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Dehumidifiers


geoffbeaumont

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My garage dehumidifier has died - I was mildly annoyed until I worked out it was about 8 1/2 years old, and had spent nearly half of that fighting a losing battle in a leaky garage. And all of it full of dust. It wasn't much over a hundred pounds, so it's payed it's way!

Anyway, looking at replacements - there are both compressor and desiccant versions. As far as I can see the desiccant ones draw a little more current, but supposedly run less so there's probably not a lot in regards running costs. They also act as a low powered heater, though that's a disadvantage in the summer, and looks like they might operate at slightly lower temperatures (1oC as opposed to 3-4oC for a compressor dehumidifier). But...I wonder how well the desiccant rotor works once it's got a bit dirty - which it will, in the garage (particularly sawdust).

Does anyone with experience of these beasts have an opinion as to which is better? The new garage is unheated, but weatherproof and naturally relatively dry, so it's not going to have to work that hard, it's more the conditions it has to cope with. It's basically just there to stop the tools going rusty.

Also - for this purpose, is it worth paying the extra for a low end commercial model (I could get one for around £300, versus around half that for a decent domestic model)?

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Hmm - after a bit of research, desiccant dehumidifiers are generally recommended for unheated areas as they not only work at lower temperatures but are significantly more efficient below room temperature.

However - some models at least seem to have a habit of failing due to dust contamination (or at least, that's the reason the manufacturer gives - probably because that puts the blame on the owner), even when used in houses. Some at least also warn in the instructions that they are not suitable for use in environments with flammable dust, presumably because of the heating element. Doesn't sound like they'd be a good idea around sawdust, so I think I'll stick with a compressor one.

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Possibly - but I'd imagine it would cost more to run than a dehumidifier? The garage has cavity walls and an insulated roller door (they're still a major cold spot) but that's it on the insulation front. It'd also be ineffective in warm wet weather (we have plenty of that in Somerset).

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I have debated for a long time heater versus dehumidifier, and come to the conclusion that an oil filled radiator would be the best for me. Living 1/2 mile from the sea, in strong westerly gales we get salt spray on house windows and the sea spray that goes with it. I feel I would be trying to dehumidify Orkney not just my workshop, things are that bad with condensation/damp that I have built a tool / equipment store that is heated by the oil rad inside my workshop.

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Well, for either a heater or dehumidifier to be effective you need the space it's acting on to be as water and draught proof as possible - else yes, you are attempting to dehumidify (or in your case heat) Orkney. Which sounds like a losing battle to me :lol: Had the same problem with the old garage here (not sea spray - but we have plenty of rain in Somerset, and the garage was neither draught proof nor water tight), and the dehumidifier ran full time in anything but unusually dry weather (or cold, when it stops working).

I ended up opting for an Electrolux EXD16DN3W - a decent domestic grade compressor dehumidifier not a world away from the old one (can't remember what that was - not a well known brand as far as I can recall, so probably Chinese made and I was just lucky with the quality). I have an oil filled radiator out there too, but only use it to take the edge off the cold in the winter - it's not powerful enough to make much of a dent.

I guess for you the inability of a dehumidifier to work at low temperatures is more of an issue (here, it doesn't often get below freezing for long - however, heating the garage to above ambient temperature in the summer would be a problem, practically and for comfort).

Although you have just reminded me that I left the oil filled radiator on at the weekend... Oops!

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Most compressor dehumidifiers (barring the cheaper end of the domestic ones) have auto-defrost, however this doesn't mean they'll work in freezing ambient conditions. Remember that they work by cooling an element (basically looks like a small car radiator) down to below ambient temperature and blowing air through it. Water condenses out on the element and drips off. At low ambient temperatures the element is below 0oC and the water freezes on it. The auto-defrost heats the condenser element so that the ice melts and runs off, allowing the unit to operate at lower ambient temperatures - not freezing though, the lower limit is generally in the range of 2-5oC.

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It's also worth noting that because they rely on being colder than the ambient temperature condenser dehumidifiers become less efficient as the temperature drops (even without taking into account the energy used to defrost the condenser element). Absorption dehumidifiers use more power, but maintain their efficiency at low temperatures as they're relying on a hydrophilic material rather than a temperature differential to remove water from the air - in theory they could be cheaper to run in an unheated building as they'll get the job done quicker, which is why I was wondering whether they were suitable for use in the workshop.

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As a side question; the new dehumidifier both displays the current humidity and allows a specific target humidity to be set (as opposed to the old one, which just had low, medium and high settings). Inevitably, this has led me to wonder what a sensible target humidity actually is?

It only needs to be dry enough to protect my tools - any drier is wasting money.

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My tool  / equipment store is 15x 9 feet and insulated with 2 inch kingspan, I dont think it will take a lot to heat but not sure what sort of temp it would need to be to prevent condensation. It really annoys me to think the tools I fetched here 12 years ago are mostly rusted, not beyond use but not as I would like them. Its only one side of the tools, that is the side facing upwards, and its funny but some spanners that are 40 years old have not rusted must be down to better steel.

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We specify desiccant dehumidifiers for a variety of clients and uses but these are typically used in virtually airtight environments - they are efficient to run and, as stated above, work by regenerating the rotor using a heater element.

 

As such, you need recycled air passing over the dehumidification rotor and also an extract to fresh air which will dump the wet output air - there is a slight 'heating' benefit in that the incoming (dry) air is heated to around 3 deg C above the ambient temperature as it passes through the regeneration process.

 

Keeping RH below 45% will virtually eliminate the risk of rust but, if you have leather upholstery or similar, may also be too dry for other components.

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  • 2 weeks later...

I got home from work on Friday evening and did my usual check of the sheds etc. and found my workshop rather damp!

IMG_0333.JPG

Thats the engine I have sat ready for the 110 - not good at all. Seems to be the lower 2' of things that have condensation on them, and there are wet patches on the fooor which leads me to believe it may be rising damp due to lack of DPC under the slab.

Gonna add some better ventilation this weekend to try and get some airflow through the building.

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