Humidity level

[David Ragan]

Our old bsmt dehumidifier crapped out, and took it to the repair guy-non-viable repair.

So, with the new one in now, and running, and the water in my shop (from all the rain here) finally going away, I am wondering--what do you all think is acceptable humidity level to maintain to keep the rust down in the shop?

Have not had rust problems here so far. dont' want any. The dehumidifier has a "continuous" setting as opposed to a humidity setting down to, I think 50%.

Do I want to just set in on continuous for the most protection possible, burn it out, and just plop down another $200 in another year or two, or is 60% humidity OK for the majority of rust prevention?

As always, thanks, D

[Eduard Nemirovsky]

David, I have humidity level in my shop 60% most of the summer. Never have a problem with rust.
About dehumidifier working continuously - I have one working practically none stop in my house and it working like that last three summers.
Ed.

[ian maybury]

So far as I know it doesn't necesarily work that quite that way David. There's no absolute cut off %RH below which corrosion stops.

Once there is moisture in the air there will always be some corrosion. The key though is that the rate of corrosion increases with relative humidity (at the metal surface) - slowly from a very low level at first, but gets going seriously above 50% RH and really motors when there is liquid water/condensation present. Protective oils or coatings to a point block corrosion, but contamination by corrosion inducing compounds like salt (from hands) increases it. So cleaning and oiling matter, and moderate the effect of the atmosphere.

The scenario that causes big problems is when machines and tools get cooled to a low temperature (e.g. in frosty weather), and then there's a thaw and warm and pretty humid air containing a lot of water moves in - because the moisture condenses straight out on to the tools.

So temperature matters too. It's probably a matter of maintaining the temperature safely high enough so that in the event of warm and humid air moving in that the steel won't be cool enough to drop the air ideally below say 50% RH temperature, or definitely not below the dewpoint temperature to cause condensation. So the room and hence the hardware probably needs to be maintained above the 50% RH temperature, and definitely above the dewpoint temperature of the wettest air likely to get into the shop. (highest specific moisture content lbs water per pound of dry air from a psychrometric chart - or highest temperature and %RH)

It's for this reason influenced a lot by the dynamics of the ventilation and air conditioning in the space too. If you have large doors that get left open so that cold air can get in to cool the metal below the dew point temperature even with the heat on then there are going to be problems. There's likewise going to be a time it takes for a dehumidifier to pull the %RH down to a safer level when this happens too. So basically the rate of leakage/the air change rate in the shop has to be matched to the capacities of the dehumidifier and heating plant.

Even then proper air mixing may be a factor - if tools or metal are exposed to a localised cold draft then that may cool them enough to cause condensation or at least cause a step up in % RH which will increase the rate of corrosion/rusting.

There's in essence mulitple variables in play - %RH, temperature, surface contamination, stability of control of temp and % RH/absence of localised cold and/or wet/damp pockets of air.

It's on average about 70%RH here in Ireland (damp), and around 40 - 50 deg F in winter - and experience suggests that provided tools are oiled (camellia work well) and the shop not let get much colder that corrosion isn't generally a big problem - even without air conditioning. If it gets frosty though (perhaps 8F below freezing outside) then it becomes important to heat to maintain this room temperature, and to prevent the ingress of large quantities of outside air - when its cold (because it'll cool the room), or shortly after a warm front comes through. (because the air will be very wet, and will increase the humidity in the room)

Adding air conditioning to this to pull the room down to maybe 50% RH would no doubt improve the situation and give a bit more cushion, but it's rarely done here. Many shops don't even bother to heat enough to give protection in after cold weather, and rusty tools and machines are commonplace.....

[Robert Engel]

100% agree with Ian. Rust comes from condensation, not humidity.
I live in a very humid area and if I keep my shop closed up at night, I never have problems in the summer, where the humidity is always 80-90% till about 10 in the morning, and never below 50.

When we start getting cooler nights that's when I start having problems because machines cooling off at night and water condensing when they warm back up.
I don't know a number, but common sense tells me you would have to keep the humidity way below 60% to avoid it.

There are all kinds of solutions to avoiding the problem which I can suggest if you want.

[Scott T Smith]

David, one other thing to consider is no so much rust on tools, but rather maintaining the RH% in your shop at the same level as your house. That way the furniture that you build will not have to acclimate further (ie wood movement) after leaving your shop.

[Al Launier]

Another consideration is the electrical service Panel. I recently had a 20 KW whole house generator installed. When installing the new electrics, the electrician noticed, and showed me, some of the wire nut connections. Most of them had a white film of corrosion on them. So, to provide protection for the service panel & the generator electrical panels, he recommended a de-humidifier that would control humidity than just the one 72 qt. unit that I had for the open basement. I bought a Fridgidair 72 qt. unit for the combined "utiliy room" and the woodworking shop. I keep the RH for both dehumidifiers at 35% % the unit removes about 2 gal./day. Now I feel better!

[John McClanahan]

I have been involved with humidity control for years at my job (not woodworking). Here in N.W. Missouri 55% is about the lowest the humidity gets in summer using air conditioning and dehumidifiers. Under 50% is almost impossible.

I have seen small dehumidifiers run non-stop from spring thru late fall for many years without failing. Kinda like guessing how long a light bulb will last.


John

[ian maybury]

Hi guys, a related question.

We get small plastic cased domestic type dehumidifiers here which if you were to believe the capacity in terms of the amount water they are rated to extract should be fine in a small shop - but in practice they are next to worthless.

They tend to throw up several issues (apart from a mouse powered fan) - they run some sort of unstated internal %RH control (which likely doesn't just target a simple set %RH number), there's as a result no option to adjust the %RH setting, and their capacity anyway falls away to almost nothing by the time the room drops to the temperature where the damn thing is actually needed in a workshop. The presumption is presumably that a house won't get that cold inside, and the refrigerant doesn't go to a low enough temperature.

There's much more robust looking industrial models sold for drying out newly built house interiors and the like, and even more expensive corrosion resistant specialised models for wood drying too - but i haven't taken a close look yet.

What's the story in the US? I seem to hear people talking of using domestic units in shops and even for drying wood - but maybe the conditions are warmer or something. What options are available, and how well do they work?

Wonder why the woodworking suppliers don't offer workshop dedicated models?

[Al Launier]

I have been involved with humidity control for years at my job (not woodworking). Here in N.W. Missouri 55% is about the lowest the humidity gets in summer using air conditioning and dehumidifiers. Under 50% is almost impossible.

I have seen small dehumidifiers run non-stop from spring thru late fall for many years without failing. Kinda like guessing how long a light bulb will last.


John

John I use a Bionaire °F/RH meter to monitor the temperature & relative humidity in my shop. Granted, it's not a highly sophisticated, calibrated unit, but seeing 35% on the meteris "good enough & satisfying" for me. The shop is ~400Sq. Ft. & the utility is about half that

[John Donofrio]

You may find this useful: http://www.dpcalc.org/

I leave the dehumidifier on during the summer and keep the shop around 50-55% RH

[Peter Aeschliman]

I'd like to add emphasis the point about condensation.

I'm in the Seattle area, so the climate is very different than GA. Our dew point is much lower, so we get high RH even though it doesn't feel nearly as muggy as GA. My point is that relative humidity isn't a terribly helpful measure, because it's relative to the dew point and the temperature, and really doesn't tell you how much actual moisture is in the air without doing some calculations.

But I digress.

My experience here has been that rust on machinery has more to do with rapid temperature changes than it has with maintaining a specific relative humidity. If it's humid, that just means their is lots of moisture in the air. It doesn't mean that there is lots of moisture on the surfaces in the room, including your tools. So, condensation causes rust. Not RH. Air loses its ability to hold moisture as the temperature decreases. Cast iron holds its temperature really well (hence why Le Creuset charges so much for their pots and pans). Air temperature changes much faster than cast iron's temp. So when your cast iron table is warm and the air temperature drops, as the cold air touches the warm cast iron, it becomes colder, is less able to hold moisture, and loses its moisture as condensation on your beautiful cast iron.

My shop (a 450 sqft out building) used to be un-insulated, and I was constantly dealing with rust. After I insulated the shop, I never had another rust problem. I don't have an air conditioner or a dehumidifier, and I only heat the shop while I'm using it. So simply adding insulation solved my rust problem, because it dramatically slowed down the rate at which the air temperature changes, thus reducing the temperature differential between the cast iron and the shop air... thus eliminating condensation.

My point is, first determine whether you have a problem to solve. Try leaving a dehumidifier off for a while and see if you get any rust. If you do, the first thing to do is insulate (if you haven't already).

Granted, you may already need a dehumidifier since the weather is so unbearable down there (I say this with sarcasm from Seattle). So it might be a moot point. And of course as already mentioned, the moisture content of your lumber should match your house to reduce the immediate expansion or contraction when you move a finished piece into your home.

So I'm really just talking about rust.

[David Hawxhurst]

What's the story in the US? I seem to hear people talking of using domestic units in shops and even for drying wood - but maybe the conditions are warmer or something. What options are available, and how well do they work?

Wonder why the woodworking suppliers don't offer workshop dedicated models?

i have a dez-air in my crawl space (2000 sqft/4 ft height) that is set at around 38%. the crawl is sealed and conditioned so it maintains about 66-68 degrees in the summer and slightly lower in the winter. in the shop (garage) its ~400 sqft/11 ft ceiling i run a Fral and its set for 45% and tends to keep it around 40-42%. the garage is fully insulated (including the garage doors) and is conditioned by a mini split with heat. obviously when you open the door(s) i get a humidity spike but the door is not open long enough for the temp of the machines to change and cause condensation. i this has pretty much eliminated any rust/corrosion problems. neither of these dehumidifiers i have are your run of the mill domestic type. they don't have and buckets to empty and are continuos drain type. they are more of a industrial type style. even with near 100% humidity outside my garage can maintain less than 45% and the dehumidifier will shut off and not run continuously like the regular small domestic type with a drain bucket.

this one is mounted on a hang platform from my ceiling in the garage http://www.allergybuyersclub.com/fra...ml?itemId=3518
this one is the one in my crawlspace http://www.allergybuyersclub.com/dez...ml?itemId=3154

the one in the garage is fairly new 2nd summer. no issues so far.
the one in the crawl had a fan go bad but they replaced under warranty. other than that its been down there for close to five years now.
both get inspected and filters cleaned/replace on a regular bases.

[Malcolm McLeod]

Peter's experience matches my own. My garage shop is insulated, the tool's don't see the same temperature swings as the air, so no condensation.

That said, air at any %RH (other than non-existent zero) will condense on a tool if the tool is below the air's dew point.

The OP's question is what to do with his dehumidifier. If you can insulate, you might not need it. If you can't insulate, then perhaps a heater to keep the basement temp a couple of degrees above the dew point. ...Not sure what kind of control might be available for on/off signal based on temp vs. dew point? The dehumidifier just helps reduce the amount of time you might need heat.

[Tony Leonard]

Just for reference....my shop is in the MidSouth, insulated, AC/Heat. I found that keeping the temperature reasonable did not dehumidfy enough, so I use a small dehumidifier. The controller died (twice), so I built my own using an Arduino. The RH/temp sensor I use is pretty accurate (RH is hard to measure). I keep it set on 50% (it drops to 47% when it runs). That seems to work pretty well. No rust for 17 years or so. I have to empty the bucket every other day this time of the year. I could probably keep the ac on a lower temp to get rid of some of the humidity. Never tried to figure out the most efficient setup though. This seems to work and it isn't costing a fortune in energy best I can tell.

As a side note, I think you can buy a humidistat switch to control the unit if it is your control that broke. I just turn the dial on the unit on full blast. My controller turns the AC on/off based on the program.

Tony

[David Ragan]

I had forgot all that condensation science.

Took a meterololgy course in college cause I needed something easy. Not. Physics of fluids. I guess they were so serious about it cause it was Univ of Okla-tornado alley.

Anyway, the shop is contiguous with the rest of basment when I leave door open during weekdays. Does get a little cold in coldest winter; fortunately, nothing in basement makes water vapor down there.

I'll just leave dehumidifier on full blast all the time.

Thanks guys