What About ARC?

Posted By AirNW Admin Feb 01 2017

What do you do when you encounter a rooftop unit (RTU) that is wasting energy but not yet at the end of its life cycle? Do you replace the RTU with a new one or do you explore other energy efficient options, such as a controls? In this post we will discuss that second option, controls, specifically Advanced Rooftop Unit Controls (ARC).

Advanced Rooftop Unit Controls

Advanced Rooftop Unit Controls are controllers applied to single zone rooftop systems with constant speed fans.  Most commercial buildings are over-ventilated because the constant speed fans are set to run when the building is supposed to be occupied.  The ARC modulates the supply fan to reduce energy consumption. Energy savings from ARC can come from four primary sources:

  1. Supply Fan Speed Controls (the majority of the savings)
  2. Integrated Air-side Economizer Controls
  3. Cooling Capacity Controls
  4. Demand-Controlled Ventilation Controls

The ARC retrofit technology is commercially available from multiple vendors and manufacturers. 

 According to a study by the National Renewable Energy Laboratory (NREL) (http://www.nrel.gov/docs/fy14osti/61072.pdf) ARC products require services to install the unit, perform O&M, and deliver BMS functionality (if applicable). There is no single business model for accomplishing this, and some ARC manufacturers work through local HVAC providers for installation and O&M services.  The following table from the NREL study offers a brief summary of a few of the existing ARC systems:

Source: NREL RM12-2703 Advanced Rooftop Unit Control Retrofit Kit Field Demonstration, March 2014, p. 62.

ARCs In Action

In October 2016, the Emerging Technologies Coordinating Council completed a study of how ARC for rooftop units (combined with standardized O&M practices) could improve energy efficiency.  Two fast food restaurants in the San Diego Gas & Electric service territory were used as the test sites.  The ARC technology included variable speed control of the supply fans, setpoint scheduling and control, DR load shedding capabilities, fault detection, and a user interface for tracking energy usage, projected savings, and unit operation. DR events were manually initiated for research purposes at both sites using two load shedding strategies: setpoint increases of 1 °F per hour for 6 hours and 2 °F per hour for 3 hours.  Data was collected to measure the indoor air quality and energy consumption of each of the RTUs at the two test facilities. 

This study found that energy consumption was reduced 8.3% and 13.2%, with additional savings secured during demand response events with demand was reduced by as much as 25.9% during tests. 

In both cases, the ACR-only costs had a simple payback of less than 5 years.  However, the study’s authors found that savings could vary greatly and that there was a need for further study before a definitive determination of the controls’ benefits could be established.  The study can be downloaded at http://www.etcc-ca.com/reports/maintenance-and-advanced-rooftop-controls .

 Next Steps

Your project might also qualify for incentives through your public utility. In order to learn more visit the ARC page on our website or look up your utility to see if they offer incentives.

Next time you encounter an overworked RTU, think ARC first! If you have more questions or are not sure if an ARC fits your project, contact us at info@airnorthwesthvac.com.