Commercial Air Conditioning Systems
General Overview of Common Systems

Many investors who are making their first purchase of a commercial building are inexperienced with larger Heating, Venting & Air Conditioning systems routinely found in larger buildings. This inexperience or lack of knowledge regarding maintenance, repairs or operating expense may come as a surprise. This same surprise can be found in tenants who accept a NNN lease.

Although it is difficult to compute an average cost of ownership, as there are so many variables, many attempts have been made to determine an average cost. One of the best studies was conducted by Washington State University and used a 20,000 sf office building as an example. Based upon this study, the cost of ownership of a light commercial HVAC system was $1.20sf over seven years or $3,500/year. This study was based upon a new system installation and does not include the initial equipment purchase cost, installation or energy expense. This is maintenance and repair. Incidentally, the total annual cost including energy/obsolescence was $46,000/year ($13,714 obsolescence/$3500 maintenance/$28,786 energy consumption or $2.30/sf).

Types of Systems

Air conditioning systems for buildings may rely either on air or on a liquid such as water as a distribution medium. Conditioned air is distributed by ducts, while chilled water is distributed by pipes. Chilled water distribution systems are most commonly used for air conditioning large buildings because it is more efficient and economical to transfer heat by water rather than by air. This is due to water's higher heat capacity, coupled with the fact that water-carrying pipes are smaller and take up far less space than air-carrying ducts. A third advantage of chilled water over air is its flexibility for zone control.

When chilled water is used as a distribution medium, it will eventually cool the air in the living or working space through air-handling units or fan-coil units. Air handlers are typically larger than fan-coil units and have more sophisticated zone controls. Air handlers are often found in large office buildings, hospitals, stadiums or meeting places. Fan-coil units are typically smaller and serve smaller spaces. These smaller units are typically controlled by simple thermostats and are often found in residential buildings, hotels, retail spaces, restaurants, and multi-family units.

Water is typically chilled using a system that features cooling towers and chillers. The operation of chillers and cooling towers is a subject unto itself. Chilled water distribution systems can use the same piping system as hot water heating systems. A water-source heat pump system would also provide heat, using the same water loop.

Two-pipe vs. four-pipe systems

When using chilled water with air handlers or fan coils to cool (and heat with hot water) a building, there is a distinction between two-pipe and four-pipe systems. In a two-pipe system, there is only one water loop, and thus two pipes (supply pipe and return pipe) at each air handler or fan coil. The building can either be in cooling mode or in heating mode. In these situations, there often is a 'switch-over date' when the chiller is shut down for the season and the boiler is started, or vice-versa. Control valves are used to switch the system from heating to cooling and back. See illustration. After the switch-over date, if there is a heat wave in the fall, it likely will not be possible to cool the building because the chiller has been shut down and the water loop has been directed through the boiler.

In a four-pipe system, there are two separate water loops used: one for heated water and one for chilled water. This allows both systems to operate simultaneously for those times when some parts of the building need to be heated and other parts need to be cooled. Additionally, there is no seasonal switchover needed. It is easier to switch between heating and cooling when the outdoor temperature is unstable. Four-pipe systems are obviously more expensive because of the additional water loop, controls and more expensive coils in the air handlers. See illustration.

Heat-pump systems

Heat pumps may be either liquid or air source. Liquid or water source heat pump systems typically consist of several heat pump units above the ceiling tiles or in a closet on each floor of a high-rise building. In a liquid source heat pump system, there is a single water loop. When air conditioning, the water loop is gathering heat from the refrigerant at the heat pumps. This takes place in the heat pump's condensing coil. The water loop must then reject the heat; this is typically accomplished through a cooling tower outside the building. See illustration. When heating, the water loop is providing heat to the refrigerant at the heat pumps. The water loop must then gather more heat; this is typically accomplished through a boiler inside the building. In some heat pump systems, heat is moved from one side of the building to the other: the heat that comes into the loop from spaces on the south side is absorbed by spaces on the north side.

This is a general presentation of the primary commercial systems. There is a great deal to consider with a considerable sum of money at stake. Both owners, investors and tenants should have a firm understanding of the current condition of the HVAC system.
I would be happy to work with you to develop the information you need for proper due diligence.

William Chandler

The Building Inspector