Why You Should Specify Advanced Rooftop Units

08/28/2018 | By Janelle Penny

Is it time to upgrade your commercial air conditioner to an advanced rooftop unit (RTU)?

The Department of Energy (DOE) recently field-tested one of just two commercial air conditioning units that met the specifications of the RTU Challenge, which urged manufacturers to create 10- to 20-ton units that could meet the DOE’s high performance requirements.

Both advanced rooftop units boasted an integrated energy efficiency ratio of more than 18, but the field test was needed to understand the seasonal energy efficiency of the units rather than relying on the units’ performance under standard lab conditions.

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What’s in a Commercial HVAC Unit?

According to the DOE, most standard commercial HVAC units typically have a rated energy efficiency ratio (EER) of 8-12, with higher numbers indicating more efficient performance. Standard rooftop units also typically fit this profile:

  • Fans: Constant-speed supply, regardless of whether it’s in heating, cooling and/or ventilation mode.
  • Compressors: Also constant-speed, which means it can’t modulate depending on the specific load in a space. Instead, it cycles on and off, which is inefficient.
  • Stages: RTUs with capacities over 7.5 tons usually have two or more stages of cooling featuring two or more constant-speed compressors of equal size.
  • Heating: Most conventional RTUs use a gas furnace for heating, though some use electric resistance.
  • Economizers: RTUs that are 7.5 tons or more are usually required to have air-side economizers, unless they’re located in hot and humid climates.

The reliance on cycling fans and compressors on and off rather than modulating to meet a space’s needs leads to expensive inefficiency.

Advanced RTUs like the one tested by the DOE modulate the supply fan speed when the unit is in ventilation mode and may do so during heating or cooling as well. A variable- or multi-speed fan and variable-speed compressors are better suited to meet variable space loads and can make a considerable dent in your HVAC-related energy costs.

Advanced RTU Performance Predictions

Before field-testing the unit, advanced energy modeling was used to predict the tested unit’s performance in Houston, Los Angeles and Chicago vs. three different standard units.

Against existing RTUs already in the field: 

The advanced unit in the field test would reduce HVAC electricity and gas costs by about 45 percent in Houston and Los Angeles and 33 percent in Chicago. The savings on HVAC electricity consumption was about 50 percent in all three places.

Compared to RTUs on the market that meet the 2013 federal regulations for commercial equipment: 

The tested units would reduce HVAC energy costs by about 37 percent in Houston, 40 percent in Los Angeles and 29 percent in Chicago, with electricity cost savings of 40 percent, 42 percent and 50 percent, respectively.

Using the latest ASHRAE 90.1-2010 requirements, which require two-speed or variable speed fan control for cooling capacities over 11,000 Btu/hour: 

The tested units would reduce HVAC energy costs by about 27 percent in Houston, 18 percent in Los Angeles and 15 percent in Chicago, with an electricity cost savings of 30 percent, 19 percent and 28 percent, respectively.

In all three simulations, the tested unit that met the RTU Challenge specifications would significantly reduce energy use, costs and greenhouse gas emissions if it was widely adopted. The DOE research team didn’t estimate a payback period, because the cost of the tested units wasn't widely available when the study was conducted.

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How Advanced RTUs Really Perform

The advanced RTU tested by the DOE was a 5-ton unit instead of the 10-ton model that was entered into the challenge. Smaller units are typically less efficient, but the 5-ton unit was chosen because it had the same capacity as the existing Rheem unit that was already installed at the test site, an office building in Fort Worth owned by the General Services Administration.

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The standard unit had one compressor for staged cooling, a constant-speed fan and no economizer, while the tested unit had a variable-speed inverter-driven compressor, composite condenser fans with a variable-speed motor and an economizer that introduced outdoor air to the space. The two units served side-by-side office spaces with similar footprints to enable easy comparisons.

The Pacific Northwest National Laboratory, which conducted the data gathering and analysis on the RTU retrofit for the DOE, measured an average daily EER of 8-17 Btu/Wh for the standard unit and 12-17 for the advanced unit, with the EER increasing for both as the outdoor temperature air decreased. The advanced unit’s daily EER averaged about 18 percent higher than that of the standard unit.

However, when the research team accounted for the energy consumed by the advanced RTU’s ventilation mode, they discovered that the EER difference between the advanced and standard units jumped to 26 percent in the advanced RTU’s favor. This is likely because the advanced unit always introduces at least some outdoor air, compared to the standard, which generally recirculates return air, the study notes.

When to Upgrade Your RTU

Investments in commercial HVAC equipment can be substantial, so it may be best to wait until it’s almost time to replace your existing RTU before you invest in an advanced model. The savings can be substantial, but depending on your area and what type of unit the advanced RTU would replace, it may not be cost-effective to remove a unit that still has some life left in it even if the new model is more efficient.

The advanced model used in this test was also heavier and had a larger footprint than the standard Rheem unit it replaced, which required structural changes to the building to complete the installation.

Make sure the commercial HVAC vendors you work with are aware of any disparities that could crop up with respect to weight, physical footprint or other hurdles that might make it tough to replace an older commercial HVAC unit.

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