The terms “deferred testing” and “seasonal testing” are common throughout the building systems commissioning world. So common, in fact, that there is often very little discussion about what they mean and how they are applied. Recently, I was drawn into a discussion with someone who had a different understanding than I did, thus leading me to question my own interpretation. I requested a cursory, nonscientific literature review of nine different industry standards and guidelines and learned the following about deferred and seasonal testing.

Deferred testing (also infrequently referred to as delayed testing) appears to be universally defined as testing that cannot be effectively or conclusively performed during the construction phase acceptance period. The reasons for this include the following, all related to proper loading of the systems to demonstrate performance under near-design conditions.

  1. Insufficient building occupancy;
  2. Insufficient process equipment installation or operation;
  3. Inappropriate weather conditions; and
  4. Construction Phasing

Individual projects may also have other unique reasons for not being able to complete testing prior to substantial completion. I suspect all commissioning professionals have experienced plenty of deferred testing situations related more to schedule (i.e., the construction schedule ran out of time before the contractors finished the systems) than to lack of sufficient load conditions.

With respect to seasonal testing, almost everyone defines that as functional performance testing at near-design heating and cooling weather conditions. ASHRAE Guideline 1.1-2007 takes it a step further by including intermediate seasons (presumably spring or fall) as a potential third “season.” Although most typically applied to HVAC systems, daylighting controls and domestic hot water systems may also be influenced by different seasons.

Not all systems need to be tested for each different season or for other post-construction load conditions. The commissioning professional needs to collaborate with the project team and owner to determine which systems or sub-systems will need post-construction testing to properly demonstrate their compliance with the design documents and owner’s project requirements. This would be in lieu  of simulating the desired load conditions during construction phase functional performance testing.

The Building Commissioning Association’s Handbook defines deferred functional testing and seasonal testing to be exactly the same. The ACG’s commissioning guideline refers to “off-season” testing at maximum heating and cooling loads but does not address “deferred” or “delayed” testing. Otherwise, the other seven standards or guidelines treat seasonal testing as a subset of deferred testing.

Interestingly, none of the sources state that seasonal testing is intended to include an equipment capacity test. The focus of seasonal testing is to demonstrate dynamic functional performance of the equipment under extreme weather conditions. For example, in the heating seasonal test, it’s important to observe multiple boilers smoothly staging up from one to all boilers operating as loads are added to the system. Similarly, summer seasonal testing would include staging of multiple chillers and the smooth transition from minimum to maximum loading of cooling towers. We would want to observe the cooling towers increasing capacity from 100 percent  bypass to full water flow with minimum fan speed to full water flow with maximum fan speed.

Although inadequate equipment capacity may be a deficiency that manifests itself during seasonal functional performance testing, those tests are not intended to positively confirm that the installed equipment will perform to its design and factory-certified heating and cooling capacities. Field testing equipment capacity in an occupied/operating building is extremely challenging and potentially very expensive to orchestrate. It typically requires design weather conditions to coincide with maximum (or minimum) interior loads (people, lights, equipment, etc.) and for both of those to coincide with the availability of the commissioning professional. These are not naturally concurrent events and would require careful planning, utmost flexibility, and orchestration of building-wide occupancy and equipment loads or the imposition of an artificial load on the tested system.

If installed equipment capacity testing is required by the owner, it may be most effective and meaningful to perform that testing with artificial loads prior to building occupancy. This would be costly and impact schedule, but it may be the best way to maintain tight control over the parameters needed to match the equipment’s design operating points.