Sep. 01, 2025
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After getting some rather interesting questions on dehumidifiers from my original blog entry, I figured I would try answering some here as well as some other dehumidifier related questions I saw around the web. Please note than I am not an expert on these, so if in doubt, please ask a confident professional, such as at a DIY centre.
Most manufacturers love to make claims of 10 to 20 litres per day on most small domestic dehumidifiers and up to 50 litres on larger models. However, these figures are under extreme conditions such as 85% relative humidity at 30C. The exception is with desiccant based dehumidifiers, which give a near-consistent extraction rate regardless of the temperature and humidity level.
From my experience, for 60% RH at around 20C, most refrigerant (compressor) dehumidifiers remove between 30% and 40% of their capacity rating. For example a 10 litre dehumidifier will remove between 3 and 4 litres per day and a 20 litre dehumidifier will remove between 6 and 8 litres per day. This is with the dial set to “Continuous” and where the humidity remains roughly between 55% and 60% while the dehumidifier is in operation.
These figures don’t vary much for a few degrees higher/lower, however, the extraction rate falls significantly once the relative humidity goes below 50%. When the temperature falls below about 18C, refrigerant (compressor) based dehumidifiers will form ice and perform periodic defrosting cycles. This drastically reduces the extraction rate. For this reason, desiccant dehumidifiers are better suited for unheated rooms, which also provide background heat.
In room temperatures above about 15C, refrigerant dehumidifiers have the lowest running cost per litre, particularly when drying laundry. As such, they are better suited for heated homes, particularly during the summer where a desiccant dehumidifier can make the room uncomfortably warm. Higher end refrigerant models with hot-gas defrost improve the running cost down to around 10C.
Desiccant dehumidifiers are better suited for unheated rooms. They can also be put to use in place of an electric heater, which can actually reduce heating costs compared to using a heater alone. For reference, a 8 litre desiccant dehumidifier extracts about the same rate as a 20 litre refrigerant dehumidifier at 20C 60%. The desiccant dehumidifier will maintain this rate at 10C 60%, while most refrigerant dehumidifiers will extract 1/3 to 1/2 of what they would at 20C for the same humidity level. They are also lighter, quieter running and less bulky than refrigerant dehumidifiers.
Non-electric dehumidifiers, also known as moisture absorbers or damp traps typically use calcium chloride. This absorbs moisture out of the air, dissolving the calcium chloride in the process. These are best suited for small rooms that lack electric power, such as a motorhome. The ongoing cost is very high for the amount of water they collect, typically requiring a complete refill by the time the container has filled up. For extraction rate comparison, the Unibond Aero 360 (non-electric) claims to remove the first drop in 12 hours (above left image), whereas the pint glass above shows what my desiccant dehumidifier removed running continuously in just two hours!
This depends a lot on the room size or the building size if the room door is open and how damp the house is. For a damp building, it can take a few weeks for a dehumidifier in continuous mode to bring the dampness under control. Once under control, a few hours a day will take care of spikes such as after a shower, cooking or while drying laundry on an aerator. Humid weather will require additional runtime.
For the aprox. 3m x 4m x 2.5m room with an Amcor DC930-H (15l/day rating), with the door closed and starting at 72% RH, it goes as follows: It will bring the humidity down to about 65% after 15 minutes, 60% after an hour, around 55% after two hours and 50% after about 8 hours of continuous operation. These figures are approximate, as a very humid day may prevent it even reaching 50% even after 24 hours of continuous use.
With the door open and a 4 bedroom house and starting at 72% RH, the DC930-H would bring the humidity down to about 65% after an hour. After this, the rate totally depends on how humid the day is, whether there is any cooking, clothes drying, etc. With a more powerful 20 litre dehumidifier (e.g. Trotec 75S or Meaco Platinum 20L), it can bring the humidity level below 60% after an hour and about 55% after two hours.
Besides reducing dampness in the home, a dehumidifier will rapidly dry clothes on an indoor aerator. Depending on the load size and dehumidifier capacity, light clothing such as shirts will dry in around 6 hours and heavier items such as jeans and blankets will dry overnight.
As clothes dry on an aerator, it takes roughly 0.63kWh of heat to evaporate 1 litre of water due to latent heat. This is why a large load on an aerator can make a room feel chilly as the drying clothes effectively acts like an evaporative air cooler. A dehumidifier reverses this process by condensing the moisture from the air and releasing the latent heat back into the room. This also explains why the dehumidifier vents noticeably warmer air when running near an aerator full of wet clothes.
Without a dehumidifier, the moist air will eventually make its way out the house, bringing on a second side-effect in colder weather. As moisture penetrates an outer wall/ceiling, it starts by condensing on the cool surface, in turn drawing away additional heat as the moisture makes its way out. This also reduces the effectiveness of insulation much like wearing damp clothes in cold weather. This means that drying clothes on an aerator without a dehumidifier silently increases the heating cost or makes the house feel cooler.
Unlike a tumble dryer, clothes do not face wear & tear drying on an aerator, just like drying clothes outdoors. There is also a much lower fire risk as there is no worry about forgetting to empty the lint filter after each load. Dehumidifiers just require vacuuming its dust filter every couple of weeks. Dehumidifiers also do not create a strong laundry small that most condenser tumble dryers emit. Finally, unlike vented tumble dryers, dehumidifiers do not vent any heat out of the home.
In fact, a modern energy efficient heat pump tumble dryer is effectively a powerful refrigerant dehumidifier running in a closed cycle in place of the heater and heat exchanger. By running in a closed cycle, the tumbler dryer can take advantage of the higher extraction rate by heating the interior air to around 40C. This lets them dry clothes about as quickly as a conventional condenser dryer, while using 1/3 the power.
From my experience with various refrigerant dehumidifiers in a closed room and running continuously, the lowest the relatively humidity will reach is between 40% and 45%, higher with a small 10 litre machine. With a desiccant dehumidifier (Trotec 55E) on high with a 2 hour test run, the humidity level fell to 35%. Unlike refrigerant dehumidifiers, desiccant dehumidifiers maintain a high extraction rate even at very low humidity levels. For this reason, desiccant dehumidifiers should be run a timer or humidistat to prevent over-drying in a closed room, such as while drying laundry.
In one case where I left a small 10 litre refrigerant dehumidifier around the clock in a car to dry up a spill, the humidity level bottomed out at 35% and was extracting less than a litre per day. So basically, if the humidity is around 50% or lower, a refrigerant dehumidifier is not going to reduce this much further.
From my experience of using several models, the power consumption varies according to the rated extraction rate. For refrigerant models, multiply the extraction rate by 15 for an approximation of the wattage. For example, a 20 litre dehumidifier will typically draw about 300 watts. For desiccant models, multiply the extraction rate by 45 for low or 90 for high. For example, a 7 litre desiccant dehumidifier will typically draw about 315 watts on low and 630 watts on high.
The water collection rate is roughly relative to the wattage (for the same room temperature/humidity) in continuous mode. While a more powerful dehumidifier may consume more power, in theory, it will have similar power consumption per litre as a less powerful dehumidifier. Based on my experience of various refrigerant models over the years, the extraction rate is around 0.7 to 1 litre per kWh, maintaining roughly 20C 60% RH. Low Energy dehumidifiers get around 1 to 1.2 litres per kWh, such as the Meaco Platinum 20L. Half these figures for 10C 60% RH. Desiccant dehumidifiers typically get about 350ml to 500ml per kWh, regardless of the temperature or humidity level.
For intermittent operation, such as a maintaining a set humidity level, the power consumption can vary substantially. If the fan runs continuously, a typical dehumidifier fan alone will consume 0.5-0.7kWh per day. With refrigerant dehumidifiers, it takes several minutes of runtime before condensed moisture starts collecting, so a dehumidifier that cuts-in multiple times per hour will cost more than a dehumidifier that cuts-in less often for the same water collection. To reduce power consumption with such dehumidifiers, operate the dehumidifier in continuous mode on a timer, such as two hours in the morning and again in the evening.
See this article for in-depth running costs of desiccant and refrigerant dehumidifiers, including against non-electric and silica gel packs.
Larger commercial refrigerant dehumidifiers such as industrial models that use precooling are usually advertised as featuring low-grain refrigerant (LGR). These may extract upwards of 2 litres per kWh by pre-cooling the air intake, however, I’m not aware of any compact consumer size dehumidifier that features LGR, at least not at this time of writing.
Tip: Consider using a desiccant dehumidifier in place of an existing electric heater. On high or continuous mode, it will provide similar heat to a 1kW fan heater.
It depends a lot on the dehumidifier. I have experienced dehumidifiers that are as quiet as a desk fan on low or as noisy as a paper shredder. 10 to 20 litre refrigerant dehumidifiers typically sound like a combination of a fridge and desk fan on high speed. Larger 30 to 40 litre models can be as noisy as a large paper shredder or cooker hood fan on high. When operating upstairs, the compressor hum may be audible from the floor below.
Desiccant dehumidifiers do not have a compressor, so the only sound they make is from the fan. These typically sound like a fan heater or column fan with the corresponding speed setting. Generally when run on low, they are much quieter than most refrigerant dehumidifiers.
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Ideally a dehumidifier should be centrally located, such as in a hallway or near the bottom or top of a staircase. The next best place would be near a humidity source, such as spare room or living room with the room doors left open while running. For refrigerant dehumidifiers, keep it away from heat sources such as radiators.
For operation in an occupied bedroom, avoid running the dehumidifier overnight. If it needs to be run overnight such as to tackle a mould or condensation issue, don’t set the humidity level below 55% to avoid getting an dry throat or itchy eyes overnight.
For drying clothes, aim the dehumidifier’s air outlet towards the clothes (if adjustable) and run it with the room door closed. The smaller and warmer the room, the quicker it will dry the clothes.
Most dehumidifiers can be safely switched on and off using the on-board controls. However, for switching one on/off externally, it varies depending on the type of dehumidifier and even when moving it.
As with refrigerators and freezers, a refrigerant dehumidifier should be left standing still for at least one hour before switching on after transport. If the dehumidifier was laid on its side, allow at least 24 hours standing upright before switching it on. For carrying a dehumidifier from one room to another, it is generally fine to switch it on.
If the dehumidifier is switched off or loses power (e.g. accidentally unplugged), allow at least 5 minutes before plugging it back in. If it is plugged in straight away, this will likely lead to a rotor lock up, which can potentially damage or overheat it.
Most refrigerant dehumidifiers will run fine using an external plug-in timer, particularly those with a manual humidistat knob. For electronic dehumidifiers, check if the model has auto-restart. Some electronic dehumidifiers will return to standby after they lose power, making them unsuitable for scheduled operation with an external timer.
While refrigerant dehumidifiers require precautions before switch on, the opposite is true with desiccant dehumidifiers. To manually switch off a desiccant dehumidifier, first turn it off using the on-board power button. Do not unplug it or switch it off externally until it has completed its cooling down cycle. Otherwise the hot moist air within the unit can damage its heater unit.
On the other hand, desiccant dehumidifiers are safe to operate immediately after transport, even if accidentally laid on its side. Similarly, they can be switched back on immediately after being switched off, such as after an unexpected power loss.
As desiccant dehumidifiers generally need to carry out a cooling cycle before switch off, they are unsuitable for external timer operation. For this reason, many desiccant dehumidifiers have a built-in timer, which lets the dehumidifier carry out its cooling cycle once the time is up.
Unfortunately, besides the water collected, dehumidifiers collect all sorts of airborne particles, including mould spores, viruses, pollen, bacteria and so on, making it unfit for consumption. There are dehumidifiers on the market designed for this purpose, usually called something like “Air to water” appliances. These have special filters designed to remove dust and UVC lamps to kill the germs to make the water suitable for consumption.
From my own testing with an aquarium test kit, the ammonia and nitrite readings went off the scale. As a result, there’s no doubt that this water is unsafe, at least without treatment.
I’ve read mixed reports about this and again I’d advise not doing this to be on the safe side, at least not directly from the dehumidifier. If your tap water is clear and does not have a lime scale issue, it’s probably a lot easier and safer to get a bottle of tap-safe solution from the pet store and use this with regular tap water. Alternatively, install a water butt to collect rain water.
A dedicated dehumidifier has no outdoor unit, which means that the evaporation and condenser coils are in the same unit so no heat is removed from the room. As latent heat is given off during the condensation of water and the compressor needs to be cooled, a dehumidifier does emit warm air. From my experience, a typical refrigerant dehumidifier in a closed room warms the room by about 2C after several hours of continuous operation. Double this for a desiccant dehumidifier.
Air conditioners operate quite differently: What most air conditioners do in the dehumidify mode is greatly reduce the fan speed such that more effort is put into condensing the water from the air than cooling the air. This extracted water is evaporated using the heat from the evaporation coil and this moist air is vented out the exhaust pipe. The exhaust fan speed may be operated at a slower speed to reduce the amount of heat from the room being exhausted in the process.
Basically, most air conditioners are unsuitable for use as a dehumidifier during cooler days or to control dampness and most do not even have a humidistat. On the other hand, on a hot humid day, an air conditioner will extract humidity several times quicker than a dedicated dehumidifier without raising the room temperature, due to its much more powerful compressor.
There are a few portable air conditioners that provide water collection ability in dehumidify mode, which let them operate in dehumidify mode without the vent hose. In this case, they either collect the water in a container (like a dedicated dehumidifier) or externally such as into a bucket. Unlike a dedicated dehumidifier, most of these lack a humidistat and if using an external bucket or pan, they do not switch off when it is full.
Refrigerant and desiccant dehumidifiers can experience different methods of failing when they reach the end of their life or go faulty. A completely dead machine may indicate a failed fuse within the appliance or plug. Erratic behaviour can indicate a defective circuit board, particularly in electronic dehumidifiers. Some faults such as capacitor failures can be fixed by an electrician.
Thud sound every 1-2 minutes: A dehumidifier that repetitively tries to start its compressor every 1-2 minutes (sometimes with a few seconds of buzzing) usually indicates a faulty capacitor.
Excess defrosting / ice build-up: If the dehumidifier repetitively goes into defrost mode in a warm room (20C+), first check that the filter and vents are clear of dust. If less than half the coil behind the air intake is covered in frost/ice (example on right), this indicates a refrigerant leak. When refrigerant leaks out, the left-over refrigerant will not cover the entire evaporator coil, resulting in excessive cooling in a portion of the coil.
Compressor hum, but no air flow: This may indicate a fan motor or capacitor failure. On older dehumidifiers, this can also mean excessive dust build-up within the fan motor or insufficient lubrication.
Blowing cold air / no water collection: This is the most common failure mode and usually indicates a failed heating element. It can also indicate that one of the internal motors that spin the desiccant wheel or its hot air blower has failed.
Odours during operation: The desiccant wheels will both pick up and later expel odours that pass through the machine, particularly when later run in its high setting. For example, if the dehumidifier is in or near a kitchen, the desiccant wheel will adsorb the grease vapour and aromas emitted from frying pans, woks, grilling and so on. The next time the dehumidifier runs its heater, it will emit the combination of these odours back into the air. This can usually be fixed by running the dehumidifier for a few hours on high in another area.
Operation hum, but no air flow: As with refrigerant models, this can indicate a fan motor or capacitor failure.
Note: This article was revised on the 30th November with a few additions, such as positioning and power on/off precautions.
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