Functional Performance Test Sampling

So you’ve decided on your sample size. What next? What happens when a system failure is found?

By Rebecca T. Ellis, P.E.

Last month, this column addressed the need for (and philosophy behind) sample testing certain systems when 100% functional performance testing (FPT) is impractical from a time and/or budget perspective. This month, I will review some of the details associated with sample field testing similar systems.

System Selection

The commissioning professional should select the systems to be tested, and those systems should remain unknown to the contractor until the day of testing. I do not recommend a true random selection because there are commissioning objectives to be achieved by choosing certain systems. For example, if the intent is to sample 25% of the VAV terminal units in a project, the commissioning professional will want to be sure the test sample includes:

VAV terminal furthest from the AHU
VAV terminal closest to the AHU
VAV/reheat terminal furthest from the reheat hot water pump
VAV/reheat terminal closest to the reheat hot water pump
At least one terminal unit of each configuration and control scheme, e.g., no reheat, reheat, CO2 airflow control, occupancy sensor control, radiation plus reheat, etc.

The selection process may also take into account project-specific details such as phased construction (select terminal units from each phase), changes in personnel (select terminal units installed and/or programmed by different people), and so forth.

Acceptance Criteria

Although it is relatively easy to define a sample size and select the most appropriate systems to test within that sample, the challenging part of sampling is determining what to do if the sampled systems do not pass their initial tests. Do those systems simply need to be corrected and retested, or do more systems need to be tested as well? The following details must be worked out ahead of time and documented in the commissioning specification.

Define what constitutes a “failure.” Is failure to pass any individual step of a FPT procedure grounds for failing the entire system? Are some steps more critical than others when it comes to acceptance? Are there some types of failures that the contractor could not have anticipated and/or prevented through better preparation and pre-testing?
Define the initial sample failure threshold that will trigger testing additional systems. If there are 100 VAV boxes and 25% are sampled, how many of those boxes would need to fail their functional tests to force the contractor (and commissioning professional) to test additional boxes? Just one failed system? Two failed systems? A specified percentage of the total initial sample?
Define how many additional systems need to be tested if the first sample set fails to meet its acceptance criteria. Another 25%? The remaining 75%? Something in between?
Last, but not least, the commissioning specification needs to address who is responsible for the additional sample testing time and costs. This includes the commissioning professional’s fees for facilitating, documenting, and following up on the additional tests.

In summary, once the decision is made to not functionally test 100% of the commissioned systems and the sample percentages are determined based on the owner’s priorities and budget, the hard work begins. A successful process at the end of construction requires that the commissioning specification include a comprehensive and unambiguous definition of how the sampling strategy will be applied and administered for that particular project.