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IT Room still too hot with 38,000 BTU AC

Posted on 2010-08-17
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In a small IT room, approx 17w x 17d x 10h, running 5-6 servers, 8-9 UPS, and 3/4 full rack of networking equipment, the temperature ranges from 70 degrees to 88 degrees.  There is no main building AC overnight or on weekends, so temps peak at those times.  

We have three separate portable AC floor units in the room, all venting into the walls/drop ceiling (no windows or exterior access): 12,000 BTU, 12,000 BTU, 14,000 BTU  (38,000 total - or is some lost from three separate units).

I ran a simple estimation calculator (without entering specific wattage of all devices) and came to approx. 28,000 BTU needed for a room that size with those machines/batteries.  

The three AC units are positioned in 3 corners of the room at varied heights (floor to ceiling - on shelving) and are aimed in varied locations.  Adding on the second AC unit maybe brought the average down 5-6 degrees, the third AC unit only 2-3 degrees.  Never dropping below 70 degrees, the AC units have run 100% of the time since they day they were installed.  Our oldest unit occasionally only blows out warm air now - due to old refrigerant? (approx 4 years)

Any thoughts on what could be wrong?  Air flow?  Venting?  Ideally I could keep the room at low 70's 24/7.
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Question by:blueteam
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by:MojoTech
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Does the air feel cold (really cold) coming from the units? if not get them serviced.
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by:blueteam
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Yes, the two new ones are always very cold.  The original one is usually very cold - maybe once every hour or two it changes to warm air for a few minutes.
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by:truromeo4juliet
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You may not like the sound of this, but you may have to decrease the airspace in the room. Drop the ceilings, or squeeze the walls. Fill it with bricks. I imagine your warm-blowing unit to be pretty counter-prouctive as well... also, the effiency of 3 units is typically less than the efficiency of 1 unit, you'll need to figure in a margin of efficiency because you're running 3 units. While I don't imagine the margin to be more than 25%, consider that one of your units is blowing warm air. Fix the warm air problem from the single unit, then recheck the temperature of the server room.
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by:truromeo4juliet
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Also, consider the process of opening/closing the door to that room. If you don't have a middle-room, the very act of opening the door to go in and out can be enough to fluctuate the temperature of the room.
Rotational doors are more efficient for air cooling than conventional hinge doors, and having an entry hall is ideal.
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by:truromeo4juliet
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By efficient, I mean when someone passes through the door
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by:blueteam
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The room is entered maybe 1-2 times/day.  I don't think the old unit is sending out the warm air very often.  From what I gather so far - no one seems concerned about air flow...right?  Meaning, would it make any difference if the 3 units were all in the front of the room on the floor - vs. the current spread out, corner configuration.

As far as "shrinking" the room - it's already very tight in there with two racks, some other shelving, etc.  It can't get much smaller.  My other concern was perhaps the exhaust heat is just going up the walls/ceiling and either returning to the room, or heating it from above/sides....sort of like a heat barrier all around the server room.
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by:MojoTech
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I had a comms room that was of a similar size and had 0 air flow, only 1 unit and twice as many servers, we had the same issues and it was down to the air con unit, once serviced it was fine but it needed to be blowing air cold enough to give you brain freeze, if you can stand in front of your units without getting brain freeze then they are not cold enough.


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by:blueteam
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How many BTUs was you single AC unit?  Wall mount?  Portable?  
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by:MojoTech
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Wall mounted and unsure of the BTU.
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mmaris earned 42 total points
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In plain terms:
Place all units to where they will recirculate each others air, an a/c works by condensing a refrigerant thereby forcing an endothermic situation, your hot air then enters the unit via negative pressure from a drum fan(for wall type units.) The air is then cooled via the refrigerant lines running nearby.

If you apply the coupling effect you should notice a bit cooler air returning from the unit, the hotter the air entering the more stress is applied on the refrigerant, as well the less the airs potentiality to be cooled.

As well, you either need to place fans to blow the hot air down or pull the cool air up for more thorough air recirculation. this room is only 107cu. yards of entrained air. Placment around the room is a mute issue except for you might want to place them as close to the ceiling as possible.

Also is this a suspended ceiling? If you so might want to add some small CPU fans into the suspended material to suck the air at the top of the room out and put a thin layer of insulation material no the top of the suspended ceiling.

Any gap under the door is again a mute issue as the cool air sinks, uless teh rest of the buiding is warmer. if so simply staple soft rubber to the inside back of the door(interior.)
-Hope this helps
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by:aleghart
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I have the same size room.  On single 3-ton unit in the room.  Sounds like a combination of problems:

1. is your plenum space moving the exhaust away?
2. where are you putting the condensate?
3. where are you drawing make-up air
4. with a 10-foot room height, what is the max exhaust duct length supported?

Those are the biggest problems.  If your plenum only leads to a small HVAC unit, then you don't have sufficient volume for your hot exhaust to mix with building air.  You stack up hot air in the plenum (drop ceiling), allowing it to leak back into the rooom.

Some cheap units drip their condensate onto the pump, making water vapor.  This goes with the exhaust, and will fill your plenum with warm, moist air.  Moist air stores heat, which means that air leaking back into your room may be "warmer" than just dry air.  Also, it a terrible thing to do...condensate should be collected and disposed, taken to a drain, or exhausted directly outdoors.

Makeup air is what comes into the room when your exhaust leaves.  You can't live in a vacuum, so the air come from somewhere.  If you don't have registers designed specifically for this, then your existing return-air grill (which leads to the plenum) is now letting hot air back in, instead of sucking it away when the building HVAC is running.

Small units like those typically have no more than 6-foot exhaust ducts.  Can't reach the ceiling from where you are.  So, are you extending the exhaust (which means it's not all getting exhausted)?  No bends, kinks, right-angles, etc.

Also, keeping the room at 70F is pretty unreasonable.  I keep mine at 76-78 before the supplementary air kicks in.  The house HVAC keeps it 74-76F during most of the day.  Occasionally, my supplemental will kick in.  At 6-7pm, my unit takes over completely.  76F keeps my electrical consumption down considerably.  No need to refrigerate everything.  Just keep it withing operating specs with a little bit of reserve.

That reserve (lower than 100F) is necessary for when the AC unit cycles or fails.  You've got a room at 76F, so it can survive for XX minutes/hours until the ambient reaches 100F, starting some shutdowns.  I understand the old-school idea of 68-70F computer rooms...I used to have one.  But, keeping one small room at 70F cost many thousands in burned or frozen units, plus maintenance.  On top of that, since it was a single unit, and not redundant, it was cycling off...bringing the room temp up into the 80s anyway.

Be realistic...why did you choose 70F?  Magic number?  If you stick with it, you need to have your HVAC professionally designed and installed.
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by:greglwhite
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make sure your exhaust/vent pipe is insulated so it is not heat exchanging before it clears the room.
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by:greglwhite
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what is the temperature above the drop ceiling?  is there any insulation between this room and the adjoining rooms? what are the temps in those rooms?  any windows? what direction do the windows face?
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by:blueteam
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Answers to questions from previous posts:

1) It is a suspended ceiling - approximately 3-4 feet of space above for wiring, duct work, etc.

2) We can't tell how well the plenum space is moving the exhaust away - there are no fans up there that I know of, so it's very possible the hot air is sticking around.

3) Condensation drains into three separate buckets - but we've noticed that as we've added units, there is significantly less water (almost none).

4) Make-up air comes from the plenum and anything that gets past the small gap between the door/floor

5) We don't extend any exhaust tubes past 5-6 feet.  Two units are placed near the ceiling (on shelving) and the one on the floor exhausts into the wall.

6) 70 degrees is just a number I threw out there....if we could keep the room mid 73-75 consistently, that's fine.  The main problem is the consistent peaking to high 80's - sometimes low 90's when the building shuts off their AC every night.  (this used to peak 100+ with a single unit)

7) Exhaust tubes are not insulated, they are just the standard tubes supplied by the manufacturer.

8) The temperature in the plenum reaches 100 degrees, but can be as low as mid 60's (depending on outside temp and if building AC is on).  Surrounding offices get into the same temp ranges when the AC shuts off (80's - 90's depending on outside temps)
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by:greglwhite
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insulate your room better. especially the drop ceiling, if 100 degree air is leaking (or pouring) in, your 3 units don't have a chance. fiberglass bats at right angles to the tiles will do a good job. its not a big room so cost will be minimal.
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by:blueteam
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The problem is there's a ton of wiring laying on top of the dropped ceiling that was put there by installers/building management over the years.  Would that all just lay on top of the insulation?
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by:mmaris
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1.You can shoot loose fiberglass or shreaded (mulched) cardboard over the suspended ceiling.
(if you wish to put a very thick layer add small retainer bearing clips to the drop frame to support the panels)

2.Add the small fans in the corners to the suspended ceiling .

3.Direct the exhaust air from the units above the suspended ceiling.

4.Put salt in the drip buckets to reduce humidity, it acts and a cheap natural dessicant.(optional)
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by:blueteam
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Should there be a fan in the plenum to direct hot air elsewhere (already pre-installed by the building) or is it possible air just sits there.
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by:greglwhite
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You can either put the wires on the insulation or the insulation over the wires.  I would not blow in insulation, if you have to replace a panel it will be a insulation snow storm.

a fan by itself is just going to stir up the warm air, if you don't have a place to vent it to the outside it does you no good.
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by:mmaris
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Trying to redirect previously intalled wires over an install of insulation would be a nightmare, typ the wires have either been cut to size or are already wrapped, the risk of unpluggin or striping out a single circuit is too high to risk, if you pull up on a wire and a ground for example comes loose somewhere you risk hardware. replacing panels in a room only used by IT is a one in 30year or so need, unless you break it the looks should not force a change in the panel. And if its just a network room there shoudl be no reason it would get broken, shooting insulation is very cheep from a material and labor standpoint. trying to lay insulation under cabling is very cost prohibitive in the labor standpoint, as well for over the cables you have to get into the suspended ceiling and cut the insulation(requiring measuring as well) to fit around the cables so as not to induce pull out loads on the cables themselves. But...insulation is probably not necessary, making an exit for the exhaust air would be more practical(if you have some outlet for air above the suspended ceiling and you have ample make up air) as well as the previous mods i have suggested.
but simple steps:
1. placing units higher in the room, placing them closer to each other (so as to take advantage of the coupling effect,)
2.nstalling a few small fans that direct the upper layer of air in the room to the entrained air above the suspended ceilling.
3. directing the cold air coming out of the units towards the area most necessary.
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by:aleghart
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>1.You can shoot loose fiberglass or shreaded (mulched) cardboard over the suspended ceiling.

In a return-air plenum?  That would clog the filters of the HVAC system.  And, an issue for the building engineers, definitely not the tenant.  A drop ceiling with gypsum/fiber tiles is already a halfway decent insulator for an active plenum space.

If ambient air is 80-90 degrees, then your makeup air is requiring conditioning.  If the bulk of the make-up air is drawing from your plenum space...then you're cooling your own exhaust.  Think about the physics of that.  You shouldn't be trying to cool your own exhaust.  The net result is far less than 3-tons of cooling.  Add to that the additional make-up air is 80-90F, and your net cooling is probably less than a ton.

You're fighting an uphill battle until you can properly exhaust your AC and get a normal flow of make-up air into the room that _doesn't_ come from the plenum.

What is the relative humidity in that room?  Why did the condensate go from buckets to near-nothing?
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by:greglwhite
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because he is reconditioning his exhaust which has already been dehumidified 3-4 times.
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by:aleghart
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I thought about that.  But, there should be _some_ condensate from the initial moist air volume of the room + plenum.

It definitely points to a re-circulation problem.  Make-up air at 80F should have plenty of moisture.  The plenum feeding the room is a big problem.

Our plenums don't have vent fans.  There's just a lot of volume in the plenum, and vents to other spaces, plus the HVAC room.
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by:D_Vante
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What is the temp infront of the racks and what is the temp behind the racks.  If there is a greater than 10° difference then the air is not flowing correctly.  
Turn off unit #1
Place Unit #2 and Unit #3 behind the racks, to utilize the air condition return to pull air across the racks
Extend the AC vents from Units #2 and #3 to the front of the Racks.
Run the exhaust of Units #2 and #3 as far away as possible
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by:blueteam
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The temp in front of the rack (where one sensor is sticking out of the front of a PC) is approx 6-7 degrees warmer than behind the racks (outside) where the AC unit has a thermometer directly above rack height.  

For example - on a good day, when temps outside are 70's and the AC in the building is on (plus our three portables):

The AC unit behind (outside) the racks may read around 72
The thermometer inside the back of the racks may read around 75
and the thermometer in front (inside) of the racks may read 78-79
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by:greglwhite
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This is a symptom.  My opinion is you need to keep the 90+ degree plenum air out, pull air into the room from a cooler source (maybe a vent in the door to the adjoining room).  Once that is solved, you may not need to do anything else, or add a fan or two for air circulation.
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by:D_Vante
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Where are the vents of the AC units directed to?
The front of the racks or the rear?
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by:blueteam
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two are pointed to the front (one at the front of the racks, one over the racks - up on a shelf).  The third is pointed perpendicular to the back of the racks, blowing air across the back.
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by:aleghart
aleghart earned 83 total points
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You shouldn't point cold air at the back of the racks.  It's a waste.  The hot air will mix with your cold air and waste the net cooling effect.

There should be a return air grill above your racks that leads to the plenum space.  An HVAC engineer should have designed this into the room.  The house HVAC will push cold air into the room at ___cfm.  The air must escape (to equalize pressure) through a return air grille (which leads to the plenum or an adjoining space which leads to the plenum)  Or the excess pressure will blow out under your doors.  (Good for keeping dust out).   But, if the return air path is not sufficiently large, you can't get enough house air into the room due to pressure.  You'd usually notice this by a whistling door, hard to open, or a blast of air when you enter the room.

Did a mechanical engineer setup that room, or did you just inheirit it as-is?
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by:blueteam
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We've set it up on our own.  

So all three AC units are best in front of the racks pointing through them?  Or above them?  Varied?

There's approx 2.5' - 3' of space between the back of the racks and the wall.  The racks are enclosed on the sides, open on the back, and the front is perforated.

There are fans in the top of the server rack that shoots the air directly at the ceiling, which is solid tiles, so they only way for hot air to escape is via the AC vent tubes.

The door does not make any sounds and is not difficult to open (that I know of).
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by:aleghart
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Sounds like you have captured the air in that room.  So, you have a "net" cooling effect.  3tons cooling for 3tons heating, without exhausting any heat.  Maybe the 1ton or so heat the AC units are exhausting to the ceiling.  If you didn't have that, you'd be net negative cooling, and your room would always be overheating.

So, you don't have a return air opening anywhere in that room?  How did you ever get a certificate of occupancy to move in?  That's a big no-no for inspectors.  You need to have air in + air out, with enough CFM to get air exchanges (4-10) per hour.

Sounds like you need an slight overhaul.  Mechanical engineer would be best.  But, if the building/facility manager is OK with doing it themselves, ask them to help with return air to the plenum.  Then you need make-up air.  Probably a grille in the bottom of the door would be easy, since you don't have to worry about pipes and live wires in the wall.
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by:blueteam
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I'll have to take a look again at the ceiling to see if one of them is a return tile or not.  There's a fan of some sort there, but I had assumed it was an old AC supply that was no longer working - since it was never supplying cold air - perhaps it's a return.  It sounds like majority feel the main problem is removing the exhaust/hot air from the room to the plenum to allow the cool air to do its job.  And then on top of that, if the plenum has no where to go and is just sending the heat back into the room - it's a separate issue.
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by:aleghart
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Somtimes a little Columbo sleuthing helps!  Keep the logic simple before applying formulas to calculate cooling.  If you can't get rid of the hot air, all the cooling in the world will not help.  :)
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by:blueteam
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Also, two of the units are dual vent (intake/exhaust) and one is a single tube exhaust.  We have the two intakes "capped off" with the included cap in the AC kit.  I'm assuming it's still pulling in air from the vents directly below where the tube would normally be connected and a direct intake line to the outside is optional.
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by:D_Vante
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Can you send the model numbers of the units, I would like to look them up.
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by:blueteam
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Whynter/Sno ARC-14S
Whynter/Sno ARC-14D
Amana APN14J-E
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by:blueteam
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I would like to award points to the suggestions that helped us the most.  Please reopen question.
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