Rate of Humidity Change in Server Room

Posted on 2014-09-09
Last Modified: 2016-11-23
Hi All,

After starting a new job, I immediately noticed that the server room's humidity seemed all out of wack. The room is 12'w x 25'l - drop ceiling tiles w/ hard wall to roof. The room also has anti static floor tiles. There are two telco racks with patch panels and 6 switches. One full rack containing 2 APC 3000's , 6 Dell servers ( all R610's and 710's. ) and a Dell SAN. I'm told the room has a dedicated AC unit - just not sure how big ( tons ). The thermostat is set to 68. Typically, the temp in the room will rise to 72, the AC kicks and starts cool and the room returns to 67/68. I'm OK with that. The issue that caught my attention is that there were times that I went in there and it felt like a jungle. My body was telling me that it was at least 80% humid. I ordered a APC sensor ( AP9335TH ) the the UPS. After one day, the data tells me the following:

Humidity Low Point = 35% RH
Humidity High Point = 72 % RH

Now, I'm not overly concerned with the % but what I am concerned is that the humidity fluctuates from low to high almost 4 times per HOUR.

So. Should I trust this APC sensor? and should I be concerned with the rate of change in Humidity?

Any help/info would be grateful.

Question by:Brian
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Expert Comment

by:Nagendra Pratap Singh
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Only one sensor in that room?
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Accepted Solution

Syed_M_Usman earned 500 total points
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Thank you for posting question on EE. kindly note Humidity may wary if;
•      Room is very cool, and data center wall facing building outer side.
•      Room is big and you have fewer servers.
•      I am not sure about your ac brand and model but many CCU’s support humidity and when they notice it is low they push humidity, in that case you may need to check your cooling system.
•      If rack is not properly located for server heat air flow.
•      Rack empty spaces are not closed from front side etc.

Controlling humidity is hard nut to crack, I suggest you to spend time monitoring HW, Cooling System, Rack , UPS, Power Distribution Units… and have at least 1 more sensor for cross verification.    

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by:Nagendra Pratap Singh
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Frankly I don't have much idea in this regard now (I used to be an HVAC Engineer once) but I would second what Syed said.

Using multiple sensors is better than facing server failures. If you can feel it yourself then I assume that some ACs etc have settings misconfigured.
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Humidity - basically the warmer it is the more humidity you have. So my thinking is when the air is not running the room temp rises and the humidity does as well. Cold air is dry - so when the cold air comes in it dissolves a lot of humidity in the room. I am surprised it is that big of a difference, is the sensor very close to an ac vent or something where it would swing that much? Might be time to look into a de-humidifer if you are super concerned and have a drain near by unless you want to empty a bucket!
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What kind of AC?   What kind of air return do you have?  What is ambient RH?

If you have a portable unit or in-ceiling, then you may be introducing the condensate to the room after the compressor turns off.  The AC will normally lower the humidity, collect the condensate, and drain it out to a drain line or outside the building.

If it's a portable unit without a catch bottle or plumbed drain line, then the condensate is being dripped onto the compressor housing and boiled off into the exhaust flow.  If this is discharging into your plenum space, then during off hours (after the house HVAC shuts down), then the humidity will be cycling back into your room as makeup air drawn from the warm, moist plenum.  Also, if you have normally high ambient relative humidity, then your off hours will be running higher humidity air through your room.

A likely source is an overstressed PAC unit.  I had a 3-ton ceiling unit that was set by the HVAC contractor to 68F.  The unit wasn't built for it, and frequently iced up on the roof.  This would shut down the compressor, but the fan would keep blowing 24/7.  It would take a while for the ice to melt.  So, while the ice block was forming, air could not condense water on the coils, blowing it into the room with moderately cool air.  As the ice melted and overflowed the drain line, more water was being blown into the room with warm air.

Author Closing Comment

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Hi guys,

Sorry for the lack of response. I called in the AC guys. They tell me the unit is functioning normally. My onsite facilities guys tell I'm being overly sensitive. I still think there is a problem so I ordered a secondary Hygro-Thermometer / Datalogger and placed in the room today ( 10/6 ). I'll monitor for a few days and move to another location in the room - closer to the AC thermostat for a few days.

Thanks all for the input.

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