Humidity & low temp in basement shop

[Rick Schubert]

I'm trying to keep the relative humidity (RH) in my basement shop below the level of causing rust and mold/mildew. I've heard I want to keep it at 50% or lower at 70 degrees to accomplish this.

My question is: I keep the basement unheated at around 46-50 degrees when I'm not working in it. I understand as the temperature goes down the RH rises. So what should my RH goal be at 50 degrees?

My dehumidifier is running so much trying to keep RH at 50% at 50 degrres that it is freezing up even though it is a low temp model supposedly good down to 43 degrees. Is it broken or just overloaded running constantly to try and reduce RH to an unrealisticly low level?? Would RH at 65% at 50 degrees still prevent rust and mold?

Thanks, Rick

[Kent Fitzgerald]

Rick, the "relative" in relative humidity accounts for differences in temperature. As far as wood equlibration, mold, and rust are concerned, 50% RH at 70 degrees is equivalent to 50% RH at 50 degrees.

The problem, as you've found, is that dehumidifcation is tough at low temps. You may have to keep the basement heated up 5 or 10 degrees to achieve your goal.

The other thing to look at is where the moisture is coming from. In my house, improving the grading and drainage and keeping the gutters clear has made a big differecne in basement dryness.

[Stu Ablett in Tokyo Japan]

In my Dungeon in the summer, due to the high humidity here in Tokyo, the aircon/dehumidifier runs 24/7, and I get a lot of water out of there, but I do not have you cool temp problems.

I wax all of my old iron, and I find taking a shop paper towel, and spraying it with WD-40, folding it up, and putting it in the tool drawers helps keep the rust away.

Cheers!

[Chuck Saunders]

The only problem with the dehumidifier is that the air blowing over the condensate coils is not warm enough to prevent the frost. This can be corrected with higher ambient temperature or higher airflow over the coils. Fans are cheaper than heat so you may try to direct a fan over the coils to keep the frost at bay.
Chuck

[Ian Abraham]

You may have a fault with your DH unit. I have an old unit here that seems to have the same problem and I have traced to it a sensor on the coils. This should detect when the unit gets too much ice in it and turns off the compressor but leave the fan running. This eventually melts the ice (as long as the ambient is above freezing) and the compressor starts up again. If that sensor is kaput the unit just ices up under heavy use.

This is obviously going to reduce the amount of water the DH can collect at low temps, but it does let it operate at a lower temp. This de-ice cycle is probably the only difference between the standard and 'low-temp' models.

Cheers

Ian

[Aaron Montgomery]

I had the same problem with ice forming on my dehumidifier. Because I didn't need it to run 24/7, I put it on an electronic timer and cycle it every two hours to prevent ice formation. YMMV

[Jason Morgan]

Rick,

I too have a basement shop and have had luck keeping the RH ~50 with a dehumidifier cycling. My kenmore model wil cycle on and off at 2hr or 6hr intervals. I keep it at 45-55%RH at 60-65degrees and do not get rust and only minimal mildew in the corners of the walls.

Now on to your question. Rust formation should be better at colder temps. (forgive me if you already know this, but Im a physical chemist so its my thing). The relationship between water, temperature and RH can be a little tricky. %RH is the % of maximum moisture the air can hold. That is, if the air at a given temperature can hold a maximum of 1lb/ft^3 and it actually contains 0.5lb per cubic foot at the same temperature, then the RH=50%. Thats why its relative humidity, its relative to the tempearture. A sealed boz with air and water vapor at 65F will have a higher RH than it will if you heat it up to 85F. This is because cold air cant hold as much moisture as warm air, so for a constant amount of moisture, say 0.25lb/ft^3, as the temperature drops,the RH goes up (until it saturates at the dew point). So, bottom line, the colder you keep your shop the better for rust (less total water in the air), but the harder it is to dehumidify (less of a temperature differential results in more freezeups).

Hope this helped.

[Greg Sznajdruk]

Rick:

Residential dehumidifiers usually work down to 55 grains. Grains is how the actual water content in air is mesured. So that you can only remove a limited amount of water from the air with these units. Commercial dehumidifiers (LGR) Low Grain Refrigerant will work down to 35 grains an down to temperatures of 33 degrees. They also cost about $2,000 US. You could also get into exotics like Desicant Dehumidifiers but they cost even more.

This is a Psychrometric problem. If you have the time or interest you can study Thermo-dynamics specifically the second law.

The relative humidity has already been address by one of the posts. What you need to determine is " dew point" this is the point where water vapour will condense out. If you go on the net there are a bunch of site where they have on-line calculators. If you temp and rh are relatively consistent then you can determine dew point. The on-line calculators will give you an idea at what temperature you need to keep your shop so that you do not achieve dew point.

Hope this helps.

Greg

[Rick Schubert]

Jason, I'm not sure I understand the last half of your last sentence about colder air being harder to dehumidify. Are you saying it is just difficult mechanically for the dehumidifier because they have to work so hard that they tend to freeze up? Or is it something more complicated?

Greg, Do you what 55 grains would translate into RH at about 50 degrees?

Thanks to all so far! Rick

[Cliff Rohrabacher]

My solution is to get a bad assed dehumidifier. Sadly they are about as expensive as some big stationary tools.

[Greg Sznajdruk]

Rick:

Temp Rh GPP

50 50 26.4

50 100 53

You initial post indicated that your temp was about 50 degrees and you were trying to keep your relative humidity at 50%. This gives you a GPP of 26.4. Your dehumidifier only works down to 55 GPP ( Grains per pound). So ineffect your dehumidifier is only making noise and using electricity. AHAM ( American Home Appliance Manufacters) rates dehumidifiers you could check there web site to see what your unit is rated. At 50 degress you would have to have a Rh of slightly over 100% before your dehumidifier would condense water vapour. The reason dehumidifiers work better at warmer temperature is that warm air has more capacity to hold water vapour. We all experience this during July and August where the humidity in the air makes us uncomfortable.

Greg

[Carl Eyman]

Rick: IMHO you've been given lot of very good info on dehumidification. But short of dessicant removal of water vapor thus lowering the dew-point and RH you need to heat up the air. This will heat up your tools so they stay above the dew-point and thus there is no condensation deposited on them. This is one heck of a lot cheaper than trying to get a de-humidifier to work in your difficult envirement. I don't have a psycometric (sp?) chart at home, but I guess that raising the dry bulb temperature about ten degrees would do the trick.

[Christopher Stahl]

You could try a small heat pump for your basement. This will control temp and humidity and they are relatively inexpensive. Depending on the size of your shop, probably around $1000. This would seem like the best alternative for you.

Look at the small Sanyo heat Pumps.

http://www.sanyo.com/industrial/HVAC/Wall_Mounted/Heat_Pumps/

[Brent Smith]

Hi Chris......what's involved in installing a system like that in a shop?

Brent

[Christopher Stahl]

There's not much to setting one up. They come in different styles, but probably a wall unit would work for the shop. So hanging the wall unit and running the 2 copper lines out to the heat pump. You'll want to sit the heat pump on a level pad. You'll also need a dedicated 120V circuit to it. Then adding the refridgerant and bleeding the lines. Probably easiest to get an HVAC person to stop in for that, but should only take a few minutes.

These units are extremely efficient. So they work great year round and don't cost much to run. You'll just need to clean out the filters on the wall unit.

[Jason Morgan]

Rick,

Sorry it took me so long to get back to you.

What I meant about the dehumidifier having a hard time is that it works by condentation and that condensation is driven by the temperature differential. The larger the difference between the cold coil temp and the air temp, the more efficiently it will dehimidify.

[Rick Schubert]

Greg and all,

I'm probably beating this to death. May it is my dense-ness.

Do you think I can figure RH levels in my basement up 70% would be OK at 50 degrees for mold and rust control? I'm just trying to figure at what point of RH at 50 degrees I need to use the dehumidifier and possibly raise the heat to help it do its job.

[Kent Fitzgerald]

Rick, I tried to address this in my first resposne, but I guess I wasn't convincing. You want to keep RH about 50% or lower, regardless of temperature.

[Rick Schubert]

Kent, See Jason's post #7 in this thread. RH of 50 at 50 degrees is a lot different than RH of 50 at 70 degrees. That is why I was asking for clarification.