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What is a System?

In two previous Getting it Right columns, I addressed the differences between traditional quality control and commissioning (December, 1999 and May, 2002). The crux of the issue is that quality control is focused on equipment and commissioning is focused on systems, i.e., groupings of individual pieces of equipment designed to dynamically operate together.

We talk a lot about “systems” in the commissioning world, but I think we sometimes gloss over the word too quickly. One of the key aspects of commissioning is being able to define the systems to be commissioned in a manner that will achieve comprehensive performance verification as efficiently as practical.

As noted above, systems are not pieces of equipment, so defining a “system” as a single pump, fan, boiler, etc., is typically not appropriate for commissioning purposes. (Exceptions to this may be unitary “equipment” from single vendors that are actually pre-packaged systems, e.g., computer room units with fans, filters, compressors, controls, etc.) However, how far do you go in defining what is in a particular system? There are no hard and fast answers except for the requirement that, whatever is decided for a particular project, it is necessary to be clear and unambiguous.

It is important to define system boundaries. This is best accomplished by drawing bubbles around schematic diagrams. For example, in a heating hot water system you will probably include the central heat exchangers, pumps, and controls. However, are the end users (unit heaters, finned tube radiation, duct-mounted coils, etc.) part of the “heating hot water system” or are the terminal units individual systems with their own components and controls?

Another method of defining systems is to name each system and list the equipment associated with it. This is a bit more risky than the schematic diagram method (a picture is worth a thousand words), because you risk leaving some components out.

I strongly believe in having a single functional performance test procedure for each “system” and that functional performance test should be conducted consecutively from beginning to end without interruptions greater than overnight. This is something that should be taken into account when defining the systems early in the project. Do not define a system that is so large and far-reaching that it is too cumbersome to manage testing in a reasonable time period.

This constraint is one of the reasons I prefer to define terminal units as their own separate systems. On large air or water distribution systems, it is impractical to schedule testing of the central system equipment and all of the associated terminal units in one solid block of time. Because the central systems (air handlers, boiler plant, chiller plant, etc.) can often be completed prior to completion of all associated terminal units, it is helpful to test the central systems first to keep them off the project’s critical path. Individual terminal systems can be subsequently tested in relatively small time periods as they are completed. In addition, multiple terminal unit system tests can often be performed simultaneously to expedite the process.

Another factor to consider when defining system boundaries is how much inter-system testing will be required. Regardless of how you divide up a building’s systems, the integrated nature of today’s facilities will likely necessitate inter-system testing of some type. These are some of the most important commissioning tests, because they are often the most neglected aspects of non-commissioned projects. For example, think carefully about how you will test the communication between and performance of the following system interfaces.

Air Handling System – Fire Alarm System

Lighting Control System – Security System

Heating Hot Water System – Heating Hot Water Terminal Units

Chilled Water System – Air Handling System

The more interactions there are between systems, the more efficient overall building operation tends to be. Smartly defining systems to allow for efficient but effective inter-system functional testing will minimize commissioning costs and schedule requirements.


Engineered Systems, June, 2005

Rebecca Ellis, PE, LEED AP, CCP, CxA
Questions & Solutions Engineering
1079 Falls Curve
Chaska, MN  55318