4. The System Concept

The information and testing procedures covered in the guide are viewed from a system perspective rather than a component perspective. This is especially critical for functional testing and the overall success of the system. The performance of the system is dependent on four areas of interaction:

       The individual components in the system.

       The components with each other as a system.

       The system with other systems in the building.

       The building with the ambient environment.

Successful performance is only achieved when these interactions occur in a harmonious fashion in the real world-operating environment. For instance, all of the components associated with the operation of a preheat coil may check out satisfactorily when tested for sequencing logic (i.e. when the control system thinks itís moving the preheat control valve, the valve is actually moving and moving in the correct direction), proper action, and correct arrangement. These pre-functional verifications must be correct for the functional test to be successful.

The goal of the functional test is to look at what happens to this properly installed preheat coil when it is called upon to operate as a part of a complete system. A properly written functional test may uncover the fact that the preheat coil controls properly during a steady-state operating mode, but experiences problems at start-up, when the coil must deal with flow variations and other shifting parameters. Instability in the system flow control loop may cascade into instability in the preheat coil control loop. The system needs to be tested and adjusted so that the components can respond to changing conditions and restabilize without excessive oscillation or damage to the equipment while maintaining tolerable conditions. The functional testís ultimate goal is to identify and correct these system level problems. Thus the guide will take a systems perspective.

4.1 The System Diagram

Two important tools to supporting the use of the system concept in the commissioning process are the system diagram and system sequence of operations. The system diagram is a drawing that shows the entire system under consideration in schematic format, not just portions of the system. The system diagram should not be confused with the system schematics and riser diagrams often found on construction documents. While the schematics and riser diagrams included in most construction drawings are useful tools and often are good starting points for developing a system diagram, they often lack important features and/or only show a portion of the system and its components. To gain a better understanding of these differences, compare Figure 2 with Figure 3. Both figures are drawings of the same system, but Figure 2 is the schematic that was presented on the contract drawings and Figure 3 is the system diagram, which was developed from this information as well as other information in the contract documents.

Figure 2: System schematic from the mechanical drawings

Notice how Figure 3 shows the complete system associated with the air handling equipment, not just the equipment itself. This allows the user to see the entire process and visualize the potential interactions without having to flip between multiple documents. A well-developed air handling system diagram will include the following features:

       The system will be depicted in a logical arrangement but not necessarily a physical arrangement. Relatively passive elements, like elbows, are simply not shown so that the system flow path can be shown in orderly, non-contorted manner.

       The complete air flow path, from point of entry in the building to point of exit is depicted along with all significant system components such as dampers, coils, filters, and fans. Terminal and zone control equipment is shown to the extent necessary to allow a complete understanding of the operation of the system. For simple systems or systems with widely varying terminal equipment arrangements, this may mean that every zone is shown. For more complex systems or systems where one zone may be a typical representation of several other zones, only a typical zone of each type is shown.

       Equipment operating parameters are indicated including flow ratings, horsepower ratings and other pertinent operating data.

       Final control elements that can affect the system operating parameters are shown. In most cases, it is also advantageous to show any sensors associated with the system on this diagram.

Figure 3: Full system diagram for the air handler in Figure 2.

On projects where a system diagram does not exist, developing one is a good first step in any commissioning process. When retro-commissioning, the development will familiarize the developer with the system. For new construction projects, the commissioning provider should work with the design team to make the system schematics into system diagrams. This process aids in understanding and documenting the operation of the system. Once completed, the system diagram can serve as the schematic on the contract drawings, an illustration in the System Manual, and as the graphic for the system in the DDC operatorís terminal.

4.2 Detailed Sequence of Operations

A detailed system sequence of operation or system narrative goes hand in hand with the system diagram in documenting the overall operation of the system. Many times, the sequence provided on the contract drawings and duplicated in the specification provides a good overview of how the system is intended to perform, but fails to address critical details which can make or break the success of the installed system. Consider the following fairly common air handling unit sequence of operation:

The control system shall modulate the economizer dampers, heating valve and cooling valve in sequence as required to maintain the discharge set point of the system. The discharge set point shall be reset from 55įF to 70įF as the outdoor air temperature varies from 80įF to 0įF.

While on the surface, this statement appears to reasonably state the system requirements, there are many details missing. The missing details must be addressed for the sequence to function efficiently in a real-world operating environment. A fully developed version of this sequence is included in 18Functional Testing Basics: Supplemental Information.

There are several commissioning related benefits associated with developing a detailed sequence of operations. The most obvious is that the sequence provides a good description of how the system is intended to work under all operating conditions. Working with the design team to develop such a sequence allows the commissioning provider to clearly document the design intent of the system. The detailed sequence is essential for a systems manual and serves as a firm basis for the control system programming. In retro-commissioning applications, taking the time to develop this sort of information based on existing project documents, reviewing program codes, and observing system performance via functional testing and trending provides excellent documentation of how the system is currently functioning.

If the design and construction team do not address the details prior to development of the control program, then most of these details will be addressed in response to a start-up or operational problem; a reactive and potentially costly approach. Scenarios include:

       The problem will be caught and corrected by an experienced controls programmer but will not be documented in the project construction documents.

       The problem will be caught by an experienced controls programmer but will not be corrected without a change-order from the Owner. If the Owner does not issue the change order, then the sequence will be programmed incorrectly.

       A less experienced programmer who wonít recognize the problem will program the sequence as written.

       The commissioning provider will catch the problem during functional testing. Correction may require a change order.

       If the functional testing plan does not identify the problem, then the problem will most likely show up as an operational issue.

The bottom line is that creating a detailed system-oriented sequence of operation is important for a successful commissioning process. This sequence will provide a firm basis for functional testing, the systems manual, and a re-commissioning plan, all of which are important parts of the commissioning process.