De-humidifiers

Unlike mobile air conditioners which usually draw in air from outside because they blow the heated air outside from the condenser, dehumidifers only work on the air in the room/workshop, so only that volume of air gets treated. As long as no new air is drawn in, the humidity is lowered.

We have a largish portable airconditioner here, and the output hose is dangled outside a window and gets really hot, but of course the air it exhausts is replaced by warm air coming back in again...

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:

I think they are excellent I look after a holiday house in Sussex, 15 years old with dg windows. To all intents & purposes it's hermetically sealed! The DeH is run 3 hours daily and it consistently drags out over 5 litres of water every 2 weeks or so, more in winter& That is when there is no one there. The house doesnt have a "damp" problem, just dont know where the moisture comes from . It is an Ebac unit and it works by drawing air over a fridge type chiller unit. It has variable setting to increase or decrease extraction

Heartily recommend use of these

They are very good at keeping the workshop dry, and cheaper to run than a heater. Water gets in the air from various sources. Any sources of damp, such as leaks. Obviously air gets in from outside through draughts, open doors and windows, etc. However, the dehumidifier takes the water out faster than the damp air can replace it. That is why you have to run them continuously.

Hope that makes sense

Cheers Mark

I have always run a de-humidifier in my workshop and an oil filled radiator turned down low, my workshop is within a larger shed, and is well insulated...walls and ceiling. it is also well sealed, I even have a strip of that brush stuff on the bottom of the door. I really should knock off the de-humidifier while I am in, but I usually forget to switch it back on when I finish up.

It's nothing fancy just a small one from Curries and in a fortnight it has pulled about 2 gallons of water out, of course I am only in there part time, and when the door is shut (when I'm not in) it runs until it reaches the preset level and cuts out of course. I think it takes a couple of hours get get the room dried out again.

Nothing ever rusts in there, I bead blasted a couple of bits off the Landrover on Friday, put one in the W/shop and left one in the bigger shed, the one in the W/shop is still perfect but the other one will need a quick going over again before I paint it.

Anything like paper put in the W/shop gets nice and crisp after a day or so, and it's not the first time I have stuck a wet jacket in and by the next morning...bone dry!

I wouldn't be without it.

Rich

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The efficiency drops dramatically with temperature - the performance figures (gallons/hour) on my B&Q box are quoted at 30C. Millitres at 3C from experience.

Steve

Crunch a few numbers.

At about room temperature, say 20 deg C, saturated air can hold about

20 grams of water vapour per cubic metre. It's not an exact figure, but close enough for now.

Let's say that your holiday home is about 100 cubic metres internal volume, and your DeH is set to control at 40 percent RH.

You need to remove 100 x ((1 - 0.4) x 20) grams of water, = 1200 grams or 1.2 litres to get the humidity down to the set level, if the air is saturated.

The rest of the 5 litres or so will come from natural air changes, so we can estimate these as being 5 - 1.2 = 3.8 complete air changes per

2 weeks or so. This actual figure will be slightly more than this, depending on the initial RH.

This is a very low rate of exchange! I suspect that 1 change per hour is nearer the mark for the average house; your one must be sealed tighter that a duck's rear end.

The danger of deterioration in an empty property is largely due to humidity cycling - something that should be avoided. Below about 40 percent RH means that wood shrinks and paint tends over time to flake off due to this; above about 60 percent spores and fungi become active. Cycling outside these limits can be destructive.

My suggestion is generally to run the DeH 24/7, set at 45 - 50 percent, and avoid all sorts of problems, but with such a low rate of air exchange in this particular case possibly not necessary.

As far as workshops are concerned, the problem with humidity is slightly different - the aim is to stop or slow the deterioration of tools and machinery. In this case one needs to avoid the situation whereby condensation occurs, which happens when the local temperature drops below the dew-point.

Typically, this arises when a very cold spell is ended by a fast rise in temperature and humidity; the metal temperature lags behind that of the air, and at some point falls below the dew point. The metal surface becomes saturated with liquid water, until such time as its temperature increases by the relevant amount, and then it's a question of gaining enough heat to evaporate the water. Deterioration takes place as the liability of the metal to rust is at its greatest.

A dehumidifier that recirculates workshop air will be preferable to one that exhausts warm air to the outside, as that warm air can be used to offset the effects of the cold snap, as well as taking out atmospheric water - a double benefit.

What a DeH can't really do is stop any deterioration due to the remaining water-vapour; there will always be some tendency for metal to react; the lowered concentration of atmospheric water-vapour will slow that, of course. But getting down to zero would be difficult, especially as workshops tend to be occupied by human beings - a prolific source of water-vapour.....

At 3C the moisture-carrying capacity of the air is much lower, so there is simply less water to be extracted. The DeH is not (necessarily) less efficient - it simply needs to remove less water to reach the target RH. Measure the effectiveness of your DeH by the level of RH it maintains - and its efficiency by the amount of electricity it uses to do it. In cold weather it should maintain a low RH with hardly any effort at all. Don.

The other point about cheap dehumidifiers is that they are incapable of defrosting themselves at low temperatures because they rely on heat from the surrounding air to melt the ice. If I don't turn mine off below 5º the heat exchanger accumulates an enormous block of ice.

Cliff Coggin.

Just for the sake of accuracy and completeness:

Temp Saturated air g/m^3

0 4.87 1 5.22 2 5.58 3 5.97 4 6.39 5 6.83

10 9.44

20 17.37

So, if you set your dehumidifier to maintain 100 x 5.97/17.37 = 34 percent, your local dew-point will be 3 deg C. Keep an eye open for those cold snaps.

I used to have this problem with mine, solved it by putting it on a timer and having it switch off for 30 mins every four or five hours. I also have it plumbed so that the water drains through a pipe into the flower bed so that I don't have to empty the collecting container. I managed to flatten a

15mm pipe so that I could pass it out in the mortar gap between two bricks in the workshop wall.

Don Valentine

Don't understand the math or the DH RH stuff, but observation shows that my "stuff" is at risk from condensation in winter when its cold and damp and not in the summer when its warm and dry, yet the de-humidifer performance is the inverse, as Cliff pointed out they rely on ambient air temperature for the defrost cycle.

Steve

I suppose another plus in winter, is they do blow warm-ish air into the workshop, and so keep it just above normal air temp. I keep mine next to the lathe, and helps keep it above dew point. Then again a 60w bulb in the cabinet would probably be cheaper in winter, be we seem to get alot more "warm/damp" days these winters.

Joules

I don't need to watch out for cold snaps, as a) the dehumidifier chucks out heat (though not a lot!) and b) my main workbench surface is over the family deep freeze, which also chucks out heat - again, not a lot, but useful in the cold weather!

I used to have to drain off gallons from the tank each week, especially if working in the workshop - I forget how many a person can exhale per day, but its a lot! Now, the drain goes through a small hole in the floor to the concrete below, and the tank has been dispensed with for good. If its really cold, I run a small room-sealed gas convector, which keeps all the machinery from cooling to dew point. An electric fan heater can bring the room temperature upto a comfortable working level very quickly in frosty weather.

speedy

Dave,

I also plumbed my dehumidifier rather than fill up the tank, but every now and again get a very odd problem where it floods. Theoretically impossible as there is a shallow tray under the condeser with the exit pipe in its base going straight down and outside to a drain. Every now and again gravity fails in the vicinity and the water condensate won't flow out and the tray overflows. A quick blow though the pipe implies nothing was wrong and it's fine for a few weeks. I >>think

I occaisionally get the same problem with mine definitely caused by dust. Blowing up the drain line simply pushes the plug of muck into the condensate tray from where it can run back into the drain to block it again. Sucking the muck out as you have done should be a better solution. I can open mine up to get at the tray and wipe off the dust from the inside of the casing.

Incidentally the fan bearings seized up recently because of dust and I was delighted to find a company on the Internet that supplied me with two new sealed bearings for about a fiver including p&p. A very cheap repair if it gives the machine another four years of useful life.

Cliff Coggin.

My solution is that the new workshop walls and floor will have a thermal resistance of 30W/K so that the domestic file server, web server and firewall in the basement, in conjunction with tightly controlled ventilation and heat-exchangers, will keep the workshop temperature at a comfortable value. The air conditioning will control the maximum temperature and humidity. The forty-odd tons of concrete in the floor will limit the rate of change of temperature. I could have got it wrong, but I'm aiming at somewhere that's comfortable for me _and_ the tools all year round.

Mark Rand RTFM