. Remove both filters and clean them periodically with soap and water—or replace them—to keep the appliance working in top form. You may need to consult the manual to determine how to access the filters. If you don't have a manual, you can find one on the manufacturer's website or by doing an online search of the make and model of the dehumidifier followed by the word "manual."
Empty the Reservoir
Almost every dehumidifier comes with a built-in reservoir that you can use instead of hooking the unit up to a drainage hose. When the reservoir fills up, a switch shuts off the condenser to avoid an overflow, and a warning light illuminates on the control panel. Empty the reservoir, replace it, and the unit will start running again.
Defrost the Coils
When the temperature in the room is below 65 degrees Fahrenheit, frost may form on the evaporative coils, and it can eventually turn to ice. Paradoxically, a layer of ice on the coils hampers their ability to cool the air and extract moisture. If you suspect the coils are iced over, unplug the machine before making an inspection. If you find ice, leave the unit turned off until it melts.
Check the Fan
If you can hear the condenser running, but you can't feel any air coming from the front grille, the fan may be stuck or broken. Unplug the unit and disassemble it to access the fan. Remove any obstruction you find. If you can't find an obstruction, but the fan won't turn, you probably need to replace it.
Place the Unit Properly
If you put your dehumidifier too close to the wall, air can't get into the refrigeration chamber, and the unit won't work properly. Ideally, the unit should be at least a foot or two away from the wall behind it, but at a bare minimum, make sure the distance is at least 6 inches. Make sure no curtains or other furnishings are interfering with air circulation.
Call for Service or Replace the Unit
When you can't narrow the problem down to something obvious, you're usually dealing with a malfunction of the refrigeration system. There could be a refrigerant leak, or the compressor may have a bad seal that prevents it from developing enough pressure to vaporize the refrigerant. You can't repair a refrigeration system yourself—federal law requires HVAC technicians who work with refrigerants to possess a Type 1 608 certification.
Considering that a minimum service charge for an HVAC technician is in the neighborhood of $150 even before any repairs are completed, it's probably not cost -effective to repair a unit with a compromised refrigerant system. It usually makes more sense to dispose of the unit according to local bylaws and buy a new dehumidifier.