This help is a collection of topics for the features on the calculator's Controls page. These help topics can be accessed by clicking on the question mark next to a feature's name. This opens the help window and positions the corresponding topic at the top.

Help topics are outlined as follows:

Name: Following the feature's name is summary content that is similar to the pop-up tip that is viewed by positioning the mouse cursor over a question mark (and waiting a second or so).

Discussion: General tips on usage will be first in the discussion section. Following this may be qualitative discussions of expected behaviors when changing the values of this feature.

Web Browsers: The RTUCC is tested and fully functional in the Mozilla Firefox, Microsoft Internet Explorer, and Google Chrome web browsers. Performance and display characteristics are best in Firefox. Starting with calculator version 4.3, WebKit-based browsers such as Google Chrome are supported.

At this time the Microsoft Edge browser is not fully recommended because of the following issue:

Submitting the Controls page (and saving the Results page): Click one of the "Submit" buttons or any of the gray divider rows to submit the Controls page to the calculation engine.

The Controls and Results pages can be saved as local web pages to record run parameters and performance results. Note that a parameter-summary table is included on the Results page (after the "Results" table and chart).

Discussion: To capture the details of an RTUCC run, do a "save-page-as" from the browser and then pick the "web-page, complete" option. This works well from the Firefox and Google Chrome browsers. Google Chrome does the best job capturing all formatting details. Tables, images (graphs), and everything will be captured. (Note that currently the MS Internet Explorer browser does not properly capture the Results page.) From the saved local page, or directly from the original page, you can drag your cursor over tables and paste them into Excel for additional analysis.

The Controls and Results pages can also be printed to multiple sheets of letter size paper. Print from the "File / Print" menu option and not the "File / Print preview" option.

Defaults: Use the "Restore" button to change all feature values back to the defaults shown in the far right column. If a feature has been changed from its default value, the background color of the corresponding cell in this column changes to a slightly darker shade. This color change is intended as an additional reminder to the user as to which features are actively being changed.

Most of the default values are static. However, some defaults are calculated (and updated) depending on the settings of other features. "Ventilation Rate" and the three "Power Inputs" features are examples of defaults that depend on other features.

The "Power" button is displayed next to the "Restore" button if the "Advanced Features" are turned on. The "Power" button acts to reset the power draw values. Clicking the "Power" button will cause the three power fields (evaporator, auxiliary, and condenser) to be set to default levels as calculated based on current settings of the "EER" and "Capacity" features. (See additional discussion in the help topic for the Power Inputs feature.)

Building Type: Select the building type which best represents your building. Each building type has an associated load model that is used to predict RTU loads and mechanical ventilation as a function of outdoor temperature. Each load model is derived from EnergyPlus analysis on a prototype ASHRAE building.

"Ventilation Rate" is calculated as the building type is changed. Manually entered values for "Ventilation Rate" will be overwritten whenever the building-type selection is changed.

To input custom load-model parameters, select the "Advanced Features" option, then edit the parameter fields below the "Building Type" feature. Refer to the help section on custom load models

If the load-line is locked, the building type (and two of the parameters in the custom-load model) will be locked (disabled). When locked, ventilation levels can be changed directly or by the ventilation parameter in the custom model.

Discussion: There are two supporting PDF documents that can be downloaded: (1) the description of the prototype buildings, see pages 8-13, and (2) a description of the corresponding EnergyPlus analysis done to develop the load models (see Appendix A in the User Manual).

Custom Load Model: To define a custom load model, edit the three parameters and then click the "Apply" button. This calculates new values for associated features and must be done before submitting the form. (The "Advanced Features" option must be selected to show the parameter fields for the custom load model.)

The parameter values for the custom model are initialized to the default parameters for the currently-selected building type.

When entering custom load-model parameters, the units are KBtu/HrF for the slope (S), KBtu/Hr for the intercept (I), and dimensionless for the ventilation-slope fraction (VSF). After editing these custom parameters, the "Apply" button must be used to calculate the corresponding new values for the "Ventilation Rate" parameter.

If the load-line is locked, two of the parameters of the custom-load model are locked (disabled) and cannot be changed. If locked, ventilation levels can be changed directly with the "Ventilation Rate" feature or with the ventilation parameter in the custom load model. The "Apply" button applies the ventilation levels set in the load model and overwrites any changes manually entered through the "Ventilation Rate" feature.

Discussion: A custom load model is defined to describe the approximate linear relationship between the sensible-cooling load on the HVAC system and the difference between the inside and outdoor temperatures. The model is characterized by three parameters: (1) a slope parameter accounting for conduction, ventilation, and infiltration through the building envelope, (2) an intercept parameter accounting for internal loads, and (3) a ventilation-slope fraction parameter that specifies how much of the first parameter is caused by ventilation. Refer to the help section on building types for additional information. Appendix A in the User Manual PDF further describes the concepts and analysis behind this approach.

The slope and intercept of the load model are scaled to produce a sensible load that is in balance with the sensible capacity of the RTU at design conditions. In a sense, the building is scaled (sized) to match the capacity of the cooling system. This scaling operation produces the final load-line that is processed in the binned analysis.

State/City: The "State" feature affects the lists of cities appearing in the "City" feature. The "State" and "City" features are used together to query weather data for this location. If your city is not listed, select the city whose climate is most similar yours (usually the closest city).

Discussion: The calculator uses weather data to conduct a binned energy analysis for the RTU in cities across the United States. Weather tape data (outdoor dry-bulb and coincident web-bulb) was binned in 5 degree increments and filtered by the selected occupancy schedule. The result is a database of hours (in each bin) and coincident wet-bulb temperatures for each city and schedule combination.

Schedule: This defines the time during which the building is assumed to be occupied. For the unoccupied periods outside of this schedule, the air conditioner is assumed to be shut down (no runtime) or the thermostat is set back. The system behavior during unoccupied periods is determined by the “Indoor Temperature” and the “E-Fan and Condenser” features.

Discussion: The "Schedule" feature filters the number of hours that can accumulate in a weather bin. Shorter occupant schedules translate into fewer potential hours where the cooling system can be subject to a load.

The hours outside of this schedule are considered to be unoccupied if the "Indoor Temperature" feature is set to "Cond. Off."

Indoor Temperature: This is the thermostat setpoint. The second feature, "Setback," acts to increase the cooling setpoint temperature during unoccupied hours. The cooling setpoint, during unoccupied hours, is calculated as the sum of these two features (base setpoint + setback = higher setpoint during the unoccupied hours). Additional information for this feature can be found in the topic for the E-Fan and Condenser feature.

This "Setback" feature acts to perform a double run. The load-line is established (locked) in the primary run (for occupied hours) at non-setback conditions. The same load-line is applied again in the secondary setback run for the unoccupied hours. The sum of the two runs is used to populate the energy categories in the "Results" table.

A special case is considered when "Setback" is set to "Cond. Off." In this case the condenser and the evaporator fan are not allowed to run during unoccupied hours and during these unoccupied hours only auxiliary energy is calculated (and totaled). In this case bin calculations are not displayed for the unoccupied hours.

For all normal setback cases, both condenser and fan energy (and aux energy) are calculated during unoccupied hours. Turning on the "Show Bin Calculations" option will display separate bin calculations for these unoccupied hours. This is in addition to (displayed after) the occupied-hours bin calculations. (Note: there will be a link above each of the two "Loads and Hours" plots for the candidate unit. Clicking this link will scroll/jump the page back and forth between the occupied and the unoccupied-hours results. Just clicking this twice will put this jump into the browser's page history and then you can use the Alt-arrow keys (hold Alt key down, then try the left and right arrow keys) to toggle back and forth. This can be a useful trick for visually comparing the occupied and unoccupied results.)

Note that for staged systems using normal setback, the evaporator fan will cycle with the compressor during the unoccupied hours. However if the special case of "Cond. Off" is selected, the condenser and the evaporator fan are always off during unoccupied hours.

The "Schedule" (Scheduled Hours) trace is shown in the "Loads and Hours" chart. This illustrates the occupied hours where the condenser is allowed to operate as established by the selected schedule. The "Total Hrs" trace shows all the hours (sum of occupied and unoccupied). Note that for normal setback, the "Schedule" trace in the "Loads and Hours" chart for unoccupied hours will represent the unoccupied hours (i.e. the hours where the condenser is active in the setback run for the unoccupied times).

Discussion: When interpreting the impact of changes to the thermostat "Indoor Temperature" setpoint feature, consideration must be given to whether the non-ventilation load-line is locked or not. If the load-line is locked, this feature will give the most intuitive results. However, in the default state (load-line un-locked), the impact of this feature may be less than expected. Consider the following example:

Indoor Temperature Candidate Unit (kWhrs)
75 10,798
70 12,807
70L (line locked @ 75) 14,670
Note: All other RTUCC parameters are at default values.

For the locked case (third row in table above), the primary driver behind the larger increase is that the non-ventilation load-line is not automatically adjusting to accommodate the more severe conditions at the lower setpoint (see discussion on the locking feature). So the effect of the original non-ventilation load-line (based on delta-T) is clearly increased by the lower setpoint.

In the contrasting un-locked case (second row), the load-line is defined to be in balance with the capacity (see methods page). So the change in setpoint has a less direct impact on the consumption. This is because the lower return temperatures adversely affect mixed-air conditions and the corresponding system performance. And to achieve balance between total sensible load and the reduced capacity, the total sensible load must be reduced. Therefore, the primary driver behind the increase in consumption, for this case, is not directly loads, but rather seen most strongly in two associated effects:

  1. Mixed-air conditions: The lower return temperature affects the load-line calculation (at design), the capacity (at bin conditions), and the power (at bin conditions) used while satisfying the load. The overall effect is an increase in power usage (see similar discussion of the un-locked case for the "Ventilation Rate" feature)
  2. More cooling bins: The lower setpoint essentially extends the cooling season. There are more bins where the cooling loads are satisfied by the economizer but still add to energy consumption because of fan power.

Note that the setpoint affects the mixed-air conditions (for both locked and un-locked cases). The lower return temperature adversely affects the sensible capacity which dominates the performance-correction effects (see the bin data for the overall correction factor (OCF)). The OCF factor is higher for the lower setpoint cases. Higher OCF factors correspond to higher consumption.

Indoor Relative Humidity: Uncheck the "auto" feature to specify a fixed humidity level. In "auto" mode the inside humidity reflects outdoor conditions. In "fixed" mode, the indoor relative humidity is assumed to be controlled to a fixed value.

Discussion: The "auto" mode models the interior humidity as tracking the outdoor conditions. Tracking is equivalent to assuming a moisture mass balance if moisture gains (from people and processes) equal moisture losses from condensate. This approximation is a simple way to model humidity conditions if no other information is known about moisture dynamics in the building. The tracking mode calculates the interior relative humidity and wet-bulb by assuming the moisture content of the interior air is equivalent to that of the exterior air on a humidity ratio basis (i.e., mass of water per mass of dry air is equivalent inside and out). The tracking mode is constrained to allow indoor humidity of values no less than 20% and no more that 65%. (The "auto" mode generally overestimates the interior humidity ratio. This assumption tends to overestimate associated latent loads. There is a discussion of this assumption in Section A.7 of the User Manual.)

Alternatively a "fixed" mode is used if the "auto" feature is de-selected. In "fixed" mode, the interior humidity ratio is assumed fixed (at the user selected level) and is not affected by the outside conditions.

If you know the typical humidity levels in your building during the summer, use the "fixed" mode (especially if you have an associated piece of equipment that controls the humidity). If not, use the "auto" mode. Note that the calculator does not determine the energy usage of any associated humidification or dehumidification devices. It strictly calculates energy estimates for the air conditioner.

The following reference discusses empirical validation of simplified methods for modeling humidity: Miller, J.D., "Development and Validation of a Moisture Mass Balance Model for Predicting Residential Cooling Energy Consumption," ASHRAE Transactions, Vol. 90, pt. 2, 1984, (275-293).

Note that a sensible load analysis best represents a thermostat's sensible nature and is key in supporting the RTUCC's calculations that are affected by humidity and latent loads. A sensible thermostat responds to temperature changes and sends requests to the unit to remove the sensible cooling load. The effects of humidity are accounted for in how the unit's sensible capacity is diminished by humidity in the entering air (mixture of outside and return).

The change to a sensible analysis (in revision 2.0), from the original total-energy analysis, changes the question the calculation answers from:

Total energy question
Given the unit's total capacity at the specified entering conditions, AND assuming that all sensible and latent loads are met by the unit, how much energy would it consume in satisfying the total load?

To this one:

Sensible energy question

Given the unit's available sensible capacity at the specified entering conditions, AND that the unit is controlled by a thermostat (a purely sensible device), how much energy would it consume in satisfying the sensible component of the total load?

Total Capacity: Select the AHRI net cooling capacity of the equipment being used (or equivalently, the cooling load of the building).

If a variable-speed system is selected using the "E-Fan and Condenser" feature, the "Number of Stages" feature is ignored. In this case all the bin calculations are reported in the first stage.

Discussion: This is the total (sensible and latent) net (including evaporator fan energy) cooling capacity at AHRI test conditions.

In all cases (unless the spreadsheet interface is used), the units are represented with this AHRI rated capacity, the EER, and DOE-2 correction factors.

Note that changing the capacity value automatically changes the default power values for the blower fan and the condenser.

Number of Stages: The number of stages (capacity levels) in the RTU.

This feature determines whether the unit will process the load with one or multiple stages. A multiple-stage system minimizes part-load cycling losses and evaporator-fan energy (see methods page on equipment response to loads).


If a variable-speed system is selected using the "E-Fan and Condenser" feature, the "Number of Stages" feature is ignored.

A 2-stage unit with a single-speed fan can be modeled by setting the "Number of Stages" feature to "2" and the "E-Fan and Condenser" feature to either of the "1-Spd" options.

Capacity levels for each "Number of Stages" setting are set as follows:

Stages Capacity Levels
1 1.0
2 0.5, 1.0
3 0.4, 0.6, 1.0
5 0.2, 0.4, 0.6, 0.8, 1.0
10 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0

Oversizing Factor: Percent that capacity exceeds load at design conditions.

Discussion: This factor serves to diminish the load so that the unit is effectively oversized at design conditions.

Blower-fan savings associated with variable-speed systems (see "E-Fan and Condenser" feature) are larger for oversized units.

S/T Ratio: Ratio of sensible to total capacity at AHRI test conditions.

Discussion: The S/T ratio (@ AHRI conditions) is used in estimating the sensible capacity of the unit. This along with the rated total capacity of the unit are the starting point for calculating sensible capacity and system power draw at conditions other than AHRI test conditions. See the methods page on sensible capacity and also the page that calculates a table of steady-state system performance values at conditions other than AHRI test conditions.

Candidate Unit: Energy Efficiency Ratio (EER), Cost (in k$), and Maintenance Cost (in $) for the candidate unit.

Note that resetting the EER automatically changes the default power values for the condenser.


Note that IEER (Integrated Energy Efficiency Ratio) values are not to be entered here. IEER values are a demographically and operationally weighted average of a set of four representative part-load EER values. An IEER projects a unit's performance to a national level. These weighted averages are intended to rank units on a national basis. While IEER values are very useful in sorting units nationally, they are not intended for evaluating a unit for a particular climate and operation strategy. However, the raw EER part-load data that goes into the IEER calculation can be very useful for characterizing a unit's performance under specific conditions.

(Refer also to the help section on the spreadsheet interface. The modeling of the full and part-load data is described there.)

The spreadsheet interface serves to adapt this supporting EER data (originally intended for the IEER calculations) for use in modeling work:

  1. A superset of the four points: The four points are expanded to a table of sixteen. These added points give sufficient data for regressing the data as a function of part-load fraction and outdoor dry-bulb.
  2. Separation of evaporator and condenser power draws: Removing the evaporator fan power from the EER data set allows for independent modeling of the condenser unit and the evaporator fan.

Specific Candidate Unit: Select from a list of three specific high-performance RTU systems.

Each of these three options corresponds to a commercially available rooftop unit or an add-on control system. The basic features are summarized below.

  1. Advanced Controls: This is a retrofit fan controller that minimizes fan usage for any RTU which has a single-speed fan.
  2. Three Stages: This is a high-performance RTU with three stages. Capacity levels are 40, 60, and 100 percent. This unit uses a three-speed evaporator fan.
  3. Variable-Speed Compressor: This is a high performance RTU which uses a combination of staging, variable-capacity condensers, and a variable-speed evaporator fan. This unit first uses its variable capacity to meets the lower loads; at higher loads the additional stages are used.


The setting of the Setback and E-Fan and Condenser calculator parameters significantly affects the assessment of the Advanced Control system. Please also refer to the help topics for these two calculator parameters.

The following outline lists the fan levels set by the Advanced Control system. For two-stage RTUs, the first-stage cooling runs the fan at 75% and second-stage cooling runs the fan at 90%. Single-stage RTUs address all calls for cooling by running the fan at 90%.

Selection of either the Three Stages or the Variable-Speed Compressor RTU options invokes corresponding sets of capacity and efficiency correction curves for that unit. The "Advanced Controls" option only affects the behavior of the evaporator fan and therefore uses the default corrections curves.

Note that selection of a specific unit acts to configure associated features for the candidate unit. For example, selecting the "Three Stages" option sets the "Number of Stages" feature to "3" and the "E-Fan and Condenser" feature to "N-Spd: Always ON." Specific performance curves are used in the calculation engine depending on the selection.

However, a selection here does not automatically set either the "EER," Total Capacity," or "Power Inputs" fields. It is necessary for the user to set these manually.

Standard Unit: Energy Efficiency Ratio (EER), Cost (in k$), and Maintenance Cost (in $) for the standard unit.

Discussion: See discussion under Candidate unit section.

Note that the "Standard Unit" cost features can be used to investigate the economics of replacing an existing unit (with indefinite life remaining). In this case, the standard unit is not new and represents an existing unit. For example, starting with default settings, set the standard (existing) unit to an EER of 8 and a cost of 1k$ (assumed cost for an anticipated compressor replacement). Adjust maintenance costs to reflect the need for more annual service on the existing unit.

Power Inputs: Specification of all the power inputs at AHRI rating conditions (in kWatts). This includes the evaporator fan (blower), auxiliary power (power needed for control electronics), and condenser unit (fan and compressor). 

These three fields are initialized to default levels based on the total capacity and EER of the unit. Changing these three values will cause the EER to be recalculated. A helpful editing pattern is to first edit the "EFn" and "Aux" fields, then re-enter the "EER" value. Clicking the "Power" button (top or bottom right) will cause the three fields to be set to default levels based on current EER and Capacity. Note that blower power can be determined from manufacturer's performance data as the difference between gross and net capacity when expressed in kWatts.

(Note that condenser-fan power can be specified with the Condenser Fan feature.)

The corresponding three categories of energy consumption are shown as columns in the bin-calcs table (the "Show Bin Calculations" option must be on). Their annual sums are shown in the "Results" table (the "Advanced Features" option must be on).

Condenser Fan: Condenser-fan power as a percentage of the total condenser power (at AHRI rating conditions).

This feature has an effect on energy calculations ONLY in the case of variable-capacity condensers. This feature is disabled if the "E-Fan and Condenser" feature is set to a non-variable-speed mode.

Discussion: This feature acts to split out the condenser-fan power for use in accounting for the reduced-power draw of variable-speed condenser fans in the calculation engine. It has impact ONLY in the case of a variable-speed system. Fan-affinity laws are used to estimate the reduced (from full-load) power consumption of the condenser fan. Full-load condenser-fan power is calculated as this feature's indicated percentage of the AHRI-rated condenser power (as specified in the "Cnd" field of the Power Inputs section).

E-Fan and Condenser: Characteristics of the evaporator fan and the condenser.

This feature is dual purpose in that it allows the user to select: (1) the fan type for single or multiple-stage systems or (2) a variable-speed system (i.e. variable-capacity condenser and a variable-speed evaporator fan).

The following options are available:

The "Cycles with Compressor" fan modes will cycle the blower fan with the compressor during both occupied and unoccupied hours. In this case the fan is completely off whenever the compressor is off. An exception to this occurs when the economizer is active. When economizing, the blower fan always runs at full speed.

The “…Always ON” fan-mode choices for staged units will cause the blower fan to be “…Always ON” during occupied hours, but will cycle the fan with the compressor during unoccupied hours. In other words, the "...Always ON" mode runs the fan at all times during occupied hours, but cycles during unoccupied hours. The fan is effectively in "Cycles With Compressor" mode during unoccupied hours.

The behavior of the system during unoccupied hours is affected by the "Setback" feature which determines if the condenser is allowed to run during that time. When the "Setback" feature is set to "Cond. Off," then DX cooling and the evaporator fan are kept off during the unoccupied hours. When the "Setback" feature is set to any value other than "Cond. Off," then DX cooling is allowed during unoccupied hours and at loads as determined by a reduced thermostat setpoint.

A special case where the condenser and the evaporator fan are OFF during unoccupied hours can be investigated by setting the "Setback" feature to "Cond. Off." This will set everything OFF during unoccupied hours independent of how the "E-Fan and Condenser" feature is set.

(For clarity, the following two paragraphs restate the information above from a different perspective: condenser ON or OFF.)

Blower-fan energy, when the condenser is OFF, is handled in three ways: (1) If a “Cycles…” fan mode is in effect, the blower fan is completely off when the condenser is off, (2) if a “…Always On” fan mode is in effect, the unit will reduce fan speeds if it can (e.g. an N-Spd or V-Spd fan) to levels that support ventilation, (3) whenever the economizer is active the fan runs at full speed.

Blower-fan energy when the condenser is ON depends on the type of the unit, but basically the fan fraction goes up and down with the capacity fraction for any unit except those with single-speed fans. The main exception to this is when the economizer is active; then the fan always runs at full speed.

The "N-Spd:" fan-mode choice is a valid option only when the unit has two or more stages. If the "Stages" feature is not set to "2" or higher, a warning message is displayed when the user attempts to "Submit" to the calculation engine.


A noteworthy special case is a system with two stages and a single-speed fan. A single-speed fan system runs at one speed (full speed) during both first-stage and second-stage compressor operation. This system, if there are two stages, is assumed to have a row-split evaporator design (i.e., essentially two evaporator coils in series, one after the other, subject to the same air flow from one blower fan). The row-split design yields high sensible (less latent) performance when only the first-stage is active as compared to when both stages are active. In first-stage mode, the coil will be warmer and therefore yield a more sensible output (high fan flow coupled with lower condenser cooling cause warmer supply temperatures). This higher sensible capacity in the first stage can be seen in the calculator by looking at the S/T values in the tables and charts for bins where the first stage is active without the second stage.

The following table illustrates the variation in evaporator (blower) fan energy usage as affected by parameters in the calculator: (1) economizer mode, (2) setback mode, (3) condenser mode, and (4) fan mode. The results shown here are for the candidate unit. All parameters not represented in the table are set to default levels. Fan energies ran from the high of 3,167 kWhrs, to the low of 397 for a variable-speed fan that adjusts to reduce fan speeds when additional flow is not needed.

Generally, the table shows reductions in fan energy for systems that can reduce fan speeds when full-flow is not needed. Also, the control option of allowing the fan to cycle with the compressor reduces the fan use and energy consumption. When the economizer is active, the fan savings of a "V-Spd" and "N-Spd" system are reduced (because the fan runs at full speed while economizing). The "1-Spd" fan consumes the most fan energy because it's always running at full speed.

It helps to consider a few specific comparisons in the table:

Evaporator Fan Energy (kWhrs)
Always On Cycles
Off Cond. Off 1-Stage 1-Spd 1,498 771
Off Cond. Off 2-Stage 1-Spd 1,498 1,030
Off Cond. Off 2-Stage N-Spd 552 484
Off Cond. Off V-Spd V-Spd 397 NA
Off 5 deg. 1-Stage 1-Spd 2,088 1,361
Off 5 deg. 2-Stage 1-Spd 2,347 1,879
Off 5 deg. 2-Stage N-Spd 794 726
Off 5 deg. V-Spd V-Spd 558 NA
On Cond. Off 1-Stage 1-Spd 1,498 1,229
On Cond. Off 2-Stage 1-Spd 1,498 1,417
On Cond. Off 2-Stage N-Spd 958 953
On Cond. Off V-Spd V-Spd 756 NA
On 5 deg. 1-Stage 1-Spd 3,167 2,898
On 5 deg. 2-Stage 1-Spd 3,283 3,202
On 5 deg. 2-Stage N-Spd 2,320 2,315
On 5 deg. V-Spd V-Spd 1,782 NA

Capacity unloading can be achieved by multiple stages or other variable capacity methods. Consider a two-stage unit with a two-speed fan. A two-speed fan system has a lower fan speed when running only the first stage. Based on fan-affinity laws, power consumption at the lower fan speed (1/2 the air flow of the high-speed mode) is assumed to be one eighth ((1/2)^3=1/8) of the high-speed power specified in "EFn" field on the Controls page. Note: this feature is only visible when the "Advanced Features" are turned on. When only ventilating, the two-speed system runs at its lower fan speed.

The variable-speed choice represents a system that has a variable-speed evaporator fan and a variable-capacity condenser. Variable capacity is achieved either though many stages or other capacity-unloading methods. Detailed part-load condenser-performance behavior can be represented through the spreadsheet interface. The spreadsheet data represents the condenser fan and the compressor at part-load conditions and can be used to account for condenser fans that can reduce their speed. When either of the variable-speed options is selected, the "Stages" feature is effectively disabled and all bin results are reported in the columns for the first stage. The cycling-degradation factor is set to zero when either of the two variable-speed modes is selected; it is assumed that the variable-speed unit does not cycle.

Both variable-speed and staged-condenser systems run the condenser and evaporator fan at capacity levels needed to satisfy the sensible load. At reduced load, capacity and fan air flow are reduced together, yielding different sensible-latent splitting (as determined by the apparatus dew-point and bypass factor method) of the total capacity. These systems are assumed to have either a row-split or interlaced evaporator design (serial air flow from one multi-speed fan).

Degradation Factor: Part-load degradation factor (percent).

This value controls how much the unit performance is degraded as a function of load fraction. At no load, performance is degraded by this percentage. At full load, the degradation is zero. A linear relationship is used for load fractions between these two extremes. 

N Affinity: N (exponent value) used in fan-affinity law calculations.

This value sets the exponent in the fan power calculations. For example, if the fan runs at one half of its flow capacity, the power it consumes will be (1/2)^N of its full-flow power draw. If N is 3, then at one half flow it will consume (1/2)^3 or 1/8 of its full-flow power.


An N of 3 is considered to represent the theoretical fan-affinity law behavior for an ideal fan and yields a high-end estimate of the fan energy savings associated with a multi-speed fan. At lower fan speeds this value is considered to overestimate fan savings. An N value of 2.5 is typically accepted as an exponent that best represents overall fan behavior.

An N of 3.0 was assumed for all RTUCC versions before 4.3.

Also see the discussion in the E-Fan and Condenser topic.

Demand Cost: Use these features to set the monthly demand rate ($/kW) and the number of applicable months.

The calculation engine keeps track of the peak power draw during the season. The demand charge is calculated as the simple product of this peak value and the rate and number of months values entered in the two fields. Demand costs are shown in the "Results" table if the "Advanced Features" option is on.

Spreadsheet Data: Detailed performance data from the specification spreadsheet.

The purpose of the spreadsheet data is to facilitate detailed modeling of the full-load and part-load performance of the RTU's condenser (compressor and condenser fan). The RTU's evaporator fan is explicitly modeled in the calculator and its part-load behavior is not characterized by the spreadsheet. Some characteristics of the RTU at rating conditions are imported from the spreadsheet and serve to populate some of the features in the calculator (e.g. S/T ratio, Power Inputs...).

Note: the sample spreadsheet referred to in this topic can be downloaded in a zip file which includes the DetailedPerformanceData_VSCD.xlsm spreadsheet. This spreadsheet contains Visual Basic (VB) code. To enable the VB code (sometimes referred to as macros) the user will usually need to click a button titled "Enable Editing" and then a button titled "Enable Content" at the top of the Excel interface.

Copy (select with a mouse drag or use control-a to select and then control-c to copy) all data from the "Data Summary" sheet of a detailed specification spreadsheet and paste (control-v) it in the "Spreadsheet Data" text box. Then click the check box next to the text box. This feature is a mechanism to import detailed performance data to the calculation engine of the web application.

The web application produces four regression models from the pasted data:

A sample spreadsheet is provided to illustrate and test the import feature. Right-click on the link and then choose to open or choose to save ("Save Link As...") the Excel file to your computer. Note that this spreadsheet is version marked to insure compatibility with the web application. Paste operations from older versions will not be accepted.

This sample spreadsheet has two "scratch" sheets that illustrate how supporting data be can be found in PDF format on manufacturers' web sites and used for this feature. One of the scratch sheets shows the raw images of the supporting tables (from a Lennox PDF). The other sheet shows intermediate tables that were assembled by pasting data from the PDF into the spreadsheet.

The "Part-Load Performance" sheet has detailed part-load performance data that serves to characterize the part-load behavior of the condenser. The upper table of data on this sheet corresponds to the part-load data that is used to determine a unit's IEER value. As a confirmation of this, the cells on the diagonal of the upper table are used to calculate the unit's IEER (the IEER result is shown on this sheet). For modeling work, the off-diagonal cells in the table are needed to characterize the part-load behavior as a function of load fraction and the outdoor dry-bulb. The lower table specifically represents the condenser unit and has the evaporator-fan power and evaporator-fan heat subtracted out of the EERs in the upper table. These special EER-Cond values are needed to represent the condenser's part-load behavior (including the condenser fan). The equation for calculating EER-Cond is given on the "Part-Load Performance" sheet. Only the YELLOW cells on the "Part-Load Performance" sheet feed into the regression models. As mentioned above, the part-load behavior of the evaporator's blower fan is accounted for separately with fan-affinity laws.

Note, however, that the "Part-Load Performance" data will almost never be available on a manufacturer's web site. In the future, fully populated spreadsheets, including the part-load data, may be available for download from the RTUCC site. These will be provided to us from manufacturers for a selected set of rooftop units. But for now, leave this data set to NA or try out the feature using the test data that is provided with the sample spreadsheet.

Steps for importing the spreadsheet data:

  1. Copy:
    Copy all the data on the "Data Summary" sheet. Using control-a is one way to select a whole sheet. Another is to click the little corner cell in the upper left, or simply drag with your cursor over all the active cells in the sheet. The part-load data (last four rows of the Data Summary sheet), may optionally be excluded if the manufacturer's part-load data is not available.
  2. Paste:
    Turn on the "Advanced Features" on the web site first. Paste this data into the appropriate "Spreadsheet Data" cell on the Controls page. There is one spreadsheet cell for the Candidate unit and one for the Standard unit. After pasting, the cell may appear empty (blank); not until after step 3 will there be visible text in the cell (so don't double paste). Note: to remove data from a "Spreadsheet Data" cell, put the cursor in the cell, then use control-a (to select all the content in the cell), then use control-x (to clear it).
  3. Click the check box (just to the right of the spreadsheet field):
    This sets features to reflect the corresponding values in the spreadsheet (e.g. EER, net capacity, evaporator fan power, aux power, condenser power, and S/T ratio). Note, after pasting spreadsheet data, the check box is checked automatically if you click anywhere (outside the spreadsheet cell) on the page. After clicking the check box, the model number of the unit will be displayed in the spreadsheet cell.
  4. Submit:
    Click the "Submit" button. On the report page, you'll see the raw sheet contents parsed and reconstituted by the engine and displayed. There will also be a table for each of the four regression models that have been developed (the table indicates the model form and the model coefficients and their corresponding T values). If data is insufficient for modeling, a warning message will be displayed where the regression model table would normally be displayed.

After pasting in the data, the cell background color for the "S/T Ratio" and the "Degradation Factor" features will be a light yellow. This is a warning that these features are effectively disabled if the modeling process succeeds. The corresponding S/T model and degradation model are absolute and do not use these values. It is best to think of the values for these features as backups that will be used if the corresponding part of the modeling process fails (or if insufficient data is provided). The report page will indicate which of the models have succeeded. Again, there will be corresponding warning messages for those that fail.

All the other features on the Controls page are still active and can be changed by the user. Note that by unchecking the check box (also see step 3 above), you are then free to edit the nominal values for any of the parameters brought in by the spreadsheet (i.e., EER, net capacity, blower power, aux power, and condenser power). Re-checking the check box then brings back the nominal spreadsheet values and displays them on the Control page. Note that the "Power" (top right) button (see help for the power fields) will act to uncheck this check box and set the the power fields to their default levels; re-checking the check box will again display the values from the spreadsheet.

The regression models will be in effect as long as there is spreadsheet data in the spreadsheet field (regardless of the check box state). To remove data from a "Spreadsheet Data" field, put the cursor in the field, then use control-a (to select all the content in the field), then use control-x (to clear it).

Ventilation Rate: Specify in CFM or as a percent of the fan capacity. Remaining fan capacity (not used in ventilation) is available for use by the economizer.

Ventilation rates are calculated automatically when a building type is selected. Ventilation rates are calculated so as to produce the same fractional contribution to the slope of load-line as was determined in a corresponding EnergyPlus analysis of these building types (see the Building Type feature). Updating any feature that affects this calculation will cause the ventilation rate to be updated. Manually entered values are allowed but will only persist until a ventilation-rate update is triggered by changing the value of a related feature (for example the city/state).

An alternate method for establishing the ventilation level is by adjusting the ventilation fraction in a custom building-load model (see the Building Type feature).

When the Ventilation Rate feature is in the "% of fan capacity" mode, the ventilation parameter can be set anywhere between 0% (entering mixed air is composed completely of return air) to 100% (entering mixed air is composed completely of outside air) of the fan's capacity. This parameter range is equivalent to changing the fresh air damper from fully closed (0%) to fully open (100%).

When in the "CFM" mode, values above 100% of fan capacity may be allowed. CFM values in excess of primary fan capacity are assumed to be supported by a secondary fan.

For both the "% of fan capacity" and the "CFM" modes, ventilation values are not allowed to exceed levels where the non-ventilation load would be less than the internal load (i.e., a negative conduction load is not allowed; see Methods page). An error message is reported to the user in this case. Another way to look at this issue is to remember that the design load is calculated to match the design capacity and that as ventilation is increased the non-ventilation loads must decrease to maintain the match. However, ventilation levels are not allowed beyond the point where the non-ventilation load model would have a negative slope (an envelope better than a perfect insulator).


Effect of location (city/state) on calculated ventilation levels:

You may notice that the calculated ventilation rates change as the city and state change; the milder the climate (i.e., cooler summer design temperature) the higher the ventilation rate. The milder climate causes the unit to have a larger effective capacity (better coil performance) at the cooler design conditions, and because our building auto-sizes to balance loads with capacity at design conditions, this mean higher loads from all contributing components, including ventilation. Our building model assumes that ventilation contributes a certain fraction of the slope in the load model. This fractional contribution from ventilation is only dependent on building type and does not change with location. However, even though the fraction stays constant, the actual ventilation CFM will increase as the design loads are increased for milder climates. You can think of this as a larger building (to match the higher capacity) requiring more ventilation because it has more square footage.The ventilation portion of the load is specified for each load model (see Appendix A in the User Manual) and is also apparent in the "loads-and-hours" bin-calc plots as the difference between the "total" and "non-ventilation" lines.

Impact of ventilation changes as influenced by competing effects:

As intuitively expected, increasing ventilation levels generally causes higher consumption (and higher savings) to be reported by the calculator. But there are several competing effects that can cause unexpected changes in consumption as ventilation increases. Settings of associated parameters in the RTUCC determine the relative impact of these competing effects. The following outline discusses these associated parameters from within the context of a locked or unlocked load-line:

Ventilation rate
Candidate Unit (kWhrs)
1 24% 10,798
2 50% 11,654
3 50% (locked @ 24%) 10,777
4 100% (locked @ 24%) 10,954
200% (locked @ 24%) 12,758
Note: All RTUCC parameters are at default values.

The table above contrasts the savings as affected by changes in ventilation levels (all other settings are at default values). The first row is the base case of 24% ventilation flow. The second row shows the results when the ventilation feature is manually set to 50%. The third row shows the result when the RTUCC starts at a ventilation rate of 24%, then locks the non-ventilation load-line, then changes the ventilation to 50%. A comparison of rows 1 and 3 illustrates that an expected increase in consumption can essentially be nullified if the ventilation is increased when the load-line is locked (see explanation below in the "locked" section). Higher ventilation rates (rows 4 and 5) show an expected increase in consumption.

If non-ventilation load-line is un-locked

If non-ventilation load-line is locked

Economizer: Allows economizer to assist in meeting loads.

Discussion: The economizer feature enables the calculator to simulate the use of an economizer. The economizer is simulated by effectively increasing the ventilation rate from the specified rate to 100% of fan capacity. If enabled, economizer is activated in any weather bin where increasing the ventilation acts to decrease the air conditioning load.

Electric Utility Rate: Enter your local electric rate. If you do not know the rate, you can find it on your bill or call the local utility provider.

Equipment Life: Number of years of life before equipment is replaced.

Number of Units: This is a simple multiplication factor used in calculating costs for a group of units.

Chart discounted costs: If checked, discounted purchase and operating costs are calculated (the present value). If unchecked, undiscounted costs are used (simple payback).

The discount rate is the rate ABOVE inflation (not the nominal discount rate that INCLUDES inflation).

Discussion: The discount rate is an interest rate that is adjusted to remove the effects of actual or expected inflation. Specifically, it is an interest rate that is used to calculate the present value of expected yearly costs excluding the effects of inflation.

Note that the discount rate in the calculator is not a nominal discount rate. Nominal discount rates include the effects of inflation. The discount rate is always lower than the corresponding nominal discount rate.

Show bin calculations: Show the detailed calculations for each outside dry-bulb bin.

Discussion: Detailed results include engineering calculations on system loads and performance, tabulated and charted by outside dry-bulb bin temperature.

Lock load-line: Preserves the current non-ventilation load-line for use in any submits that follow.

Locking the non-ventilation load-line is equivalent to fixing the characteristics of a fictitious building that is the source of the cooling loads. Locking causes loads that are associated with internal gains to be fixed. Loads associated with conduction through the envelope are still driven by weather conditions, but the relationship to the temperature differential is fixed (slope of the load-line is fixed). Ventilation loads can still be adjusted after locking either by directly editing the ventilation value or by changing the building type or the ventilation parameter of a custom load model.

Discussion: In the default (unlocked) mode, loads are calculated so as to balance the unit's capacity at design conditions. Total sensible loads automatically adjust to balance capacity (also see the Methods page on the non-ventilation load for additional discussion). This principal assumption in the calculator can lead to counterintuitive (but correct) results in some of the features. For example, starting from the humidity feature's default "fixed mode" (not auto mode) value of 60% relative humidity, and then changing to 40%, results in a significant drop in consumption if the load-line is locked (when at the original value of 60% humidity). A strong drop results when the load-line is locked because sensible non-ventilation loads stay fixed as sensible capacity increases at lower humidity levels. The result is lower run times.

Inside Relative Humidity (% R.H.) Candidate Unit (kWhrs)
60 11,074
40 (line locked @ 60) 9,932
40 11,190
Note: All RTUCC parameters except "Fixed" inside relative humidity are at default values.

Locking the non-ventilation load-line can be useful in testing the sensitivities to a parameter. Locking changes the "question" that is presented to the calculator. Consider, for example, the ventilation level parameter:
  1. If locked the question is: For fixed building characteristics, what is the energy impact of changing the ventilation level from:
    1. an initial state where loads and capacities balance at design conditions,
    2. to a new state (different ventilation) where total loads are free to be higher or lower (non-ventilation load is locked) than capacity at design?
  2. If not locked the question is: For an "adjustable" building that can adapt to be in balance with loads, what is the energy impact of changing the ventilation level from:
    1. an initial state where loads and capacities balance at design conditions,
    2. to a new state (different ventilation) where loads and capacity are again constrained to match and where the matching is again achieved by adjusting the non-ventilation load (the conduction component of the "adjustable" building)?

Or more simply put, once in locked mode, the non-ventilation building characteristics are fixed. When not locked, the "adjustable" building adjusts (conduction loads) to achieve balance between the specified ventilation load and the capacity of the unit.

Note: when using the locking feature, it may be best to use some "oversizing" to allow for extra capacity to accommodate load changes. The oversizing feature can help to minimize the number of bins where runtimes exceed 1.0. When locked, runtimes exceeding 1.0 are highlighted with light yellow background.

APD/BPF: Demonstration Page for the Apparatus Dew Point and ByPass Factor Method (Carrier et al. 1959)

Discussion: The first row of features serves to establish the capacity, flow rate, and sensible to total ratio (S/T) at AHRI rating conditions. These can be used to calculate the corresponding bypass factor at AHRI conditions. The bypass factor can be projected onto other mass flow rates using the expression BF = exp(-A0/massflowrate). A0 can be determined at AHRI conditions and then the expression can be applied at other mass flow rates. The second row of features, capacity and air flow rate, establish a baseline for a projection exercise as illustrated in the output table below the features. The table displays S/T results for a variety of operating conditions, all at the capacity and flow rate established in the second line of features, all dependent on the A0 term established by the first row of features. By default the second line is set to equal the first line whenever a first-line feature is changed.

Advanced features: Unhide advanced calculator features and initialize them to their default values.

Advanced features are indicated by a tan background color in the features title cell.


Advanced features were new with the second release of the calculator. The new features in that release and those after are listed in the calculator's revision history.