We have a small server room and the humidity level inside the room is far from the ideal. We have been measuring it between 60% and 68% for 3 months. Ambient room humidity inside the building is about 50%.

Using dry mode of the air conditioner did not help to reduce the humidity. I heard that using a dehumidifier can cause static charge buildup in the room.

What are my options to solve this issue?

The layout of the server room is as follows:

enter image description here

  • Generally speaking, HVAC lowers the humidity in a conditioned space whether in heating or in cooling mode. My guess is that the cooling unit is too large for the space, thereby introducing a "short-cycling" condition whereby the unit is unable to cycle enough air and extract humidity, before meeting the cooling requirement. Lots of factors could be at play here, however. Is this an area that your AC guy considered? Jun 30 '20 at 20:23
  • 2
    I've had the opposite problem - too low a humidity encouraged Electrostatic discharge, so simply walking around the rack could cause a spark when touching the earthed racks. We ended up putting in a humidifier to add moisture to the air. Question are you suffering any problems? Or is the humidity sensor simply disconcerting?
    – Criggie
    Jul 1 '20 at 0:50
  • "I heard that using a dehumidifier can cause static charge buildup in the room." - this happens because of low humidity. Using a different machine than a dehumidifier won't avoid it.
    – user253751
    Jul 1 '20 at 9:33
  • @MRamazoodle AC repairman just says that the air conditioner does the cooling so it works as expected. Jul 1 '20 at 9:39
  • 3
    60%RH is not horrible actually, assuming you're at about 18-20C for your room temperature. That's well below the point where it's going to encourage microbial growth, and still enough that you're not likely to get damaging static discharges unless you go out of your way to do so. Pushing it down to 40-50% would be good, but much below that is not likely to significantly impact corrosion rates because it's such a small amount of water in the air already (about 1% by mass for 60% RH at 18C). Jul 1 '20 at 13:54
  1. humidity level inside the room is far from the ideal.

    That depends.

    "Ideal" usually means 0%, as no water based corrosion will be possible at all. Of-the-shelf servers, switches and equipment usually does operate at ~65% relative humidity without any problem, when the temperature is high enough to prevent condesing.

    From the HPE ProLiant Guides:

    [...] Operating relative humidity: 8% to 90% (non-condensing)

  2. I heard that using a dehumidifier can cause static charge buildup in the room.

    This is somewhat true; the static charge buildup is not directly caused by the dehumidifier, but by dry air (and moving parts inside your servers). It is very common in plastic industries, for example, to prevent electrostatic discharge with high humidity.

    Constant air humidity can decisively contain static electricity environment. Above an air humidity of approx. 55%, the conductivity of the air is sufficient to discharge the electrical charge into the atmosphere - without causing any damage.

  3. What are my options to solve this issue?

    From your drawing, I conclude that your server room has about 20m³. To get that ~60% humidity out of the air, you can use electrical dehumidifiers or a chemical one. While the chemical one is usually good for one or two drying-sessions (around 3$ for ~500ml, good for 20m³), for long-term usage I would go for the electrical one. It's cheap, around 100$, and you can just get one from your local hardware store. Most "at home" devices are made for ~100m³ and will fill your needs (close the doors, windows and ceiling fans).

    My personal opinion is, just leave it. A humidity level of 50-80% is very common in server rooms (of this size) and most of the usual equipment does not have any problem with it. Just keep a close eye on the temperature; you want to prevent condensing at all cost.

  • $100 seems like such a small price to pay in order to increase your margin of safety. After all, the OP will want to decrease temperature as much as practical to maximize thermal performance.
    – jpaugh
    Jul 1 '20 at 20:13

I heard that using a dehumidifier can cause static charge buildup in the room.

Then the server room was designed by an idiot. ANY server room I have seen in the last 30 years had the racks and (this, automatically) all relevant equipment grounded. Once grounded, and buildup charge is automatically discharged to the ground.

Also, dehumidifiers can cause charge mostly because dry air supports build up charge. Given that your server room is more like a humidor that is sort of a wanted state, you know.

How the heck do you end up with a wet server room to start with? With no window open, all entries normally closed and air conditioning running I would seriously investigate where the water is coming rooom. Unless the door is open all the time OR totally unsuitable for a server room - the humidity inside should slowly go down.

  • We have been facing this problem for 3 months. Before that the humidity level was normal. I didn't design the room but I know that the rack is grounded and the door is always closed. Jun 30 '20 at 11:40
  • 1
    Ok, then where the h**** is the moisture coming from? Especially if it is higher than outside. Broken pipe somewhere?
    – TomTom
    Jun 30 '20 at 11:56
  • A/C repairman says that the air conditioner works without any problem. Can the air flow inside the room cause such an issue? Jun 30 '20 at 12:00
  • Well, the humidty comes from SOMEWHERE. WIth the door closed, it does not come from the outside. The air flow inside will only circulate whatever water is there - and drying mode will remove water. So, where is the water coming from?
    – TomTom
    Jun 30 '20 at 12:01
  • 5
    Well, this is the one thing I would investigate. If you have a leaking pipe somewhere - it is a SMALL change to a flooded server room. THis is the riddle I would demand to be solved because of high danger.
    – TomTom
    Jun 30 '20 at 12:33

You need to add heat to the server room. Enough heat to make the A/C work harder to maintain your set temperature, but not so much heat that the temperature rises above the desired range.

A well-designed HVAC system for a server room will include an air cooling device, a separate air heating device, a temperature sensor and a humidity sensor. If the air cooling device and air heating device are within the same air duct, the air cooling device MUST come first. During times of high humidity, the A/C will cool the air below the desired temperature and remove a lot of humidity, then the heater will boost the air temperature up to the desired temperature, resulting in much dryer air than an A/C alone can give you.

If you are using a mini-split HVAC and using the "Dry" mode, this simply reduces the fan speed to improve dehumidification. Ultimately, it will not give you the dry air that you desire. A standard mini-split is the wrong equipment for this room.

Try a one-day experiment: forget about "Dry" mode. Use "Cool". Set your air conditioner for the desired temperature, then plug in the biggest electric heater that you can safely run in the server room. Monitor the temperature and humidity. You will be pleasantly surprised. I predict that you will see a dramatic drop in humidity within the first 1-2 hours with no change of temperature.

The permanent solution is to hire an HVAC contractor that knows how to control temperature AND humidity, and this will involve both heating and cooling at the same time when it is hot and humid.

  • i agree; if the existing AC is not doing it it's likely because the AC is either oversized or there's not enough heat load in the server room; the AC is short cycling; For the AC to effectively remove humidity it needs to run for a significant amount of time not just blast cold air every for 5 minutes once an hour to maintain a temperature
    – ron
    Jul 1 '20 at 14:09
  • for a ~10 sqft room, a small cheap electric humidifier would probably work; I don't think you are going to go from problem=humidity to problem=static electricity. Unless you ridiculously oversize the humidifier. Get a small one, run it periodically. Simply running a dehumidifier does not always result in static electricity problems
    – ron
    Jul 1 '20 at 14:13
  • I'd also suggest making sure the condenser coils aren't overly dirty as well as checking the drain for being clogged. I've seen both happen in a regular setting and it can be nasty/dirty to fix the problem. Jul 1 '20 at 19:47

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