Retrofitting packaged HVAC systems can lead to huge energy savings, says DOE

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by Shane Henson — May 11, 2012—U.S. commercial building owners and facilities managers seeking to substantially reduce their energy bills and operate more sustainably may want to take a look at improving their heating, ventilation and air-conditioning (HVAC) systems. According to a report from the Department of Energy’s Pacific Northwest National Laboratory, they could save an average of 38% on their heating and cooling bills if they installed a handful of energy efficiency controls to make their HVAC systems more energy efficient.

The DOE report, Energy Savings and Economics of Advanced Control Strategies for Packaged Air-Conditioning Units with Gas Heat, examines options for improving the efficiency of commercial rooftop systems called packaged HVACs, which combine compressors, fans and heat exchangers into one unit.

For the report, PNNL researchers conducted a study in which they considered implementing four different control methods to existing rooftop packaged HVACs:

  • Air-side economizers use cool outside air to chill the building instead of creating cool air with the HVAC compressor. Some building codes already require cooling systems to include these, unlike the three other controls examined by the PNNL team.
  • Supply fan speed controls slow or speed up the ventilation fan that circulates the building’s air based on whether or not a desired temperature or amount of fresh air has been reached, instead of continually running the fan at full speed.
  • Cooling capacity controls run the HVAC compressor at different speeds based on need.
  • Demand-controlled ventilation slows or speeds up fans and air intake based on carbon dioxide levels inside the building instead of running ventilation fans at a constant rate.

The team tracked the effects of using these methods with a building energy simulation software called EnergyPlus. The software created computer simulations that took into account 15 climate zones in 16 major U.S. cities.

The team studied four types of commercial buildings: small offices of 5,500 square feet; stand-alone retail buildings of 25,000 square feet; strip malls of 22,500 feet; and supermarkets of 45,000 square feet. More than 1,400 different simulations estimated the potential savings in electricity used to power fans and cooling compressors, as well as the gas used to produce heat. Energy savings were then translated into dollars and cents.

In general, the researchers say they found that installing a multi-speed fan control had the greatest impact on energy savings in hot cities such as Miami. And demand-controlled ventilation created the best possible energy savings in colder cities such as Chicago, Duluth and Seattle.

The team reasoned that because ventilation fans generate some heat when they move, slowing fans with multi-speed fan control in hot climates could reduce the amount of chilling needed. And in colder climates, they suspected that demand-controlled ventilation prevents unnecessarily sending warm air outside, which then prompts the HVAC system to create more warm air to maintain desired temperatures inside.

When the research team added up all the numbers, they found the best possible percentage cost savings was 67%, which could occur when all four controls are added to a rooftop packaged HVAC at a small office building in San Francisco. And the minimum percentage cost savings was 28% and could come from adding all four controls to a supermarket in Seattle.

But savings weren’t limited to cash and energy use. The team also found that a substantial amount of carbon emissions could be avoided if HVAC energy efficiency is increased. As many as sixteen 200-MW coal power plants—which generate enough energy to power 3,000 to 4,000 American homes—could sit idle if just half of the nation’s packaged rooftop HVAC units on commercial buildings were retrofitted with controls, the simulations revealed.