An Overview Of Building Diagnostics
As noted by the National Research Council, building diagnostics is a process whereby a "skilled expert, drawing upon available knowledge, techniques, and instrumentation, predicts a system's likely performance over time." The diagnostic procedures may be used during the planning, design, construction, and operational (i.e., actual operation of the building or system) phases of a building or system to focus on the links between occupant responses and overall building system performance.
As these techniques have evolved, they have been used to:
Building diagnostics is a convenient tool for identifying and isolating mold/moisture problems, which are often related to envelope problems (e.g., leaks through roofs, walls, foundations, windows, and doors), minor and catastrophic piping failures (e.g., plumbing and appliance leaks), and sewer backups and floods. Excessive duct leakage can create pressure imbalances across the envelope that may also create moisture-management problems, including mold growth.
In today's litigious environment, contractors are increasingly being held accountable for mold/moisture problems that in many cases may not be directly related to the HVAC system. Building diagnostics is a practical tool that can be used to identify the source of the problem and determine what steps to take, or recommend to the owner, for corrective action.
The Difference Between Building Commissioning And Building DiagnosticsBefore turning to the elements of building diagnostics, it is useful to emphasize the difference between building commissioning and building diagnostics.
When the concept of building diagnostics was defined by the National Research Council, four essential elements were described:
1. Knowledge of what to measure;
2. Availability of appropriate instrumentation;
3. Expertise in interpreting the measurements; and
4. Capability of predicting likely performance
As the concepts of building diagnostics and building commissioning have evolved, confusion has arisen regarding the similarities and differences in these processes. The distinction between the two is as follows:
The commissioning process is inherently flexible with regard to the extent and intensity of the performance evaluation that may be provided under an "agreed upon scope of services". Unlike the building diagnostics process, however, the commissioning process does not intrinsically define criteria to objectively measure and evaluate the building performance, nor does it provide guidance for investigating occupant complaints, for linking occupant performance or productivity to building system performance, or for assuring preparedness of the building and its systems for responses to extraordinary incidents.
Under some circumstances, one or the other process may be preferred, whereas in other circumstances, the two processes may be combined to achieve desired results. If objective evaluation or scientifically valid and reliable measures of building performance are expected or required in the planning, design, construction, and/or operational phases, the building diagnostics process should be incorporated into the building commissioning process.
Also, the building diagnostics process should be used when a consistent set of evaluation criteria will be needed for evaluation of the building's performance over time. With these distinctions in place, it is time to describe several important concepts central to building diagnostics.
Concepts Of Continuous Degradation And Continuous AccountabilityThe framework of building diagnostics consists of three integrated concepts: (1) continuous degradation of the building stock and systems within the building; (2) continuous accountability, which is a process by which the continuous degradation can be intercepted; and (3) the building diagnostics procedure, which relies upon the use of objective criteria to identify the sources of problems (such as mold/moisture impact) in buildings or systems.
Buildings and systems do not always perform as intended. Rather, their performance tends to degrade unless interception and intervention techniques are employed. As this degradation can begin in the planning and design phases, and continue through the construction and occupancy phases, it is characterized as continuous degradation.
Interception and recovery from continuous degradation requires knowledge and commitment. The knowledge is obtained from diagnosing the cause of the degradation. As shown in Figure 1, building diagnostics procedures are used to identify and implement interventions, with the objective of regaining desired building performance.
Because the process of continuous degradation can start at any phase in a building's or system's life, assurance of a healthy building requires regular monitoring, interception of the degradation when detected and diagnosed, and timely implementation of intervention and recovery processes. It is noteworthy that these processes, which are similar to those practiced by health professionals, require a significant amount of knowledge and commitment.
The most important aspect for attaining healthy buildings is for the HVAC professional to make a personal and professional commitment to the stewardship of the delivered HVAC systems. The concept of continuous accountability means that the HVAC professional must stand behind his work to the degree of ensuring that the HVAC system, as delivered, meets the expectations of the owner as determined at the beginning of the project. This concept is an effective means of demonstrating to the customer that the contractor is doing a good job. It is also a means whereby the risk of liability may be minimized. Continuous accountability is an essential part of building diagnostics, and should not be minimized or overlooked.
Evaluation And Classification CriteriaIn order to implement building diagnostics (prediction of likely building or system performance), a set of evaluation and classification criteria is needed. The evaluation criteria used in building diagnostics consist of a set of objective, measurable values associated with parameters that link human responses, indoor exposures, building system performance, and economic performance to the loads that are imposed on the systems.
There are four "indoor exposures" (thermal, air quality, illumination and acoustics parameters) related to four types of "human response":
Simultaneously, each of the four indoor exposures is related to the performance of the "building systems" that control it (e.g., structural, envelope, services such as HVAC and electrical, and enclosures such as open or closed workspaces). The "building system" parameters also are simultaneously linked to the "indoor exposures" and the "sources or loads" that they must control. In addition, all of these parameters are linked to the desired economic performance of the systems in terms of first and operating costs, energy use, and productivity.
After the parameters are selected, "compliance values" are selected as part of the building diagnostics procedure. This selection is made on the basis of the consequences of making false positive or false negative errors. An example of a false negative error is not detecting a moisture incursion pathway that exists. A false positive error is measurement from a false fire alarm. If the consequences of making false negative errors outweigh the consequences of making false positive errors, the ranges of values will be more stringent.
The classification criteria are used in the building diagnostics procedure to determine compliance with the set of evaluation criteria. These classification criteria are grouped into a hierarchy of three categories to minimize the likelihood of committing false positive or false negative errors in diagnosing the performance of a building or areas within it: Problematic, Marginal, and Healthy. Within each of these categories, two or three classifications are provided that indicate progressively improved performance.
The evaluation criteria may be used at any time during the design (i.e., planning through bid periods), construction (i.e., bid, substantial completion, and warranty periods), or operational periods. Therefore, this procedure can be used as a management tool to monitor the performance of HVAC systems and to plan for timely and cost-effective system interventions. The key is for practitioners to use building diagnostics in the monitoring of system capacity and controllability over time to ascertain building performance.
This article is excerpted from Good HVAC Practices for Residential and Commercial Buildings by Bradford A. Penney, J.D., James E. Woods, Ph.D., P.E. (both with The Building Diagnostic Research Institute Inc.), and Glenn C. Hourahan, P.E. (of the Air Conditioning Contractors of America), published by the Air Conditioning Contractors of America (ACCA). Used with permission.
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Publication date: 01/26/2004