The following was saved from the old guide4home website just in case anyone was looking for it. With the help from archive.org website.
Roof top air conditioners cool more than half of America’s small commercial buildings, from schools and retail stores to hospitals, restaurants and office buildings. They range from 65,000 to 135,000 BTUs/hr of cooling capacity and are usually split system central air conditioners providing cooling through ductwork.
The outdoor unit consists of the compressor and the condenser and is installed onto the roof top. This is due to a number of reasons, most notably because of the large amount of space on the roof, but also because it is well out of the way and doesn’t cause unattractive obstructions to the building.
Roof-top air conditioners can also come as a packaged unit where all the mechanics are enclosed in one unit. These tend to be used for larger commercial buildings where the distance between the indoor air handling units and outdoor cooling units is too great causing problems with lubrication in the ductwork and making it unmanageable. These are usually known as chilled water systems.
This type of roof top air conditioner uses refrigerant to cool the water which is then pumped throughout the building via ducts and on to the air handling units. This cooled water is then used to chill the air as needed. There is no limit on the length of ductwork on these systems as there is no issue with lubrication all that is required is that they are sufficiently insulated. These types of roof top air conditioners would also be ideal for use in arid climates as the water would add moisture to the air as it cools.
Roof mount air conditioners are central systems that are used to cool entire buildings but the main problem with their position on the roof is that they are exposed to direct sunlight. This means they can get very hot during the day which reduces efficiency. Wherever possible these units should be installed in an area that gets the least sunlight, especially during mid-day.
Many roof mount ACs in use today are not as energy efficient as they could be simply because many buyers opt for the cheaper units without realizing that the cost of operating a lower efficiency model will be more expensive in the long run. More expensive variations that meet Energy Star standards may be more expensive to purchase but will almost certainly be more cost effective in the long term.
Many commercial buildings, such as those mentioned above, will have built-in ductwork as part of a ventilation or heating system, whilst others will have been built with an air conditioner system in mind. Some older buildings may not have been built with any ductwork included. In some cases it would still be possible to install a central air conditioner with a mini-duct or mini-split system. These systems have been designed specially for buildings where installing ducts would be impossible.