Controls Help

This help page is a collection of context sensitive help topics that can be called by clicking on the question marks next to the title of any of the controls on the data entry page. Clicking on a question mark both opens the help window and positions the relevant topic at the top of the help window. It is not necessary to close the help window before clicking on a different question mark. Additional question-mark clicks automatically refresh the help window to display the relevant topic.

Help topics are outlined as follows:

Name: Following the control name, is the mouse-over pop-up tip. This is identical in content to the the pop-up tip that can be viewed by positioning the cursor over a question mark and waiting (a second or so) for the pop-up. Note that presenting the pop-up tip content here can be especially useful to users of the Netscape browser (pop-up tip help is not implemented in Netscape).

Discussion: The discussion first includes general tips on usage. Following this may be qualitative discussion on behaviors to be expected (in the results) when changing the selected values of this control.

Printing and Saving Results: The controls and results pages can be printed or saved as local web pages to record run parameters and performance results.

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 capture 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 control values back to the defaults shown in the far right column. If a control 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 controls are actively being changed.

Most of the default values are static. However, some defaults are calculated (and updated) depending on the settings of other controls. The "S&I Fraction", "Ventilation Rate", and the three power controls are examples of defaults that depend on other controls.

The "Power" button is displayed next to the "Restore" button if the "Advanced Controls" 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" controls. (See additional discussion in the help topic for the "Fan/Aux/Condenser (kW)" control.)

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.

Several parameters on this input form are calculated as the building type is changed (e.g. "S&I Fraction" and "Ventilation Rate"). This means that hand entered values for S&I Fraction and Ventilation Rate will be overwritten whenever the building type selection is changed.

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 develope the load models.

To input custom load-model parameters, make sure "Advanced Controls" are checked, then select the "--SET BY USER--" building type. When entering custom model parameters, note that units are KBtu/HrF for the slope (S), KBtu/Hr for the intercept (I), and dimensionless for the ventilation slope fraction (VSF).

State/City: The state control affects the lists of cities appearing in the cities control. The state and city controls 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 Comparison 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: The air conditioner is assumed to be shut down (no run time) during hours outside of this schedule.

Discussion: The schedule control filters the number 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.

Indoor Temperature: This is the thermostat setpoint. The second control, "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 controls (base setpoint + setback = higher setpoint during the unoccupied hours). The special case of "Cond. Off" prevents the condenser from running during unoccupied hours. In this case only the evaporator fan is allowed to run (as dictated by the "Fan and Compressor" control).

This set-back 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.

When "Setback" is set to "Cond. Off" (the default), the condenser is not allowed to run during unoccupied hours. In this case, during unoccupied hours, only evaporator fan energy (and aux energy) is calculated.

For all non-zero 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: when setback is turned on, 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 useful trick for visually comparing the occupied and unoccupied results.)

Note that when the setback feature is active (non-zero), single-speed fan systems will run in the "Cycles With Compressor" mode during the unoccupied hours. This is true regardless of which of the three single-speed options is selected in the "Fan and Compressor" control.

The "ActCond Hrs" (Active Condenser 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 when a non-zero setback is chosen, the "ActCond" trace in the "Loads and Hours" chart (for unoccupied hours) will represents the unoccupied hours (i.e. the hours where the condenser is active in the secondary run).

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

Indoor Temperature Candidate Unit (kWhrs)
75 8,342
70 9,229
70L (line locked @ 75) 10,463
Note: All other RTU 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 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 affects:

  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 control)

  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 unit performance correction effects (see chart of 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: In auto mode the indoor humidity ratio is equal to the outside humidity ratio. 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 on the last page of this pdf document on the building load models.)

Alternatively a "fixed" mode is used if the "auto" control 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 RTU Comparison Calculator does not calculate 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).

Solar and Internal Gains: Specified as fraction of sensible capacity at design conditions.

The S&I fraction is considered primarily a calculated value as affected by the building type and city selection. Manual changes to this factor are allowed but not recommended in version 4.1 of the RTU CC. Manual changes support comparisons with version 4.0 of the RTU CC. Instead, please use the "user defined" building type to establish a custom load line. Also note that manual changes to this factor persist only until this form is updated. An update will cause a calculated value to replace the manual entry.

Discussion: The Solar and Internal Gains control allows for qualitative characterization of the buildings sensitivity to internal cooling loads (all loads not driven by outside temperature). The S&I loads include all cooling loads except ventilation and conduction. This includes loads such as occupancy related internal loads (waste heat from lighting and plug loads) and also solar loads. These S&I loads are assumed to be independent of outside conditions and constant through out the day.

Increasing the S&I fraction results in a less steep load line (less sensitive to outside temperature) and decreasing the S&I fraction makes for a steeper load line (more sensitivity to outside temperature). Setting the S&I load fraction to zero is equivalent to modeling a building with no internal gains or windows (e.g., air conditioned storage building). With "Show bin calculations" selected, notice that increasing the S&I factor essentially lengthens the cooling season (more cooling bins are proceeded).

Note that the S&I control has no effect if the non-ventilation load line is locked.

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

The "stages" control determines whether the unit will process the load with one or two stages. A two stage system minimizes part-load cycling losses (see methods page on equipment response to loads).

If a variable-speed system is selected using the "Fan and Compressor" control, the staging control 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 ARI test conditions.

In all cases (unless the spreadsheet interface is used), the units are represented with this ARI 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.

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 "Fan and Compressor" control), are larger for oversized units.

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

Discussion: The S/T ratio (@ ARI 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 ARI 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 ARI 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 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 units performance to a national level. These weighted averages are intended to rank units on a national basis. While an IEER is 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 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 drybulb.
  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.

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's cost controls 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.

Evaporator, Auxiliary, and Condenser: Specification of all the power inputs in kWatts. This includes: the evaporator fan (blower), condenser unit (fan and compressor), and auxiliary power (power needed for control electronics).

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. 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.

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 controls" option must be on).

Control Strategies: Control modes for the fan and compressor.

Select a control mode for a Single Speed (1-Spd), Two Speed (2-Spd), or Variable Speed (V-Spd) system.

A single-speed fan system runs at one speed during both single-stage and two-stage compressor operation. This system, if there are two stages, is assumed to have a face-split evaporator design (i.e., essentially two side-by-side evaporator coils subject to parallel air flow from one blower fan). The face-split design yields first-stage dehumidification performance equivalent to that when both stages are active.

The two-speed fan choice is a valid option only when the unit has two stages. A two-speed fan system has a lower fan speed when running only the first stage. This system is assumed to have either a row-split or interlaced evaporator design (serial air flow from one two-speed fan). 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 eight ((1/2)^3=1/8) of the high-speed power specified in BFn field on the controls page. Note: this control is only visible when advanced controls 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 partload condenser-performance behavior can be represented through the spreadsheet interface. The spreadsheet data represents the condenser fan and the compressor at partload conditions. The spreadsheet can be used to account for condenser fans that reduce speed under partload conditions. When either of the variable-speed options is selected, the "stages" control 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.

The variable-speed choice assumes the system runs both the evaporator fan and compressor at capacities proportional to the load. At reduced load, capacity and fan air flow are reduced together, in such a way that dehumidification performance is preserved and approximately equal to that at full-load operation. This system is assumed to have either a row-split or interlaced evaporator design (serial air flow from one multi-speed fan). Capacity unloading can be achieved by multiple stages or other variable capacity methods. As with the two-speed choice, fan affinity laws are used to model the reduction in fan power at reduced flow rates (e.g. at 1/3 load, the blower fan is assumed to draw (1/3)^3 = 1/27th of the full-load blower-fan power).

"Always ON" runs the fan at all times (all day, all week), regardless of the occupant schedule. "Cycles With Compressor" runs the fan only when the compressor is running. "OFF When Unoccupied" runs the fan at all times when the building is occupied.

If "V-Spd: Always ON" is selected, a minimum fan speed is set based on the ventilation control. If the ventilation control is set for 10%, then the fan will run at 10% of capacity when there is no call for cooling.

Note that when the setback feature is active (non-zero), single-speed fan systems will run in the "Cycles With Compressor" mode during the unoccupied hours. This is true regardless of which of the three single-speed options is selected in the "Fan and Compressor Controls" control.

The following control modes are available:
1-Spd: Always ON
1-Spd: OFF When Unoccupied (i.e., always ON when occupied)
1-Spd: Cycles With Compressor
2-Spd: Always ON
2-Spd: OFF When Unoccupied (i.e., always ON when occupied)
2-Spd: Cycles With Compressor
V-Spd: Always ON
V-Spd: OFF When Unoccupied

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.

Demand Cost Controls: Monthly demand rate ($/kW) and the number of months that this rate applies.

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 controls" option is on.

Spreadsheet Data: Detailed performance data from the specification spreadsheet.

Copy (select with a mouse drag or control-a, 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 units IEER value. As a confirmation of this, the cells on the diagonal of the upper table are used to calculate the units 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 drybulb. 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 manufacture's web site. In the future, fully populated spreadsheets, including the part-load data, will be available for download from the RTUCC site. These will be provided to us from manufactures 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. 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 manufacture's part-load data is not available.
  2. Paste:
    Turn on the Advanced controls 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 type control-a (to select all the content in the cell), then control-x (to clear it).
  3. Click the check box (just to the right of the spreadsheet field):
    This sets controls 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" controls will be a light yellow. This is a warning that these controls 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's best to think of the values for these controls 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 succeed. Again, there will be corresponding warning messages for those that fail.

All the other controls 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 type control-a (to select all the content in the field), then 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 control). Updating any control 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 control (for example the city/state).

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

When the Ventilation Rate control 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 complete 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 levels are not allowed to exceed levels where the available (after ventilation is subtracted) sensible capacity would be less than the S&I load (or in other words, a negative conduction component is not allowed; see Methods page). An error message is reported to the user in this case:

For the specified ventilation rate and S&I fraction, available sensible capacity... at design is less than the S&I load. This is a non-physical (equivalent to requesting that the building be better than a perfect insulator, i.e., a non-ventilation line with a negative slope) situation that can be resolved by decreasing the S&I fraction and/or decreasing the ventilation rate.


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 the higher the ventilation rate. The milder climate causes the unit to have a larger effective capacity at 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. (This ventilation piece of the load pie, can be observed in the definition of the load models, and also in the "loads-and-hours" bin-calc plots.) However, even though the fraction stays constant, the actual ventilation cfm will increase as the loads increase in milder climates. You can think of this as a larger building (milder climate) requiring more ventilation because it has more square footage.

Impact of ventilation changes as affected by competing effects:

As intuitively expected, increasing ventilation levels generally causes higher consumption (and higher savings) to be reported in the RTU Comparison Calculator. But there are several competing effects that can cause lower than expected rises in consumption as ventilation increases. Settings of associated parameters in the RTU, 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
Savings (kWhrs)
24% 1,740
50% (unlocked) 1,870
50% (locked @ 24%) 1,925
Note: All RTU parameters are at default values.

The table above contrasts the savings as affected by changes in ventilation levels. This illustrates that impact is stronger if the increase in ventilation is done when the load line is locked. First row is the base case of 24% ventilation flow. Second row shows the results when the ventilation control is manually set to 50%. 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%.

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 Comparison 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.

Discount Rate: 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 RTU Comparison 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.

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 displayed (the present value). If unchecked, undiscounted costs are displayed.

Show bin calculations: Show the detailed calcs 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: Preserve the non-ventilation load line (determined by parameters below) for use in following submits.

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 Methods pages for discussion of the conduction component). This principle assumption in the RTU Comparison Calculator can lead to unintuitive (but correct) results in some of the controls. For example, starting from the humidity control's default "fixed mode" (not auto mode) value of 60% relative humidity, and then changing to 40%, results in a significantly stronger change in consumption if the load line is locked when at the original value of 60% humidity. A stronger change 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 8,407
40 (line locked @ 60) 7,682
40 8,324
Note: All RTUCC parameters, except "Fixed" inside relative humidity, are at default values.

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 and conduction through the envelope to be fixed. Ventilation loads can still be adjusted after locking, but changes to the Solar and Internal (S&I) gain control will have no effect once the load line is locked.

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 RTU Comparison 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 controls serve to establish the capacity, flow rate, and sensible to total ratio (S/T) at ARI rating conditions. These can be used to calculate the corresponding bypass factor at ARI conditions. The bypass factor can be projected onto other mass flow rates using the expression BF = exp(-A0/massflowrate). A0 can be determined at ARI conditions and then the expression can be applied at other mass flow rates. The second row of controls, capacity and air flow rate, establish a baseline for a projection exercise as illustrated in the output table below the controls. 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 controls, all dependent on the A0 term established by the first row of controls. By default the second line is set to equal the first line when ever a first-line control is changed.

Advanced controls: Unhide advanced controls (and initialize them to their default values).

Discussion: Advanced controls were new with the second release of the RTU Comparison Calculator. The new features in revision 2.0 and releases after, are listed below: