>>> UPDATE: I have found a new dehumidifier that comes close to the requirements specified below.
It is the ProBreeze 20L Compressor Dehumidifier, Model PB-08. I purchased it from Amazon UK:
It does indeed have reverse cycle de-icing; It responds properly to mains power being cycled (by an on/off timer); It is very quiet; It responds well to the humidity sensor and switches off when the set level is reached, which saves power; However the water tank contains only 5.5L - this is because it is constrained by the size of the compressor, which is at the base of the unit, but perhaps this is a good thing because the unit is more stable than the Amcor which has the compressor at the top - this allows for a large water tank, but makes the unit very top heavy and it is easy to tip it over when being moved... Update: 23/12/2022

A Plea To Dehumidifier Manufacturers

Please Can You Make A Decent Dehumidifier?

What has a dehumidifier to do with astronomy? Controlling humidity is an essential factor in the successful storage of many valuable things, including photographic, optical, astronomical, and electronic equipment. Temperature should ideally be kept low which means an unheated storeroom is ideal, but then it is even more important to maintain low humidity, moreover the dehumidifier must have effective de-icing otherwise it is useless. Most dehumidifiers available today do not have effective de-icing.


The self-contained 10 Litre tank simply slides out and is very
easily removed for emptying

An example of a decent dehumidifier is the Amcor D-450, illustrated to the right.
I purchased this dehumidifier in February 1995 - 17 years ago as I write - and it is still working perfectly today. The issue is that I need another one, but they don't make anything like this any more (not even Amcor). I have been searching for over two months but have found nothing that comes close *. The main issues are that they don't have an effective de-icing function, and that the water tank is much too small. Here are some of the main functionality requirements of a decent dehumidifier. The 17 year old Amcor D-450 has all of these:
- Reverse cycle de-icing. This means the dehumidifier will never fail - it can be used anywhere, even during winter in unheated areas like workshops and store rooms where the ambient temperature may often drop to freezing point and below. Explanation: The evaporator coils in all dehumidifiers must become very cold in order for the unit to function effectively. This means that in ambient temperatures below 12-16°C the moisture condensed from the air will immediately freeze. Unless the evaporator coils are regularly de-iced, the ice will build up to the extent that the entire evaporator section becomes a solid block of ice. Air cannot pass through a solid block of ice - the dehumidifier will stop functioning. It must be turned off and left for a day or so until the ice has melted. In other words, absolutely useless. Reverse cycle de-icing solves this problem very efficiently - it works by periodically switching the compressor into 'Reverse Cycle Mode' whereby it injects hot gas into the evaporator coils. The cycle lasts only 3 minutes or so - it is so effective that the ice thaws almost immediately and (in the case of the D-450) all the water drains into the collection tank within about 30 seconds. Most dehumidifiers available today either don't de-ice at all, or rely on merely stopping the compressor and letting air at ambient temperature blow through the evaporator coils. This is not only a hugely time consuming process - it doesn't work at all in ambient temperatures below about 14°C. In the case of the Blyss 28L which I tested, it spent about 30% of its entire duty cycle in this process alone, during which it was not dehumidifying at all. Why is it so hard for dehumidifier manufacturers? Reverse cycle compressors are ubiquitous - they are used in all auto-defrost fridges and freezers available today!
- Decent sized water tank - at least 8 litres, 10 litres preferably. This means the dehumidifier can be left unattended for several days without it stopping because the tank is full. Continuous drainage is not an option in many places because there is no drain. Drainage into a larger vessel is not an option either because when the larger vessel fills it will not shut off the dehumidifier and water will overflow into the room. Anyway why throw away the water - it is distilled water to all intents and purposes and very useful for many things**. The simple answer is a dehumidifier with a decent sized water tank, like the 10 litre tank of the D-450. Moreover the tank is self contained with a handle that is very easily removed for emptying (see right). As well as tank full shut-off the D-450 has two indicator lights: tank full and tank 80% full. These days it is very hard to find a dehumidifier with a tank larger than 4 litres and many of them have absurdly small tanks of only 2 litres. These very small tanks cripple a dehumidifier right from the start - it's hard to imagine why they do it - do they really expect someone to be in attendance two or three times a day just to empty the tank? (as may be needed in warm humid conditions). The D-450 shows how it should be done, with it's 10 litre tank.
- Analogue Humidistat - the D-450 has a potentiometer controlled analogue humidistat. It works very effectively with a perfect hysteresis loop. When it cycles off it consumes virtually no power and starts again automatically when RH gets above the set threshold. It also means the setting is retained when power is removed.
- Starts automatically - no need for "auto-start"! Some dehumidifier manufacturers try to make a feature out of a negative with a ridiculous "auto-start" feature. If a product needs this feature it means the manufacturer has crammed it with too many redundant electronic gizmos! With a decent design like the D-450 the product will always start automatically anyway.
- Good power consumption - only 220 Watts with the D-450 (actually measured) compared to 400 - 600 Watts with many current day products.
- Very Quiet - I haven't tried to measure the dBA but the fan noise is hardly more than a whisper and the gentle hum of the compressor is even quieter. By contrast the Blyss 28L made a loud galloping thrum - thrum - thrum noise which was quite unpleasant and could easily be heard in the next room.

* Possibly the closest product to my search (and confirmed to have reverse cycle (hot gas) defrost) is the Mitsubishi, however it is very costly (~£350) and has only a 4 Litre water tank, the emptying of which appears to be quite a rigmarole. It also has a lot of 'very advanced' features all of which are redundant for the large majority of appIications - why can't they make a model with all the 'advanced' stuff removed and give it a decent sized water tank instead? I have looked at desiccant dehumidifiers but they don't appear to have the capacity, have very small water tanks, and have very high energy consumption. I have also looked at 'garage dehumidifiers' but the 'intelligent defrost system' means it is actually quite dumb: no reverse cycle defrost means they will become unusable in very low temperatures - they also have absurdly small water tanks.

** The distilled water produced by a dehumidifier is a natural by-product - it costs nothing and is valuable for a wide range of applications including:
- Optical cleaning fluid for astronomical and photographic equipment: mix 1 part distilled water with 1 part Isopropanol and a drop of dishwashing detergent
- Mixing up chemical solutions which require distilled water
- Steam Irons: the steam jets will never clog with limescale
- Windscreen washer fluid for cars: the jets will never clog with limescale
- Final rinse after washing car: it can be left to dry and will come up gleaming, no need to dry or polish the car! I store the distilled water in 2x 200 Litre water butts in the back garden and wash the car using a jet pressure washer. Saves mains water! Immune to hosepipe bans!
However, note that the water should be considered non-potable, because it may contain airborne bacteria and (in the case of a new dehumidifier) may contain metallic trace elements from the evaporator coils.

Water Extraction Rate (Litres per day)
The Amcor D-450 is rated at 10.5 Litres per day, yet in tests over a five week period it extracted more than double the amount that the Blyss 28L (28 Litres per day) extracted. How can this possibly be, you may ask? The answer is simple: The Amcor spends around 95% of its time actually dehumidifying. Only 5% is taken up with defrost because of the very efficient reverse-cycle defrost. By contrast the Blyss 28L spends only about 30% of its time dehumidifying, because it spends at least 30% on the defrost cycle alone (which doesn't work properly anyway) and at least 30% of the remaining time going through its hugely long hysteresis loop.
Perhaps it can now be understood how hugely meaningless the "Litres per day" rating is. You can be absolutely sure you will never achieve the claimed rating. It is a purely theoretical rating. It is used for marketing purposes only and can be very misleading.

Notes about the Amcor D-450
The Amcor D-450 (made in Israel) is a brilliant product. It is an extremely efficient dehumidifier, and is working absolutely perfectly today, 17 years after it was purchased. Despite its small size it maintains an average RH of 48% in the large unheated store / workshop area where it is located. It produces copious volumes of distilled water and consumes only 220 Watts of power. It cost £194.95 when purchased in 1995 - it would undoubtedly cost more today but probably not that much more - the price of electrical goods such as this has been declining substantially over recent years.
I have serviced it twice in these 17 years, by hoovering out dust that has accumulated in the heat exchanger area, and by lubricating the fan motor bearings. Air is being drawn in the whole time so, despite the filter, dust will naturally start to clog the fan motor bearings after some years. Tip: oil alone may not be the best lubricant because the accumulated gunge needs to dissolved. This may not be intuitive but try a product like 3-IN-ONE Penetrant Spray - this contains Limonene which has proven very good at cleaning out the accumulated gunge while leaving a residue of light oil as a lubricant. Fan motor bearings typically need only a light oil. This product has been very effective at keeping the D-450 performing 'as new' even after 17 years.

The reason I need another product like the D-450 is that the original store / workshop area has been expanded to twice its original size. I am waiting to purchase one now. I tried quite hard to use the The Blyss 28L as the additional dehumidifier but in the end it proved ineffective - it spent the majority of its time just blowing air. I returned it to B&Q - they have a very good returns policy. One point in favour of the Blyss model is that it does have a reasonably sized water tank - 8 Litres (though mine cut out at 7.5 Litres).

Come on, dehumidifier manufacturers! Surely you can do much better! In the market today we have a whole raft of almost useless products: in winter they will stop working altogether in any unheated space. How useful is that? In summer they will stop working up to three times every day just because the abysmally small water tank has filled up. How useful is that? I am waiting to buy - just show me something decent...




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