Dehumidifiers???

[ondablade]

Having just bought an EBAC 2650 domestic 18L dehumidifier http://www.ebacdirect.com/product.php?productid=27 to stabilise conditions in my shop (it's also for occasional domestic use) i guess i'm wondering how best to use it. (or even if it's a good idea)

According to the climate data our (Irish) average outdoors annual relative humidity is around 70% RH, which gives an equilibrium moisture content for wood of about 13%.

It gets a bit higher in the winter, more like 80% RH on average, and a little lower in Summer. (but don't forget the Summer temperature might be 65 deg F while the winter is around 45 deg F - so there's a lot more actual water in the air in lbs of moisture per pound of dry air terms in the Summer)

When heated to say low 70s deg F in a centrally heated (but not air conditioned) house this air will drop to somewhere between 35% and 60% RH in winter and summer respectively - which is equivalent a range of equilibrium moisture content for wood from about 7 - 11 %.

Air conditioning will drop moisture out too, but that's another ball game since it's very rare around here.

These dehumidifiers can be set up with a drain hose so they don't need emptying, and have a humidistat so they will come in and out automatically as required. The only real maintenance required seems to be to clean the filter monthly, and replace it three monthly.

If i was to set the dehumidifier to say 55 - 60% RH this would give around 10 - 11% equilibrium moisture content for timber that has equilibriated to those conditions in the shop.

Does this sound reasonable? Is it better to start with your timber slightly on the wet or the dry side of it's final use conditions? Some might argue the shop should maintain/bring timber down to more like 8% moisture content which is equivalent to more like 45% RH.

To put it another way. Equilibrium moisture content is not related to temperature (except slightly by about 1% lower for every 25 - 30 deg F increase in temp), or even to how much water is in the air - it's instead related to % RH or relative humidity.

Which means that a dehumidifier with a humidistat that will automatically maintain a set % RH regardless of changes in temperature will actually more or less directly control the moisture content of wood that's left for long enough to equilibriate in the space. i.e. The issue is basically to figure what's the best %RH to maintain the shop at. ..

ian

PS given the relatively wide range of moisture content (12 - 17%) through the batch of beech i've just bought for my bench, and being of a sceptical bent i'm actually inclined to wonder if most of the timber we use to build furniture is anywhere near the dryness we think it is/it's supposed to be. Most timber merchants seem to notably steer clear of specifying the moisture content for their kiln dried timber....

 

[moz]

Is it better to start with your timber slightly on the wet or the dry side of it's final use conditions?

Hi Ian,

In my experience, I would say that you tend to have far fewer subsequent problems if the timber is too dry than if it is too wet.

PS given the relatively wide range of moisture content (12 - 17%) through the batch of beech i've just bought ...

Trees, like people, are all different. They may be the same species but will have genetic differences as well as being affected by environmental factors. Specific gravity can vary quite a bit so I wouldn't take it as read that moisture content is varying just because your meter says so, particularly if the timber is not through and through as it comes off the log (though 5% seems a lot).

John

 

[ondablade]

Thanks John, i guess it makes sense that if it gets a little wetter after making that joints etc will tighten up.

On the beech the wet bits are towards the same end of the planks that were top of the bale - my suspicion is that maybe one end got rained on at some stage. That said my readings are with a pin meter, so the wetness may not penetrate all that much. (it's already dropped by 3% to the 14% EMC that corresponds to the %RH in the shop in a week)

It seems that equilibrium moisture content is very similar for all species for a given atmospheric % RH, but as you say no doubt differing woods may respond rather differently to the same % moisture content...

ian

 

[moz]

Sorry Ian, I didn't know which type of meter you were using. I use a Wagner electro-magnetic moisture meter. The readings depend on timber density; it is set to an average and the meter has to be used in conjunction with a species adjustment chart (since they all vary in density). I've never used the pin type but, since they just rely on conductivity within the small path between the pins, they can be prone to some misleading readings in places. I think it unlikely to find a 5% difference in a single board. I don't know whether this is useful

http://www.wagnermeters.com/faqs.php?la ... &pageid=29

John

 

[chipchaser]

Hi Ian,

I bought myself a secondhand dehumidifier for the workshop but haven’t yet used it as I decided the building was too leaky (draughts not rain!) and I would end up unsuccessfully trying to modify the climate of Warwickshire. However, I will try it when I have better sealed the double doors.

I agree with John, better to be a little drier if you can’t match the final location exactly. The comment by Professor Wengert about half way down the following Woodweb thread might be of help:

http://www.woodweb.com/knowledge_base/I ... nsion.html

Purely for interest, I tried testing a few different timber items in my kitchen. I found readings between zero, which means less than 6% on my meter, and 14%. The zero readings were in pine fixtures finished with polyurethane varnish 20 years ago. 10% in a pine table finished with what I suspect to be two pack because it has been so durable. 14% in a pine dresser with wax finish.

I could easily believe that the variation in readings in your beech was due to some random influence like one end being rained on. Are the readings coming closer together now it has been in your shop for a while?

Regards

Graham

 

[ondablade]

Hi John. My meter is a Lignomat MiniLigno DX/C approx 5 - 60% pin type. I don't know what the beech is like internally for the reason you say, and i guess it's possible too that's something is throwing the meter.

That said i've been testing it for about two weeks now. The readings are very consistent, but have dropped to around 14% which is close to EMC for my shop conditions, and which is now (unlike at the start) very close to equal along the length of the planks. Which suggests the possibility of surface wetting to me.

I'm glad i'm not the only obsessive on this topic Graham. Woodweb is a great source of reference for drying information. I know the theory of both psychrometrics and basic drying fairly well (i have a copy of Understanding Wood) but I must say that i was thinking your thought about drying the whole county too when i bought my DH.

I don't know if people do this on a whole shop basis, and how effective it might be in pulling the shop % RH down is one of the million dollar questions.

The theory is easy. A psychrometric chart says that my shop in its current winter conditions (around 75 - 80% RH and 45 - 50deg F) probably needs around one litre of water removed from the air it contains to get down to say 50% RH/9% EMC - which should be no problem for an 18L/24hrs unit designed to dry a complete small house.

On the other hand as you say how effective it will be all depends on the number of air changes in the shop in 24hrs, and on any other sources of moisture. The shop is actually quite well sealed, has housing standard damp proofing, and so far as i can tell is pretty dry.

I've only started the DH running continuously this afternoon (wanted to be around when it ran first to make sure it wasn't a fire risk or whatever), but it's going to be interesting to see how low it can pull the %RH.

The beech since it's destined for the shop will probably be fine at 11% MC (i'll cut a board and test the centre when it looks to be stable, and i've figured out which lengths to use for what), but it'll be interesting to see whether it can bring the shop down to that or the 45% RH or so required for 8% EMC for wood destined for use indoors.

Lower even if it's required to speed up drying - although for example i haven't much of a sense what the schedules required for this may prove to be. (to drop the MC by maybe 5%) It's not easily estimated by all accounts.

Plan B could be some sort of heated and dehumidified polythene enclosure to make a pseudo kiln, although i read on Woodweb that domestic DH units don't much like to be used for kiln drying (i.e. in hot small spaces) as they tend to suffer corrosion problems.

Next step might be to invest in some of the drying manuals published in the US by i think the USDA referenced by Gene Wengert (? - the wood technology specialist on Woodweb) to get a better handle on stuff like schedules.

This possibly sounds a bit over the top, and may well be if as i suspect could be the case most use timber at quite widely varying levels of dryness. On the other hand i can't help feeling that maintaining the shop at the appropriate level of humidity, and if necessary allowing wood to condition (in a DIY kiln if needed) might just be the best way to avoid the nasty surprises i keep on reading about...

 

[RogerS]

Can I be blunt?

Did they have dehumidifiers when Chippendale was making furniture? Do any of the professional furniture makers use dehumidifiers? What exactly is the point? Your wood is going to adopt whatever humidity it feels like depending on when and where and the relative humidity of the day or location.

I bought one for my workshop but really to minimise the likelihood of condenstaion forming on my cast iron surfaces. I've not bothered to turn it on this year and I doubt if I ever will.

Your wood is going to move far more taking it into a centrally heated location then in your workshop with a bit of moisture sucked out of the air...only to be replenished when you open the door.

Keep it switched off.

Mike G is remarkably quite on this one :wink:

 

[MikeG.]

Mike G is remarkably quite on this one :wink:

'cos as soon as I take over the running of the world dehumidifiers will be banned :wink:

As I have said before, they are a pointless waste of energy masking simply corrected flaws in a building's insulation and ventilation.

Mike

 

[RogerS]

Actually thinking a bit more about it, it might be better to use a patio heater to keep the workshop cosy and then the dehumidifier running 24/7 to remove the moisture that the patio heater will kick out. :wink:

 

[ondablade]

That's exactly the sort of feedback i was hoping for Roger - I'm just finding my way. I've already lobbed a few 'just how much of this matters' sort of questions.

That said there's some basics it's hard to escape. (unless you know better ;-)) Mr. Chippendale presumably would air dry timber for years, didn't have to contend with 70 deg F indoor temperatures in Winter, and anyway probably had a well figured game plan based on some combination of smart design and experience. (of which last it'd be dead interesting to hear of)

The happy reality for him in this part of the world though is that despite the weather seeming damp and soggy in winter and drier in Summer the average % RH doesn't actually vary by more than about 10% over the year which by the time the rate of change of the moisture content of the wood is slowed by lacquers etc is probably only the equivalent of a 2 - 3% change in equilibrium moisture content. Fires when lit in the old days presumably only warmed rooms for shortish intervals anyway.

On the other hand take that same 70% Rh at 50deg F shop air and heat it to 75 deg F continuously as in a centrally heated room, and you are looking at the %RH (which determines the equilibrium moisture content for wood) dropping to not much over 30%RH - a 40% drop, and more to the point equivalent to something like a 7% change in equilibrium moisture content from what may be your shop conditions - or approaching a 5% change in dimension in oak as it goes from 14% down to about 7% moisture content.

This is significant - as in 1 inch in 20 across the grain if the timber gets equilibriated. It probably won't make this in practice, again due to lacquers, cycling of heating etc, but it could. The other practical reality is that you can air dry your timber for as long as you like in ambient conditions (Winter or Summer), but barring some very weird weather it's not going to get much drier than 12 - 14%.

Bringing it into a 50deg F 70% RH shop to 'condition' isn't going to make a blind bit of a difference in getting it to dry further - not unless that shop is also continuously heated to 70 degF - or dare i say it it's dehumidifed.

I'd be the first to agree that dehumidifiers don't belong in properly designed and built houses that don't have damp problems, but for conditioning timber it's probably going to be a damn sight cheaper and more energy efficient than heating your shop to high temperatures. (some combination mind you might be a nice comfortable move in Winter - maybe a cheap form of heating so that the DH's work is minimised)

It's hard to argue with the basic physics of the situation Roger - i can't see how furniture to go into modern houses can be made without controlling shop conditions.

It'd be easy to fling energy at the situation though. The million dollar question is as before - are there practical ways to get around problems? Design is clearly a big part of the deal...

ian


[/i]

 

[mr grimsdale]

Haven't read it all it's a bit long but
Mr. Chippendale presumably would air dry timber for years, didn't have to contend with 70 deg F indoor temperatures in Winter,
He may well have - they had huge fires in the good old days! They also had a bigger range of conditions to deal with than we would endure today - damp houses etc
Equilibrium moisture content is not related to temperature .... or even to how much water is in the air - it's instead related to % RH or relative humidity.
Yebbut RH is related to temperature, EMC is related to RH, so EMC is related to temperature. Hence turning up the heating will dry your wood.
The trad solution to shrinkage problems is all down to design (given a reasonably dry wood to start with)

 

[mr grimsdale]

Actually thinking a bit more about it, it might be better to use a patio heater to keep the workshop cosy and then the dehumidifier running 24/7 to remove the moisture that the patio heater will kick out. :wink:

That's exactly the sort of feedback i was hoping for Roger - I'm just finding my way. I've already lobbed a few 'just how much of this matters' sort of questions.
I think he may have been pulling your leg - it's a mad idea!

 

[ondablade]

I meant Roger's comment about being 'blunt', not the patio heater!

No argument on the relationship between temperature and %RH in a given volume of air of a given specific humidity. That's why when you take outside air into a house and centrally heat it it the %RH drops so much - why as above it can cause so much trouble.

The reality is nevertheless that the equilibrium moisture content of wood is directly related to the %RH of the surrounding air, and pretty much independent of the ambient temperature. As you say %RH does change with temperature though, so if you e.g. cool a room you have to dehumidify to maintain a constant % RH.

Anyway - the question remains. Just where are the boundaries in terms of drying and conditioning of timber for practical purposes? I'm trying get to some hard information/experience here, but like most things in woodworking it's a minefield of contradictory opinions.

Mag writers when in pontifical mode will waffle on about the need for 8% or whatever, as will timber merchants/kiln dryers in sales mode. (before carefully avoiding specifying anything on the sales documents)

There's this wing of the game that goes 'aw shure don't mind all that old baloney'. Yet there's as many saying otherwise, lots talking of almost freezing workshop conditions, and lots more posting of shrinkage and warping problems.

??

ian

 

[dicktimber]

DON'T USE EM!

My mates wife tripped and let a basin of water fall on her best carpet.
She didn't tell her husband, but switched on the dehumidifier....
Later that day they went on holiday.
When they came home, the machine was full of water......
all from his aquarium, where the fish lay well and truly dried out.....

He went mad....
But the carpet was dry.

he he he he.....

 

[RogerS]

DON'T USE EM!

My mates wife tripped and let a basin of water fall on her best carpet.
She didn't tell her husband, but switched on the dehumidifier....
Later that day they went on holiday.
When they came home, the machine was full of water......
all from his aquarium, where the fish lay well and truly dried out.....

He went mad....
But the carpet was dry.

he he he he.....

I bet she went all koi.

 

[dicktimber]

In'it fun on this site??????
:lol: :lol: :lol:

 

[ondablade]

Mmmmmm. So they do work!!!!

ian

 

[moz]

i can't see how furniture to go into modern houses can be made without controlling shop conditions.

ian

It's a nice thought. I know very little about them but I believe a serious problem with condensing dehumidifiers is that they become far less efficient at lower temperatures, i.e. just the sort of conditions in the typical workshop where we would be looking to control moisture at this time of year. I wonder whether dessicant types are better and whether they use any less energy overall?

John

 

[Matt@]

using a dehumidifier will seriously improve gluing efficiency especially when using PVA.......

it also keeps the abrasive paper nice and dry :wink:

as long as you have the right size dehumidifier for the work area...

 

[chipchaser]

Hi Ian,

I couldn’t admit to being obsessive but try to understand the theory before deciding whether to ignore it! My shop is dry but unheated most of the time so timber stored there settles at 14 - 16% in the winter, too moist to suit use in a warm house. As you say, if looking at low seventies F (21C) in use, convention dictates 10% moisture content in the prepared timber and the completed joinery at delivery. I would be well upset if a craftsman-maker delivered to me a piece of furniture that shrank because he had used material at the wrong moisture content.

Chippendale’s furniture didn’t have to survive centrally heated houses and his workshop would very likely have been quite similar in climate to the end use location, i.e. intermittently heated. Huge fires in C18th houses? Lots of radiant heat when burning but they also draw in lots of cold air from outside so the RH was probably near to what it was outside. And when the fire dies down the temperature drops off quickly. The room is then cooling until the servants clear the grate and light the fire next day. You can see the damaging effect that modern temperature and humidity regimes have on historic joinery in timber panelling in country houses converted to hotels.

Unless you are drying green timber I don’t see why a domestic dehumidifier in a polythene enclosure would be overstretched. Keeping your stack of beech at 10 – 11 % wouldn’t be much different to the service it was designed for. I don’t even think it is always a profligate use of energy. As my workshop is used irregularly upgrading and heating it to maintain a steady environment would be much more expensive than occasionally running a dehumidifier in an enclosure when required. However, it would probably be worth upgrading and heating my workshop if I used it daily. It would certainly be more comfortable. Actually I do need to relinquish this space to its rightful owner as soon as I can find time to build a new one which will be better suited to regular use. Don’t think that will be very soon though.

Good luck with your beech bench. Are you making or planning to make furniture or similar? My Emir beech bench came secondhand from Corby and the top had moved, they keep their stuff (recycled from schools) in an unheated warehouse with roller shutter doors. The edges of the top are misaligned but it’s not a problem in use.

So far I have only used my dehumidifier to dry out the muminlaws carpet/ floor after a radiator valve leaked unnoticed until the dampness reached the middle of the room. The progress of drying could be seen by the amount of water in the tank each morning. Now that I have been educated by dicktimber I am disappointed that I didn’t manage to dessicate her small noisy dogs. I imagine very realistic “Staffordshire dogs” siting quietly in the corner

One of those USDA manuals on drying timber can be downloaded free. You’ve probably been here already:

http://www.fpl.fs.fed.us/products/publi ... eader_id=p


Regards

Graham