Commissioning: Presenting Pre-Commissioning
HOW ABOUT IF WE ALL KNOW EXACTLY WHAT WE'RE GETTING INTO?The commissioning lexicon currently includes terms such as recommissioning, retro-commissioning, and continuous commissioning. This month, I'd like to explore a concept that may not be new, but which I believe is an integral part of commissioning a building renovation project. By giving it a name, I'm hoping it may be more likely to become common practice. I'd like to dub it pre-commissioning.
My initial discussion of pre-commissioning is based on the assumption that the building being renovated has not been commissioned, recommissioned, or retro-commissioned. This covers a vast majority of the existing building stock.
A CONCEPT IS BORNFor HVAC systems, if existing systems are to be reused, in whole or in part, in the renovation, the performance of those systems will be measured and documented prior to beginning the design and construction process. Pre-design data collected for existing air-handling equipment includes airflows in both 100% and minimum outside air modes, fan motor electrical consumption, key distribution system static pressures, and chilled water and hot water coil flows. Pre-design data collected for existing hydronic systems includes water flows and pressure drops in maximum demand conditions, pump motor electrical consumption, and key distribution system pressures.
Keep in mind that "existing systems" include central chilled water, steam, and other central utility distribution systems. More buildings than owners would like to admit are renovated (or added) with increased demand on central utilities that don't have the capacity to support the additions and modifications. Owners have the right to build beyond their own central utility capacity, but in that case, they should not hold the project team responsible for non-performance of the new or modified systems under high-demand conditions. The onus will be on the design engineers to define under what load conditions the owner can expect decreased performance, and this is where the value of data collected during pre-commissioning will be invaluable.
In addition, it is critical that the design engineers know how the existing equipment is controlled, e.g., control points, sequences of operation. This is best learned through direct observation and testing of the systems prior to design. Reliance on control system O&M manuals for accurate and current information is always a mistake, and reliance on system operators is risky.
The baseline data collected as part of the pre-commissioning process can be used to:
- Identify operational issues that should be corrected prior to beginning the renovation.
- Define performance criteria for existing spaces served by the existing systems that need to remain operational during construction.
- Determine whether or not the existing systems have sufficient capacity to serve the renovated spaces.
DEFINING THE NEEDThe fundamental concept behind pre-commissioning is the need to understand current building system performance and operation before successfully designing and implementing modifications to those systems. I believe most facility operations people would agree that no building system performs exactly as originally specified, scheduled, and/or documented in "as built" drawings and manuals. Therefore, starting a project with the assumption that the systems are performing as originally designed is the beginning of a bad trip.
If the design team does not really know how an HVAC system, for example, is performing under existing conditions, they are sure to guess wrong - simply because there is only one set of right answers to the question of current operation and an infinite number of wrong answers (assumptions). Because the savvy design team will clearly state in their contract that their design will be based on "the best available documentation," the owner will bear the brunt of additional costs and schedule delays associated with discovering, at the end of the renovation project, that the existing systems were performing differently than expected.
Pre-commissioning is actually a subset of the full service commissioning process that starts during pre-design. A full-blown formal pre-commissioning process may not be required in buildings whose systems were recently recommissioned or retro-commissioned or in buildings with an active continuous commissioning process. In these latter cases, meaningful and reliable data should be available as a result of the existing building commissioning efforts already undertaken by the owner. ES