Real-world Examples
The following are examples of how RCx observations provided information to the energy model and how the energy-modeling results then contributed to the RCx project:
Stuck VAV Damper
One of the most straightforward examples is the classic case of inoperable VAV dampers. In this case, the RCx team observed a VAV damper stuck in the open position.
The “Minimum Flow Ratio” was set to 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.
Integrated Diffusers/Lights
The RCx team on this project noted the air-distribution method in the building was old-style diffusers that were 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 F.
This observation transferred 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. This allowed accurate calculations of the energy effects of this observation to be performed. Additionally, the effect on the equipment capacity, as well as zone temperatures, was quantified.
Information flowed from the RCx project into the energy model, and the energy model provided information to the RCx project to help guide the owner’s decision.
Infiltration
A building built in the 1920s 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 it was experiencing persistent pressurization issues between floors. The RCx team was able to determine that many of the exterior zones of the building were negatively pressurized.
An energy model was developed for this building, and the calibration process of the model to the utility data proved very difficult. It seemed the heating energy in the model just couldn’t get anywhere near the historic heat consumption of the building.
When this fact was combined with the observations regarding the loose envelope and the negative pressurization, it became apparent infiltration was driving the utility cost of the building. The question, of course, was: “How much infiltration does the building have?” The industry has not yet developed a reliable method to quantify infiltration rates.
The approach that was taken was to incrementally raise the infiltration rates in the energy model until results that were consistent with utility data were reached. This approach eventually did reach results very 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 decisions made by the owner. 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.
Although energy modeling may not be appropriate for every RCx project, it can be a valuable tool available to RCx providers. The insight provided by the model often 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.