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Deferred or Phased Testing

Deferred testing is functional performance testing performed after substantial completion. Whereas systems performance testing should occur prior to the owner accepting the systems from the contractors, there are some instances where testing at the end of construction is either impractical or not meaningful.

The most commonly cited reason for deferred testing is testing of systems with equipment that is not intended to be operated under specific weather conditions. For example, most direct expansion (DX) air conditioning systems have minimum ambient air temperature conditions below which the systems are prevented from operating. Some cooling towers fall into this category as well. On the heating side, if a building’s heating system is dependent on a central steam system which only operates during winter months, functional performance testing will need to wait for steam availability.

Deferred testing can also be driven by project phasing considerations. Core and shell construction projects are an example of this. A base building (core and shell) will often include the mechanical, electrical, and life safety systems infrastructure with minimal or no terminal units or connected loads. On the HVAC side, the core and shell systems may include a central boiler plant, chiller plant, and/or ventilation air handling units. A core and shell project will typically also include terminal units for public spaces such as entries and elevator lobbies, but these represent only a small fraction of the intended future load on the central infrastructure systems.

Tenant fit-outs or tenant improvements (TI) are the projects that finish areas of the shell building for specific occupants. At least some TI projects commence immediately after a core and shell project is complete, but a number of shell areas may remain unfinished and empty for months or years after core and shell completion. TI projects are almost always separately bid or negotiated construction projects and often involve contractors different from the core and shell building contractors.

The question for core and shell project commissioning is when the infrastructure systems should be tested. Should they be tested at the end of the core and shell construction without significant terminal unit loads? Can loads (e.g., heat, humidity, electrical demand, pressures, etc.) be simulated? Should parts of the infrastructure systems be tested at the end of construction and others be deferred?

For the tenant improvement projects, the question is whether or not the infrastructure systems need to be retested to some extent every time a major TI construction phase is completed. If so, who is responsible for that testing – the core and shell commissioning professional or the TI commissioning professional?

A third project type is one which is bid to a single construction team but is intended to be built in multiple consecutive phases. This is more straightforward than core and shell/tenant improvement project delivery processes, because a single commissioning plan can cover all phases of the project with a consistent commissioning team engaged from start to finish.

Phased projects need to consider what, if any, systems need to be tested or retested at the end of each phase. Some systems may be installed in one phase to serve future phases. Some systems may be similar to core and shell infrastructure systems whose connected loads increase incrementally in multiple future phases. If the owner intends to occupy completed phases while construction continues on other phases, it is advisable to test systems serving the occupied spaces before the owner moves in instead of deferring testing to the end of all phases of construction.

Deferred testing and phased testing can have a significant impact on the cost or complexity of commissioning. If contractors need to return to a building months after moving on to other project commitments, their attention to the deferred commissioning activities may be less focused than it is prior to substantial completion. In addition, mobilizing for testing at the end of each phase of construction is more costly (time consuming) than a single condensed period of functional performance testing at the end of all construction.

To the greatest extent possible, the commissioning specification should identify which systems will be commissioned when and, if applicable, how many times. Once under construction, if the schedule slips the commissioning professional needs to be aware of how delayed construction completion will impact the testing schedule. If there is a months-long delay, seasonal deferred testing may be opposite of what was originally specified. For example, if the specification called for the heating system to be deferred because construction was to be completed in July, a new construction completion date of December may cause the central cooling system to be deferred instead.



Engineered Systems, January, 2007

Rebecca Ellis, PE, LEED AP, CCP, CxA
Questions & Solutions Engineering
1079 Falls Curve
Chaska, MN  55318