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RTU Comparison Calculator

RTUCC: Quick Start

This outline discusses the four basic modes in which the calculator is used to represent a rooftop unit. The intent here is to describe the data requirements for each of these modes and strategies to make the best comparisons with consideration for data availability. This discussion will generally progress from low data-input requirements to high. Help on each of the calculator features below can be accessed by clicking on the question mark next to the feature's name on the calculator's Control page.


This is the default mode and uses a generic set of DOE-2 coil curves to represent all units.

  • No data; a basic demo: A simple click of the submit button will use all the default values of the calculator's features and will generate the basic summary output on the Results page. Turning on the "Show bin calculations" feature will produce a more detailed report. Clicking the "Advanced Features" checkbox displays additional calculator parameters and their default values.

  • Basic description of a single-stage RTU: This is the minimum data that is needed for an initial comparison of two single-stage units (with single-speed evaporator fans). This level of comparison can be done without enabling the "Advanced Features" of the calculator.
    • Total capacity (and oversizing factor): The nominal size of the unit in kBtuh. This is the cooling capacity at AHRI test conditions.
    • EER: This is the energy efficiency rating of the unit at AHRI test conditions.
    • Costs: Purchase cost (in units of k$: $1,000 = 1 k$) and estimate of annual costs ($).
    • Economizer: This should be checked if the unit can be configured for economizing and it is enabled.
  • The environment: This characterizes the environment which essentially determines how hard (and how long) the unit will have to work.
    • Building type: Pick a building type. This establishes a building-load model that predicts cooling load as driven by weather data.
    • Location: Pick a state and city to establish the weather data used in driving the building-load model.
    • Schedule: Pick a schedule that best represents the occupancy patterns in the building.
    • Setpoint and setback: Pick a setpoint temperature and also a setback temperature to determine control points for occupied and unoccupied periods.
  • Economics: This affects the savings and payback calculations.
    • Electric Utility Rate: Enter your local electric rate.
    • Equipment life: This is the time period over which energy and cost savings are calculated.
    • Discounting: If it is difficult to estimate a discount rate, turn this feature off to give non-discounted (simple) payback calculations.

Generic with "Advanced features":

Clicking the "Advanced Features" checkbox reveals added features which allow the calculator to do more detailed modeling of the single-stage unit. These features also support the modeling of the more advanced characteristics of multi-stage and variable-capacity systems. Please also refer to the help topics for these features.

  • More detailed representation for the single-stage unit:
    • Fan and Condenser power data: The "Power Inputs" feature allows the user to specify the three power inputs. The evaporator fan power can be estimated as the difference between the gross and net capacity of the unit (expressed in kWatts). The user will notice that the calculator recalculates the EER and condenser power if the evaporator fan or the auxiliary values are edited. A helpful editing pattern is to first edit the fan and aux fields, then re-enter the EER value. Clicking the "Power" button (upper right) will recalculate default values for the power parameters based on the unit's capacity and EER.
    • E-Fan and Condenser: Choose between "1-Spd: Always ON" and "1-Spd: Cycles With Compressor." The difference here is that one setting models the evaporator fan as running continuously and the other allows the fan to cycle off with the compressor. Refer to the help topic for this feature for additional information related to unoccupied hours and economizing.
    • Humidity: Generally it is best to leave this set to automatic unless there is data on internal humidity levels that might be affected by a separate humidity control system.
    • Ventilation: This feature value is automatically calculated based on building type and generally does not need to be edited.
    • Degradation factor: This is the fractional drop in efficiency of the unit when running at small loads. It will be difficult to get a specific value from a manufacturer. This can be edited to explore the sensitivity of the savings to changes in this parameter. Generally leave this at the default value.
    • S/T Ratio: This is sensible-to-total capacity ratio at AHRI test conditions. This should be available in a manufacturer's performance brochure. Leave at default levels if not available.
    • Demand: Generally only use these fields if you must calculate a demand charge. Refer to the help topic for this feature (click on the question mark).
  • Staged and Variable-capacity units: Four of the "Advanced Features" facilitate modeling of variable-capacity systems.
    • The features:
      • E-Fan and Condenser: Select from three different system types: (1) one- or two-stage units with a single-speed evaporator fan, (2) units with multi-stage condensers, and (3) units with a variable-capacity condenser.
      • Number of Stages: The number of stages in a unit with a staged condenser. Leave this set to 1 for single-stage and variable-capacity units.
      • N for Fan Energy Calcs: The value of n used in fan-affinity law calculations for variable-speed fans. This should generally be left at the default value. This feature has no impact for a single-stage unit with a single-speed evaporator fan.
      • Condenser Fan: The fraction of condenser power used by the condenser fan at AHRI rating conditions. This fraction can usually be estimated from a manufacturer's brochure. This feature applies only (and can only be edited) if the "V-Spd" unit is selected under the "E-Fan and Condenser" feature.
    • Examples:
      • Two-stage unit with a single-speed evaporator fan:
        Set the "Number of Stages" feature to 2. Set the "E-Fan and Condenser" feature to one of the "1-Spd" choices.
      • Two-stage unit with a two-speed evaporator fan:
        Set the "Number of Stages" feature to 2. Set the "E-Fan and Condenser" feature to one of the "N-Spd" choices.
      • Multi-stage unit with a corresponding multi-speed evaporator fan: This is the generalization of the previous case.
        Change the "Number of Stages" feature to the corresponding level. Set the "E-Fan and Condenser" feature to one of the "N-Spd" choices. 
      • A unit with a variable-capacity compressor, variable-speed condenser fan, and variable-speed evaporator fan:
        Set the "E-Fan and Condenser" feature to the "V-Spd" choice.

Three Specific Units:

In contrast to the generic correction curves that are used in the modes described above, specific performance curves and algorithms are used to characterize these three units and are written into the computer code of the calculator's computation engine. These proprietary curves and algorithms were provided to PNNL from the manufacturers. There is a methods page which provides background information on these units. Please also refer to the help topic for this feature.

  • Specific Candidate Unit: Use this advanced feature to select one of the three units. A selection here automatically sets values for several related calculator features: "E-Fan and Condenser" and "Number of Stages." These automatically-set values may need refinement depending on the specifics of the application. For example, when setting the “Specific Candidate Unit” feature to the “Advanced Controls” value this type of add-on technology can be applied to either a single-stage or two-stage unit. Similarly, depending on whether the unit’s evaporator fan is set to cycle with the compressor or run continuously will determine whether the “E-Fan and Condenser” setting needs refinement.
  • Other features: Selecting one these specific units defines its basic nature; however, additional parameters should be set to fully characterize a particular version of the unit. This includes all other RTU-related parameters that are used under the generic-mode approach described above: EER, capacity, power splits, S/T ratio, and degradation factor (if not V-Spd). Use default values if parameters can not be obtained from a manufacturer's brochure. As done above, the environment and economic parameters need to be set: State and City, Schedule, etc.

The Spreadsheet Interface:

The spreadsheet provides a general mechanism to characterize full-load and part-load performance of an RTU's condenser using tables from a manufacturer's performance brochure. Please review the help topic for the "Spreadsheet" feature. There is a methods page that provides additional information. The spreadsheet itself has additional instructions and annotation inside. Note that spreadsheet models are not allowed to be used with the "Specific Candidate Unit" feature.

There are three levels of input data. Only the full-load data (first bullet below) is required. The second and third bullets describe optional inputs:

  • Full-load total gross capacity and condenser power (required): This data is readily available from manufacturers' performance tables. Some interpolation might be required as the spreadsheet needs this data at particular operating temperatures. This data alone can be used to replace the generic coil curves. The good news is that users can build a coil model from this data even if they do not have the additional data described below. If the user prefers to stop at this data level, there are two control cells in the spreadsheet for facilitating putting NAs (Not Available) in the S/T cells (B20 on the Full-Load Performance sheet) and NAs in the part-load tables (B3 on the Part-Load Performance sheet). The NAs will cause the RTUCC to use only the full-load coil data and activate its native apparatus dew-point method for S/T modeling and its native part-load modeling of fans and condensers for estimating part-load performance.
  • S/T data (not required): The sensible-to-total capacity ratios are also generally available from manufacturers. However the native S/T modeling in the calculator is capable of accounting for changing evaporator flow and condenser capacity levels. For this reason, the spreadsheet's S/T model is only recommended for systems with a single-stage and a single-speed evaporator fan.
  • Part-load data (not required): This type of tabulated data is currently not available for the public from manufacturers. As a result this portion of the spreadsheet is only a prototype and demonstrates how this part-load data could be structured in a way related to IEER calculations. The B3 cell on the part-load sheet can be used to turn this off (fill with NAs) or show example values that are generated with consideration for the part-load nature of a single-stage unit or a variable-capacity unit.


Rooftop Air Conditioner