The advantage of recycling the air is that you will remove the moisture in the air, dehumidifying as well as cooling. Constantly running "new" air will add moisture to the system, and if humid enough, may lead to freeze up of the coils.

You're pretty close to the water there (what? 2 miles?) so you may be getting more moisture than you think. A cheap bulk polymer humidity sensor would tell you for sure.

His hot air thing sounds like hot air to me. Cost shouldn't be any different. You are cooling office air, and running that into the shop. So you are pulling fresh air into the offices. I think with the recycle, you'll be more efficient, lower cost.

Cooling air is 1/3 the cost of removing moisture. The heat of vapoization of water is high! Look of the low pressure charts on the steam tables.

Certainly no expert, but I think a closed-loop system would be more efficient. Also, aren't most A/C systems designed to function best with certain warm/cold "split" temps? Maybe he's saying instead of having an exhause fan, that it should be a (filtered) air-return? I'd certainly get a second opinion.

Wayne, as you know, everything about the way these systems are set up with where the returns and the vents are and how big they are is critical to operating efficiency and performance.

Would it be possible to get a second opinion from another company, or are you on contract with this guy? Or would he be offended?

A second set of eyes might be your answer.

And if you think your guy might be offended if he knew you got another guy to look at it, just request that #2 guy come by after hours in an unmarked vehicle/his own car and do a walk through.

The only way it will freeze up is if it runs all the time, or for very long times. Compressors freeze up all the time in Lake Havasu at 110 degrees because they always run.

So your AC guy is saying that continuously cooling the dry, 65 degree air allows the evaporator to drop below freezing?

In the first paragraph above, you seem to be saying that the warm air in the server room is sent to the warehouse...

...then you seem to be saying that the return for the server room's A/C is also in the warehouse... but after having read everything you've said, it's from a cool area in the warehouse. The same warehouse, right?

At any rate, this is definitely beyond my abilities, but I know you need for the system to cause the humidity in that server room to be as low as possible... or perhaps within certain limits.

So, I recommend the second opinion approach. You're gonna want someone who is not only knowledgeable, but who doesn't mind answering all your questions. Get a recommendation from another businessman you know in the area who has a similar setup... at least an office area and a warehouse area in the same building... sounds like that's what you have.

They will freeze up when they run constantly and if/when there is insufficient air flow based on the size of inlet or outlet (including a fouled or too restrictive filter). The really thick or dense filters will almost aways cause this...and changing to a less efficient one will often be enough to fix the problem.

Yeah, I'm thinking closed-loop as in: the air in the room IS the air you are pulling back in and re-cooling (just have a filter somewhere in the loop). Just like the forced-air (closed-loop) system you would have in your house.

I would think the closed loop would be cleaner. Even with a good filter some dust could be getting by. Isn't dust the enemy for computers ?

Yeah, I was going to suggest an array o' cool collar, but I suppose a K&N filter ought to do just as well. :cool:

Seriously, I've got to agree with finstone; increasing the airflow will transfer more heat to the coils. (stop the freeze)

I'll just add that your guy is right, that feeding the cold side warmer air will help stop the freezing, but you are certainly right that there is a more efficient solution.

Wayne, certainly no expert here, but if your pulling already cooled air in, you compressor works less. Icing up is on the evaporator occurs from a less than 32 degree temp at the evaporator. Cooler air has less moisture than warm air. So less moisture from cooled air = less icing. One of the ways A/C works best is to recirculate already cooled air. The question is "Is the air from the server room warmer than the air outside?" Given where you are located, you should have a "smart" system that will pull makeup air from the coolest place. Winter outdoors, summer maybe from the server room.

That part isn't quite right. (tho it may seem intuitive)

When air has more moisture it carries more heat-energy, and will take more energy to freeze. (Latent heat and all) Think of it this way; let's say there is ice already on the coils ... what will break the ice faster, room-temp super dry air, or room temp garden hose water?

So, with wet air, condensation may build faster but all that moisture imparts a lot of heat to the cold coils. That is, wet air will take longer to freeze than drier air. (...for a given air flow you'll have more mass flow & latent heat with additional water in the mix)

Based on your last post describing a system that has worked well for several years, I'd say you may have an undercharged system or dirty evaporator coil reducing airflow. What type of filtration are you using? Are the filters changed regularly?
jurgen

I can't diagnose this system from across the country but I agree with turbo6bar, in that your current problem is most likely not due to the design.

However, depending upon where your thermostat is located, and given a higher than normal supply air temperature, it might be possible that the compressor may run a long time. I'd suggest having a mechanical engineer (as opposed to a contractor) take a look at how your system is installed, to see if there might be a better way to do it. I have installed a bunch of systems in server rooms and telephone switches and we never use outside air, primarily as we are also needing to control the humidity within a narrrow range.

JR

In my experience freezing up is caused by not enough air flow (ie:dirty filter, not enough incoming air flow) if it used to work, that is my guess. The air in and out to the warehouse I don't think it maters on the freezing . It is probably more efficient to run closed loop because you are trying to cool the already pre or cooled air, it would take less energy to cool the already cooled air plus you get the benefit of removing the humidity and not introducing more moisture.

If it worked for several years before. It's not a design issue.
That's the lazy way out.
Low airflow or undercharge.

No expert here but seems like your compresor would run less if you recuirculate the air inside the room (draw already cooled air) than drawing outside air.

That would depend on the sensible and, to some extent, latent head loads. Without knowing the conditions of the server room AND warehouse space, it's pure wild-ass guessing. You could be entirely correct, though.

if you are pulling air from another room, you are putting a vacuum on that room, i bet the doors dont need closers. so, you are putting a load on that system. it is trying to draw air into it, and it is providing air to another room.

the server room. this unit is icing because it has cool air going into the evap. the system is deigned to remove heat from the air, so if air going into the system is ALREADY cool, there is effectively no heat load on that system even though the room is getting warm. with the unit running and no heat load, it will/may ice up.

you may connsider just running another supply and return from where you are getting the cold air now to cool the server room and use the server unit as a backup. set the T stat to auto and about 5 degrees higher than what you want and as needed, itwill come on.
i dont know what type your big system is, but you could even put a damper controlled by a Tstat in the server room to keep it cool. keeping the server ac sys off would save the system and money.