Dehumidifier Frequently Asked Questions

Dehumidifier runningAfter 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.

How much water does a typical dehumidifier extract per day?

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.

Which type should I choose – Refrigerant, Desiccant or non-electric?

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.

Moisture absorber vs desiccant dehumidifier initial effectNon-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!

How quickly does a typical dehumidifier lower the humidity?

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.

What benefit does a dehumidifier provide drying clothes?

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.

How low humidity can a typical dehumidifier reach?

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.

How much does a dehumidifier cost to run?

Meaco 20L WattsFrom 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.

How noisy is a dehumidifier?

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.

What’s the best place for a dehumidifier?

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.

Is there any precaution for switching on or off a dehumidifier, e.g. plug-in timer?

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.

Refrigerant models

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.

Desiccant models

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.

Is the collected water safe for drinking?

Meaco 20L Platinum Water LevelUnfortunately, 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.

Is the collected water safe for fish aquariums?

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.

How does a dehumidifier differ from dehumidify mode on an air conditioner?

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.

What are the typical failure modes of dehumidifiers?

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.

Refrigerant models

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.

Faulty dehumidifier icing upExcess 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.

Desiccant models

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 2017 with a few additions, such as positioning and power on/off precautions.

33 thoughts on “Dehumidifier Frequently Asked Questions”

    1. You can run the two together – This will also help improve the efficiency of the dehumidifier as the dried air is spread better throughput the room and the fan will make the room feel cooler.

  1. Hey Sean,

    Thanks for your great articles!

    I’m thinking of using a dehumidifier and air cooler combo and I want to hear your opinion.

    My plan is to direct all the heat from the dehumidifier to the areas where the air cooler draws in air. I’m thinking of making a duct so that all the heat from the dehumidifier will only go to the back and sides of the air cooler not to the air in the room.

    I’m just wondering if the heat from the dehumidifier is too hot for the air cooler to cool down?

    Love to hear your opinion about my plan. Thanks!

    1. Unfortunately, this will not work due to the physics of latent heat. When when an air cooler evaporates water, this draws in latent heat as the liquid water is converted to water vapour, i.e. elevated humidity. It takes roughly 0.649kWh of heat to evaporate 1 litre of water, so for each litre of water the air cooler evaporates, it has removed 0.649kWh of heat from the air.

      A dehumidifier does the complete reverse of an air cooler. It cools the air below the dew point or adsorbs it in the case of a desiccant dehumidifier. In doing so, the latent heat in the water vapour is released as it is converted back to liquid water again. This is expelled as the dehumidifier blows the air over the warm condenser. So for each litre of water a dehumidifier collects, it releases 0.649kWh of heat into the room in addition to the waste heat operating the dehumidifier. So if you were to duct a dehumidifier into an air cooler, you’ll end up with a rather complicated fan, i.e. the dehumidifier releases heat from the water it condenses, the air cooler removes that heat from the water it evaporates and the cycle repeats.

      There is actually a way this sort of set up can work, but like an air conditioner, it involves releasing heat outdoors. In this case, the room air goes into a desiccant dehumidifier and the regeneration process is carried out using mains gas for heat. This waste heat is pipped outside. The dry air goes through an air cooler that results in damp cool air. This is then fed through another desiccant dehumidifier stage with the regeneration process waste heat dumped outside and the cool dry air is released into the home. This approach can work out cheaper than electric air conditioning in areas with cheaper mains natural gas.

      1. Hey Sean,

        Thanks for your response!

        It looks like my plan is not a very good idea. So I’m thinking of just using an exhaust fan instead. My evaporative air cooler is made by Masterkool and it has an air volume 2,500 m3/hr and coverage up to 23 sq mtr. My room is about 20sqm in size and this aircooler is totally great I must say. But during the night I need to close all the windows due to security reasons and this is when this aircooler becomes ineffective.

        So I’m thinking of using a 10 inch exhaust fan and run it with my aircooler. Do you think this exhaust fan can keep the humidity balanced? If not, please tell me would be the best exhaust fan size.

        Thanks!

  2. Hi,

    I bought a flat that had water damage from a leaking water heater.

    the boiler (condenser unit?) has been changed, and there shouldn’t be any leaking water any more.

    We aren’t currently living there, and I leave a dehumidifier in there running all the time.
    the dial on it is set at 8, out of 9, and it still pulls about a liter of water a day, even though I’m sure the flat is totally dry now (there is not furniture or anything)

    is this normal?

    1. It’s normal for the humidity to rise as fresh air gradually replaces the air in the building, such as through ventilation, small drafts around doors/windows, etc. What is probably happening is that the dehumidifier is trying to bring the humidity too low which would not offer any real benefit over just keeping it under control, especially with the building not being occupied.

      I suggest getting a few hygrometers that shows the relative humidity reading to see what the humidity level reads, e.g. place one in the room where the boiler is and one on a table/shelf at each end of the house. Normally mould needs a relative humidity level of 65% or higher to grow. To ensure the humidity level is kept under control in surrounding rooms, 55% to 60% is generally considered the best balance between minimising how often the dehumidifier runs and keeping the humidity level low enough to prevent any further mould growth or metal corrosion.

  3. Thank you very very much for the reply, it seems to be quite difficult to get any useful info about this topic on-line.

    i’m going to get some hygrometers this evening.

  4. I hope you can help me, I had some flooding in my basement recently and to help dry things out I’ve been running a dehumidifier for the past couple of weeks. While the basement seems completely dry it’s still pulling about a liter of water a day.

    Where is this coming from? We live in UT. a very dry state. I’d love to know if I should just shut it off or if it’s pulling form moisture in the walls carpet etc…

    Thank you!

    1. What is likely happening is that it is condensing the humidity from the air that circulates the building without pulling any additional moisture from the walls and ground, since the air itself has a certain amount of moisture. So at this stage, you can shut the machine off.

      To be on the safe side, I recommend getting a digital hygrometer that shows the relative humidity reading and monitor it over the coming weeks. If the relative humidity reading climbs above 60%, then switch the dehumidifier on for a few hours to get the reading below 55%. Mould generally does not grow when the relative humidity remains below 65% and keeping it below 60% adds a bit of margin since the hygrometers can be a few percent out.

  5. Hi Seán,

    I am looking for a dehumidifier for a 4 bedroomed bungalow (approx. 111 square metre). I’m very confused at the information I’m reading on dehumidifiers as regards coverage. Most state that a dehumidifier that covers a room size of 55 sq. mt. will do up to a four bedroom house. Is this the case?

    1. I wouldn’t worry that much about rooms / coverage as the occupancy would have a greater impact on the dehumidifier capacity requirement than the number of rooms. For example, a 4 bedroom home with all 4 bedrooms occupied would produce a much greater amount of humidity than if it had just 2 bedrooms occupied.

      My suggestion for that size home would be to go for a dehumidifier in the range of 20 to 30 litres and position it either centrally (such as in the hallyway or kitchen if it’s fairly large). If you dry clothes on an airing rack, that room would be the ideal location to try running it first (or when drying laundry) as a load of laundry gives off roughly 2 to 3 litres of moisture, which would go straight into the dehumidifier rather than spread through the home before being collected.

    1. Generally this means the compressor is not running. Some dehumidifiers run the fan continuously, so when the set humidity level is reached, it switches off the compressor. Generally the air coming out with its compressor running should feel lukewarm, a little above room temperature. With a desiccant dehumidifier, the air coming out will feel warm like a fan heater.

      If it does not collect any water, try turning the dial to continuous to see whether it collects any water over the next hour. If it does not collect anything, then either its compressor has failed or it had a refrigerant leak. Another telltale sign of a refrigerant leak is if the coil behind the air filter starts icing up on one section of the coil, e.g. 1/3 covered in ice, 1/3 with water droplets and the remaining 1/3 dry.

  6. I have a new build property I’ve only been in 3 months with mould growth on every surface. I have since placed an industrial dehumidifier in the property. And I literally mean this when I say… the dehumidifier has removed over 100 litres of water in just 4 days and is still pulling about 2 litres an hour. Obvously with a new build there is alot of water that goes into the build process… mortar, plaster and not forgetting the days it rains on site during building. You can physically see the water content being drawn from the walls. Despite it feels dry and you cant smell mould anymore even just after a couple of days. It is still necasary to keep running your dehumidifier untill the dehumidifier maybe only pulls a litre a day rather than a litre every hour. It is also customary to switch of your dehumidifier if the plaster on your walls starts to crack as this is a sign your over drying your property, another sign is a dry cough… this is a sign the dehumidifier is pulling moisture from your own body. You can invest in a dehumidifier with a built in hygrometer that constantly measures the humidity so when it drops, the unit switches itself off and when humidity rises it switches itself back on, and you can usually set them to as low as 35% humidity. Its all about the rate it pulls water out of the air and any materials in your property from furniture down to even your clothes, once the steady rate dramatically drops to practically nothing each day then its about ready to switch of your dehumidifier and the job it was used for is done.

  7. Hello

    I appreciate the metric readings here as I am in Europe and the conversions are tricky…

    I am in a loft flat 85m total, one bedroom, roughly 19m, with a small 6 square metre walk in closet, living room 30 square meters, a 16 square metre kitchen and the rest either a hallway and small bathroom 12 square metres

    The ceiling is very atypical, very hard to measure in any room due to angle and ceiling, roughly 2.5m that is a eye contact average assumption

    Humidity is an issue, already at 70 – 80 percent and spikes as the building (old stone apartment 100 years plus) in the evening / night.

    The flat itself is new, say built in the last 5 years building reconditioned. Modern materials used. Roof above is in good shape. (though the building itself is much larger. I need to worry about our flat/unit)

    So the question is what size of dehumidifier to recommend?

    Any comment would be helpful

    1. If the temperature is typically 20C or warmer all year around, I would suggest a 20 litre compressor based dehumidifier. If overnight noise is an issue (i.e. prefer dead silence to sleep), you’ll likely get away just running it during the day. For the first week or two, I suggest leaving it plugged in continuously to get the humidity level under control, after which it should be able to maintain it with daytime use.

      Before you get one, it’s worth doing a quick check if there is any culprit driving up the humidity. The main culprits are drying laundry on an indoor aerator (a 5kg load evaporates roughly 3 litres of water into the air), cooking (e.g. steaming, boiling, etc. without the extractor fan running) and shower (be sure to run the fan or leave the window open for 5-10 minutes after the shower).

      1. Thanks for the reply and insight. I forgot to add we cannot currently open windows due to a cat and screens will be installed within a few weeks, This should help, most windows face north and the circulation is poor. Also we have a good fan which will help to drive the air. House temperature is about 25c. Perhaps there might be culprits in the other flats that drive up the humidity, we have not cooked or washed too much at all to have had an influence.

  8. Hello, I am struggling with dust allergy and thinking about buying a dehumidifier.i want to know if I run dehumidifier and humidity is reached like 50 percent, how long this level will continue, when I need to run dehumidifier again??

    1. Most dehumidifiers have a humidistat that will automatically switch back on once the humidity level increases. If the dehumidifier is unplugged, the level will gradually raise back up as moisture is added to the air. For example, if someone takes a shower or food is being cooked, that will quickly put the humidity level back up. With our house, I generally find it sufficient to run it throughout the day (I’ve it set at 55%) and switch it off overnight. By the morning, the level usually reaches 60% to 65% depending on the weather.

      For a dust allergy, I recommend getting a HEPA air purifier first and see how you get on. If the humidity level is not causing other issues such as mould or condensation, an air purifier will take care of the excessive dust in the air. While dehumidifiers have dust filters, most only remove large dust particles to protect the internal coils from getting clogged with dust build-up. Another option would be to get a dehumidifier with a built-in HEPA filter, this way you can use the one appliance to carry out both jobs. The HEPA filter generally lasts 6 months, particularly if loose dust is vacuumed off periodically.

  9. Hello,
    My question is how can there be so much moisture in one room in my house? The room is about 20ft by 20ft with a window and door to the outside. (closed) The first day I turned on the new dehumidifier it pulled 70 pints of liquid four times in less than 24 hours. I read all the Q&A’s above and my results seem way out of wack. The temperature reading is now 73F and humidity is now 55% (started around 68%) after the first 24 hours of use and the unit is still running.
    Thanks for any information,
    Evelyn Hoff

    1. It can take a few days for it to pull out the moisture deep inside items, such as walls, cushions and furniture. For example, let’s say someone places a damp towel under in a closed drawer or under a sofa cushion. Even with a dehumidifier running in the room, it would probably take a week for the moisture to escape for it to dry. Similarly, other moisture trapped in items can anywhere from a few days to a few weeks to fully dissipate.

      I would give it at least two weeks, by which stage it should be in its off state more often than running. If it’s still extracting multiple containers a day after 2 weeks, there is probably a more serious dampness issue such as rising damp or an underground water leak.

  10. We have purchased the silent night dehumidifier for the bedroom it needs emptying every week but since it’s been on we have had stuffed up nose and a sore throats every morning

    1. That seems like it is set too low or it is running throughout the night. I suggest switching it off overnight or running it in the hallway. If it runs in a closed room, it will concentrate dehumidification to that room. While this is great for drying clothes, it is just as effective at drying out throats if it is running with people asleep in the room.

      I suggest checking the room with a hygrometer, even if the dehumidifier has a digital one built-in. Some dehumidifiers give too low/high a reading depending on its location. 55% is generally low enough to prevent mould growth.

  11. What are normal exhaust output temps for dehu when drying out a room? I know you are supposed to compare the input to output temps to see how the dehu is working to bring RH down.

    1. It varies depending on the type of dehumidifier, its fan speed, operational mode and even the room’s humidity level. If the exhaust temperature is the same or barely higher than the room temperature, then the dehumidifier is not dehumidifying, i.e. fan-only operation. Some compressor dehumidifiers take about 5 minutes to start-up after plugging in to prevent a locked rotor, such as if it has just been switched off.

      For refrigerant (compressor) based dehumidifiers dehumidifying, the exhaust temperature is typically around 5-10C higher than the room temperature depending on the humidity level. On a low fan setting, the exhaust temperature will be a few degrees higher. When operating in defrost mode (room temperature below about 16C), the exhaust temperature will be a few degrees cooler than the room temperature as it thaws the ice build-up.

      For desiccant based dehumidifiers, the exhaust temperature is typically around 10-15C higher than room temperature on its low/eco setting and around 20-25C higher than room temperature on its high or continuous mode setting. As there is no distinct sound difference between dehumidifying or fan-only operation, the exhaust temperature can vary anywhere between room temperature and 25C higher in its auto setting.

      Probably the simplest way to check is to see if the dehumidifier collects any water. If it is connected to a drain pipe, try running its condensate pipe into a bucket and run it in its continuous mode setting for an hour. Another option would be to plug it into a power monitor. If it uses less than 100 watts when set into continuous operation mode, then it is likely just running its fan only. A typical 20 litre refrigerant compressor dehumidifier will use around 300 watts while dehumidifying. A 7-8 litre desiccant dehumidifier will consume at least 300 watts depending on its low/eco mode setting.

  12. Hi Sean

    I have a 2 bedrooms house 2 floors with 3 adults 2 kids and a small dog but we just notice a damp in the walls few weeks ago in one of our bedrooms its a bit bad we all open all windows 4 / 6 windows throughout the day for few hours ,we also have our tumble dryer on a little cupboard on the entrance hall releasing the air into outside please can you advice and tell me what are we doing wrong please ??

    As i want and need to buy a Dehumidifier ASAP how big wich type where to keep upstairs , downstairs what spec basically ,keep the windows open closed ,also need to had that the other half she is a bit dust allergic , what it doesn’t help the issue i guess from my reading above .

    Kindly i will appreacite a lot you suggestion and recommendations on these my request .

    Kindest regards and god bless

    Antero

  13. SEAN

    Also would like to had that we have one gas wall heater in each room where we keep on / off around 19 / 21 degrees , does it helps or not keep in on or off together with the Humidifier ??

    1. The gas wall is likely the source of the humidity as these generally do not have a flue to the outside, much like a portable gas bottle heater. When gas burns, it emits carbon dioxide and water vapour as by-products. Most vented tumble dryers do not release any humidity into the room as long as the vent and hose are properly fitted, so the tumble dryer is unlikely the humidity source.

      I would suggest getting a 15 litre (or larger) compressor based dehumidifier along with two hygrometers to check the humidity levels upstairs and downstairs. Place the dehumidifier in whatever room the gas heater runs most often and that should collect most of the water vapour the gas heaters emit. Unless those rooms have wall vents, I recommend periodically opening the windows for a few minutes to prevent excessive carbon dioxide build-up. You’ll probably find that you can run the heater at a lower setting or less often with the dehumidifier running as the lower humidity will reduce heat loss also.

  14. Hi, We moved into an older house rental without an exhaust in the bathroom. My husband wants to leave the window open and the bathroom door ajar to let the condensation get out. But the window faces the street, it doesn’t allow for much privacy, AND it’s freezing in the morning here in New England.

    If it were our home, we would probably invest in getting an exhaust – but don’t have that option for a rental.
    I was thinking we could use a dehumidifier in the space for when we shower. Do you think this would work? What size/type/capacity would I need to accomplish this?
    Aside from after showers, the room doesn’t have any dampness issues. Thanks!

    1. I would suggest a 20 pint dehumidifier, preferably one with a timer control (such as this). You can run it in the same way as the extraction fan, i.e. switch it on before running the shower and let it run until the moisture clears up. If it has a timer, you can set it to run for one hour, which would let it run for a while if you need to head out.

      If you don’t have enough space in the bathroom (a 20 pint model is about the size of a small carry-on suitcase), you can place it outside the room and leave the bathroom door open. In this case, you may need to run it longer (e.g. 2 hours) as it takes longer to clear the moisture when operating outside the bathroom.

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