Rooftop Unit Upgrade: Calculating the Savings

Rooftop units (RTUs) and other packaged HVAC systems are widely used in commercial buildings. Older RTUs can waste $1,000 to $3,700 annually in energy per unit, depending on size and type, according to the U.S. Department of Energy. That can really add up. By replacing older units with new, high-efficiency models, you can significantly reduce energy costs.

Even if your RTU is newer and in good condition, you can still realize significant savings by retrofitting with advanced controls. Additional benefits include an improvement in humidity control and occupant comfort, and a reduction in harmful refrigerants.

Calculating the savings

Despite these benefits, justifying the expense of upgrading to high-efficiency units or controls can be difficult without numbers to back up those claims. Pacific Northwest National Laboratory (PNNL) points the way with its free tool, the Rooftop Unit Comparison Calculator (RTUCC).

This screening tool provides a one-on-one comparison of energy and life-cycle costs between two units at various efficiency levels or a single feature comparison between two units (see image above ). For a more complicated analysis, you can use several browsers.

Though not a building simulation tool, it does offer similar features. A multi-tiered interface enables inputting of details through simple queries, including an economizer option. Outputs include life-cycle costs, return on investments, simple payback and savings-to-investment ratio. On top of some 230 climate conditions built in, you can also:

  • Calculate savings related to variable-speed condenser fans while splitting out the condenser fan in the controls, calculation engine and results reporting sections.
  • Select specific fan-control strategies and specific performance curves from manufacturers of three-stage and variable-speed units.
  • Model variations in evaporator fan speed, condenser fan speed and condenser cooling capacity to better represent staged and variable-capacity RTUs.

RTUCC is just one resource the High-Efficiency Rooftop Unit (RTU) Replacement Toolkit offers to encourage commercial building owners and operators to replace or retrofit their units. Participants have access to other resources, including an RTU evaluation methodology.

Take advantage of the calculator and other free online resources to estimate how much you can save from a rooftop unit upgrade.