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High Humidity - need advice

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dschultz307

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Discussion starter · #1 ·
I am experiencing very high humidity in my building.

system description:

Two-pipe system with fan coils in each room.

60 ton chiller (Carrier GTN-060; four capacity steps)
about 65 FCU various sizes 7500-12000 BTU

Carrier 42V FCU with electronic thermostat and manual outside air damper.

Chilled water temps at the FCU read between 44 and 52; Chiller setpoint at 45.

Reverse return piping
no circuit setters on the FCUs
all main loop water circulates through chiller during cooling mode.

humidity levels at 70%, mold on some FCU registers.
most FCUs set at 66-70 for dehumidifying.
FCUs don't seem to run long enough to remove humidity when setpoints at at 72-74
outside air dampers set to minimum.

Chiller runs on 1st stage (28%) most of the day when outdoor temps are 90.
When temps reached 100 chiller was on stage 2 (43%)

Location is Northern Virginia.


Any thoughts on causes and what to focus on?
I am the facility manager for the building, HVAC maintenance personnel have looked this over but no firm course of action at this time

Dave
 
Discussion starter · #3 ·
Summertime humidity is 40% or higher.
Today is 57% according to the NWS.

Each FCU has an outside air intake. most dampers at the FCU are set to minimum.

Restroom exhaust fans run continuously. Total of 4 fans in the building.

All the FCUs run their blowers continuously. so there is always some (too much?) outside air entering the building.
 
#4 ·
Well my experience is mainly with process coolers for computer rooms where high humidity is unacceptable, air change is rarely required in these situations. I do know however that its hard to control humidity when you introduce outdoor air. i would try to keep that to a minimum and people normally generate humidity on a normal basis. Refrigeration precess by default dehumidifies, and using free cooling might just be taking away your ability to do that.

Found this link, might help

http://books.google.com.jm/books?id...required per person&pg=PA289#v=onepage&q=air change required per person&f=false
 
#5 ·
You can't properly dehumidify without reheat. As you said, your fan coils do not stay in cooling long enough. The reheat gives a false heat load to keep your cooling on longer. Cooling is the only way to remove moisture.
 
Discussion starter · #6 ·
Thanks!

No reheat on these FCUs. They appear to be oversized for the load.
In rooms with more than one FCU we get much lower humidity when we shut one FCU off.

These FCUs have the thermostat on the unit and the sensor attached to the blower housing.

Would I get better temperature control (and maybe humidity control) if the sensor/thermostat was wall mounted on the opposite side of each room?

Other related questions:

A few techs have look this over and said that "You need to move more air if you want to dehumidify" and "CW temps lower than 45 wont help" Thoughts?


I found some FCUs with the outside air dampers stuck 100% open, I was able to fix them.

Would excessive water flow or reverse piped FCUs cause a humidity issue?
 
#8 ·
Are the chilled water valves 2-position or modulating? Modulating valves will keep chilled water flowing and do some dehumidifing.

Probably the best thing to do is dedicate a coulple of FCUs for all the outside air and put reheat on those. Let the other FCUs just recirculate.
 
#10 ·
This is a simple one , you are running your water to cold. Try raising the water temp. so that when you set the FCS's you don't cycle so fast. What you want to achieve is constant cooling over the coil to handle the humid air that is always being drawn through the coil by means of fresh air. You will have to tweek the tempertures untill you get it right. Good luck let us know how this is going.:cool:
 
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