Roof Top Unit Optimization Pilot Project- A Wrap Up
Heating and air conditioning can account for over 25 percent of energy use in commercial, retail or industrial buildings, and rooftop units serve 40 percent of the floor space within those buildings. Upgrading to a more efficient model is often expensive and, even for older units, does not achieve the savings necessary to justify the investment. CEE wanted to assess the feasibility of rooftop unit retrofits as a scalable energy-savings approach, and better understand the application and cost-effectiveness in a Minnesota climate.
RTUs are an easy option for heating and cooling in many buildings – they are reliable, have a low initial cost and work well in all climates. However, they also operate extremely inefficiently and often come with many control challenges. Since the economics of replacing RTUs doesn’t make sense for the majority of situations, CEE was interested in the practicability of RTU retrofits as a cost-effective way to increase their efficiency. With that in mind, CEE developed this pilot project to evaluate different optimization packages, gather performance data on a large number of RTUs in both winter and summer, and develop a better understanding of how optimization packages work in a Minnesota climate.
To conduct this research, CEE received a grant from the Minnesota Department of Commerce through the Conservation Applied Research and Development (CARD) program, and invested organizational matching funds to increase actionable knowledge of this important emerging retrofit strategy for the energy efficiency market.
This project is one of the largest studies on RTUs in the country and is the only study completed in a Minnesota climate. Over the course of a year, CEE research staff collected data on three different optimization packages (Premium Ventilation, Digi-RTU Optimizer, and Catalyst Optimizer) on 66 RTUs across six different sites that included offices, conference rooms, labs, warehouses, as well as manufacturing and printing spaces. Performance data was collected from March 2012 to August 2013, with optimization packages installed and monitored from June 2012 to August 2013. During that time, flip/flop test protocol was used to switch between standard operation performance and optimized operation on a weekly basis. This allowed for two years’ worth of data collected in one year, and also accounted for any changes that occurred in the building over that time period. The initial findings showed that all the technologies achieved significant electric savings, while gas savings were negative or insignificant.
Digging deeper, some key findings stood out by providing insight in to making RTU retrofits successful across the board, and where to go from here with the results of this research.
RTU optimization in a Minnesota climate
This study was the first time this technology was used in a cold climate like Minnesota’s and CEE worked in partnership with the manufacturers to develop an understanding of how it performed in extremely cold weather. All three technologies had issues within the extreme cold, and as a result of the findings, some manufacturers changed their equipment to operate more reliability and efficiently in our climate.
Optimizing every unit on a site
Not all units in the study could be optimized, but these units were still monitored as a part of the research. If a specific unit could not be optimized when all other units at a site were optimized, that unit used more energy after optimization than it did previously. This happened across the board with units that were not optimized, indicating that in order to get the assumed savings for a site all RTUs need to be retrofitted with optimization packages.
RTU retrofits are not “one size fits all”
With so many different optimization options available, one challenge with RTU retrofits is that each technology requires specific knowledge to be deployed so that it is cost-effective and achieves the listed savings. Some optimization packages work better in certain applications and in many situations an independent expertise is needed to get the most out of the retrofit.
Why the increase in heating cost?
Across all of the technologies there was an increase in heating cost, but it was not significant enough to offset the electrical savings. The data collected in the scope of this project did not provide enough information to understand exactly why this occurred. More investigation needs to be considered in order to figure out the increase in gas consumption.
There is still much to be learned! The market for RTU optimization packages is improving rapidly. Since the start of this project there are already many more options available, and the different packages that were looked at in this study have already been improved upon. A full report will be issued in early 2015, and will include details on the savings and cost-effectiveness of each of the three technologies, including the true savings of RTUs in Minnesota climate. For more information, you can access our recent IX webinar on the subject and view our RTU and economizer animations.
Image Credit: Nic McPhee