The Energy Model’s Role in Retrocommissioning

By John Bixler, PE, LEED AP, NV5

Retrocommissioning (RCx), or the assessment and optimization of building performance, is a scientific process that relies strongly on data and technology to deliver effective and reliable results. The tools required to complete the task include the review of existing drawings and energy bills, interviews with building operators, data-logging devices, as well as trends and other information from the building automation system.

Yet, one of the most effective, though often overlooked, tools in the RCx toolbox is the energy model. Energy modeling is a physics-based software simulation of building energy use. The primary intent of energy modeling is to make apples-to-apples comparisons of different characteristics of a building during design—not reflect actual energy use of a building.

A well-constructed energy model can be highly valuable in the context of a RCx project as well. Retrocommissioning is all about evaluating actual building energy use and improving operations. What better way to supplement those observations and further assist decision-making than with an energy model?

Bi-Directional Data Exchange
The value of an energy model to a retrocommissioning project lies in the back-and-forth flow of data between the energy modeling solution and actual building operational characteristics to help evaluate cause and effect.

Consider a lighting replacement project. If the RCx observations note a building has legacy fluorescent lighting and the recommendation is to replace those with LED lights, the energy model will calculate the electrical savings at the lights, as well as the effect that the change will have on the HVAC system. With the energy model calculations, the retrocommissioning agent can also quantify, with a high degree of confidence, the difference in heating and cooling loads. As well, information, such as peak-load reductions and even cooling-tower water temperature bins, can be provided as part of the RCx report, thanks to the energy model. In essence, the energy model helps make system interactions clear and quantifiable. The load information provided by the energy model can also be used to evaluate equipment sizing as part of the RCx process.

Surprising to some, developing energy model inputs can be much less intimidating in a RCx project. Questions about the building envelope, equipment capacity, motor sizes, room-usage patterns, lighting and other concerns are easily answered.

The Real Value — Informed Descisions
Once input into the model, the retrocommissioning agent is able to push beyond rules of thumb and experience and confirm sizing issues with hard data, allowing the owner to make well-guided decisions with this additional information.

Often, the less obvious observations that emerge during the RCx process are the very components that can enhance the energy model. For instance, results from functional testing can be added directly to the energy model during a retrocommissioning project. Did you find a VAV (variable air volume) damper that was wide-open? Is there an air handler that never reaches its static pressure setpoint? Have you discovered that random lights are left on overnight? These observations can all provide valuable input that flows directly into the model.

Utility usage data, gathered as part of an RCx project, also flows into the energy model. Actual figures can be compared to the model’s expectations, which can then be used to guide tweaks to the model. The model’s energy use can then be calibrated to that of the actual building.

While the energy model input and the calibration process can be difficult and time consuming, the payoff is a high degree of confidence in the model (and any savings calculations evaluated by the model). As well, the information flowing out of the model provides important insight into the building’s performance (information flowing from the energy model to the RCx project).

Onsite Observations
RCx observations and the energy model can contribute invaluable information to a retrocommissioning process.

One of the most straightforward examples is the classic case of inoperable VAV dampers. In this case, the RCx team observes a VAV damper stuck in the open position. The “Minimum Flow Ratio” was set to the No. 1 in the energy model. The model was able to quantify additional energy use from the electric, chilled water and hot-water systems in the building.

On another project, the NV5 RCx team noted the air-distribution method in a building was a network of old-style diffusers integrated with the fluorescent lights. The air blew directly over the lights as it entered the zone, picking up the heat generated by the lights. The heat gained by the airstream was quantified by measurement and averaged about 5 degrees Farenheit.

The team transferred the observation into the energy model in the “duct losses” area, where a duct delta T (heat gain from the air-handling unit discharge to the entrance of the air into the zone) can be entered. With that input, the model evaluated energy effects to the building and quantified the equipment capacity, as well as zone temperatures.

Calibrated with Confidence
In some cases, a retrocommissioning team must adjust an energy model to support real-world findings.

In one recent case, we were asked to evaluate a building built in the 1920s. The structure had a very loose building envelope, and many of the original windows were nearly 100-years old. Part of the motivation for a RCx study of this building was that it was experiencing persistent pressurization issues between floors. The RCx team determined that many of the exterior zones of the building were negatively pressurized.

We developed an energy model but the calibration process of the model to the utility data proved very difficult. It seemed the heating energy in the model couldn’t match the historic heat consumption of the building.

This fact combined with the observations regarding the loose envelope and the negative pressurization indicated that infiltration was driving the utility cost of the building. In this case, the team incrementally raised the infiltration rates within the energy model until they reached results that were consistent with utility data.

In this case, information from the RCx project flowed into the energy model. Once it was calibrated, the energy model then provided information back to the RCx project, which helped guide subsequent decisions. The owner decided to replace the windows as the first step, then address the HVAC system and building pressurization after the loose envelope was addressed.

Long-Term Benefits
Although energy modeling may not be appropriate for every RCx project, it can be a valuable tool available for owners and retrocommissioning teams. Often, the insight provided by the model is the key to understanding a complicated building dysfunction. The information flow between the RCx project and the energy model—in the hands of creative problem-solvers—can provide significantly more information to project teams and owners.

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