From an HVAC system perspective, most buildings are integrated organisms whose terminal unit performance, i.e., satisfying the building occupants, is dependent on a number of infrastructure systems. When defining a commissioning program for the partial renovation of an existing building, how far back into the infrastructure should the HVAC (this also applies to electrical, lighting, security, and life safety systems) commissioning attention extend? I believe that many people who have tried to keep the HVAC commissioning scope within the confines of the architectural boundaries of a renovation have been frustrated and disappointed with the results.
Working on the big picture
As an example, let's think about a project (or phase of a project) to renovate half of a patient floor in a hospital. Those patient rooms and support spaces are served by an AHU that also serves other patient rooms, horizontally and/or vertically adjacent to the renovated area. That AHU is dependent on steam and chilled water from a campus central utility plant. Similarly, the perimeter heat in each room is served by hot water generated by a building steam-to-hot water heat exchanger and distributed by building pumps.Essentially, the only HVAC components within the architectural boundaries of the half floor are VAV reheat air terminal units and perimeter radiation. The obvious minimum commissioning scope would be to only commission those terminal VAV and perimeter radiation units. On the surface, that's one approach to keeping the renovation scope and cost "reasonable."
However, the design intent criteria needs to reflect the central system parameters expected to be available at the time of testing. If the engineers and contractors working on the renovation are to be responsible for terminal unit performance, they can only commit to system performance over which they have control. Therefore, if the evaluation and upgrade of HVAC system components outside of the architectural boundaries of a project is not included in the project team members' scope, they must "cover themselves" by defining the base expectations for central system performance. Some examples of these include:
- Supply duct static pressure and airflow at a specific location
- Return duct static pressure and airflow at a specific location
- Supply air temperature
- Hot water supply and return pressures
- Hot water supply temperature
Spreading the responsibility
The onus of meeting these expectations will fall on the building owner/operator. The commissioning testing process will necessarily include verification that these existing system conditions are available prior to testing the performance of the renovation contractors' terminal devices. This adds an interesting twist that many owners are not used to in a project, i.e., their being accountable for aspects of the system performance. This responsibility is the "cost" of limiting the scope of the renovation team to the architectural boundaries of the renovation.This can get a bit dicey, especially if the owner simply "believes" or "hopes" that the existing systems will perform as documented in the DID. It will come as a rude surprise if the owner's systems don't perform up to snuff at the end of construction. Therefore, I advise owners to take this responsibility very seriously.
If a building owner wants to separate out existing systems commissioning (e.g., retrocommissioning or the process of confirming that the existing central systems are working properly) from a limited renovation scope of work, that's the owner's prerogative. However, in order to expedite on-time final testing and completion of the renovation, the owner should put forth a concerted effort to verify that he can hold up his end of the bargain (central system performance) at the end of construction. This can be done with the owner's in-house staff, an outside commissioning consultant, or the commissioning professional working on the renovation project.
This specific example represents a common situation commissioning professionals deal with every day. Another significant example affecting new construction is a building planned for a campus served by central heating, cooling, pneumatic, and/or electrical systems. While there is no single correct answer to the question, "How far back into the existing support systems do we need to take commissioning?" the key is to discuss it with the building owner, designers, and contractors during the commissioning planning process. As with most commissioning issues, addressing challenges early helps to mitigate every team member's risk.
