You need to decide the criteria you will choose for the location. An outside wall, as you say, makes it easier for the air-conditioning, ground floor makes it easier to install equipment, as you don't need to carry it upstairs, and central is easiest for the cabling.
You also need to consider the security. Dependant on the construction of the building, the first floor may be more secure as well.
I suspect all the locations that have been chosen are, again as you say, valid depending on the criteria.
From a network point of view, I usually try to choose the most central location, so all the cable runs are as short as possible.
The reality is that the room is usually chosen as an area that can not be used for anything else productive. In a small building it is usually not an issue as to where it goes. Air-conditioning pipes are quite small these days, so that is also not really an issue.
Not much help, but those are my (rambling) thoughts ;-)
My preference is ground floor center.
Some suggestions from the <a href="http://docs.sun.com/source/805-5863-13/">Sun Microsystems Data Center Site Planning Guide</a>:
"Whether a dedicated facility or part of a multipurpose building, the physical location of the data center is extremely important. The raised floor space, air conditioning support, uninterruptable power supply (UPS), generators, and related support equipment must be coordinated with the other areas of the building and properly positioned within the building perimeter in order to optimize their interaction and the overall support of operations. The location of the data center within the overall facility should be based on numerous criteria, including the following general considerations.
* Isolation from contaminants; isolate the computer room from contaminant-producing activities. Influences from print rooms, machine shops, kitchens, loading docks, or any area with high levels of contaminant generation or operator activity should be avoided. Ensure the exhaust from generators or other sources does not directly enter the intake of air handlers serving the computer room.
* Access; ensure adequate access for hardware from loading dock, freight elevator or appropriate entrances. This will include appropriate door sizes negotiable corners, ramps and smooth floor surfaces. In addition, it is important that proper access is provided in support areas to allow for service or replacement of UPS, chillers and other large items. As a facility grows or changes, access paths are often eliminated or changed.
* Security; provide secure points of entry to the computer room. This helps secure sensitive data, limit the possibility of employee vandalism, minimize exposure to inappropriate psychrometric or contaminant conditions, and control the possibility of failures caused by inadvertent actions of untrained personnel.
* Raised flooring; design the raised floor computer spaces in convenient proximity to the support equipment (UPS, chillers, etc.). It is often appropriate to locate the data center on floors above the support equipment in order to consolidate cooling and power trunklines.
* Air conditioning; consider the type of air conditioning to be used. Chilled water units will need to be connected to chillers located in the building or an adjoining support facility, and may require cooling towers. Due to noise and structural issues, chillers are normally located in the basement of the facility or in a separate wing of the main building. Direct expansion air conditioners require condenser units located outside the building. In addition, the roof or outside pads should provide adequate structural stability to accommodate the condensers.
* Risk of leaks; avoid locating the hardware areas beneath potential liquid leaks. Do not run the air conditioner piping through the ceiling void of the computer room. Do not locate the data center beneath kitchens, workshops, or other areas that have a high potential for leaks. Locating the computer room below building grade adds the potential for leaks from outside the building. In addition, locating the computer room on the lower floors of a multistory building, particularly one with multiple tenants, runs the risk of leaks associated with a sprinkler discharge in the floors above. Expansion joints, conduit or pipe penetrations, cracks and other breaches can all allow for water infiltration.
* Proximity to tenants; avoid locating the computer room near areas leased by other tenants. While the current application of the neighboring room may be appropriate, this could change dramatically should the lease change hands. In addition an area with a short-term lease may change hands frequently, necessitating potentially disruptive renovation activity.
* Room to expand; locate the computer room in an area that offers the potential for future expansion. Even though technology changes tend to make hardware more space-efficient over time, the ability to expand, either within the current footprint of the building, or through additions, should be available to accommodate possible growth as the room evolves. If growth is anticipated, constructing surrounding offices on a preinstalled raised floor will facilitate the conversion to hardware areas. If growth is not anticipated in the near future, but is still possible, applications that can be easily moved should be considered for the surrounding areas. Avoid land-locking the computer room. While the expansion need not be directly connected to the existing areas, it is often easier to share support equipment, such as chilled water loops or security, if they are located in close proximity. "