The HVAC Sizing Preferences dialog lets you set a single, reusable set of design-oriented overrides that apply during system-level sizing, without disturbing the settings you have authored in your HVAC systems for dynamic simulation. It gives you one place to switch between your design intent and your simulation intent, rather than editing many individual controls each time. The preferences apply only to the ApacheHVAC System Loads and Sizing run, invoked from the ASHRAE Loads dialog. They influence only the sizing of systems and equipment and do not directly affect the dynamic simulation.
The dialog has two tabs. The Preferences tab holds a set of yes/no options for parameters such as economizer bypass, energy recovery bypass, supply air and supply water temperature reset, and fan motor heat. The Oversizing Factors tab holds eight global oversizing factors. Each tab has its own Enable checkbox, and both are unchecked by default – the preferences are opt-in.
The HVAC Sizing Preferences dialog can be opened from three locations:
1. Loads Calculations and Equipment Sizing Navigator
2. Global HVAC Overrides
3. ASHRAE Loads (With Analysis type set to ApHVAC System loads and sizing)
HVAC Sizing Preferences are saved to the registry in the same way as other preferences. They persist per user, system wide, across all models on that installation of the VE, until you choose to revise them. They are not saved to any individual model. The preferences do, however, drive the setting of the existing sizing-specific parameters and oversizing factors within the ASP file during sizing calculations.
All options on this tab are unticked by default, with two exceptions that match the existing default state of the System Parameters interface: Bypass outside air Economizer and Bypass energy recovery pre-conditioning of outside air are ticked by default. These two are shown as ticked the first time you open the dialog after installing VE2026. Thereafter your settings persist in the registry and are not reset between installations.
Tick this box to bypass the outside air economizer on all systems during the System Loads and Sizing calculation. The economizer operation is unchanged for dynamic simulation. Ticked by default. This may be relevant when internal gain loads peak during an economizer period causing a peak system size during economizer operation. This will remove the assumption that weather conditions meet economizer enable logic and provide system sizing without the assistance of economizer.
Tick this box to bypass energy recovery pre-conditioning of outside air on all systems during the System Loads and Sizing calculation. The airside energy recovery setting is unchanged for dynamic simulation. Ticked by default. Energy recovery often assists cooling and heating load on coils and engineers may want to calculate heating capacity assuming the energy recover is non-existent should it break during peak hours or be value-engineered out of the project.
Tick this box to disable the System SAT reset during sizing calculation, so that the system holds its design (lowest) supply air temperature whenever the cooling coil is active. Your input value is kept for dynamic simulation. Unticked by default.
Tick this box to set the Motor airstream heat pickup factor to 0.00 % for all fans that are not within a multiplex, for the sizing calculation. Your input value is kept for dynamic simulation. Unticked by default. In IESVE Software, the fan heat is important for dynamic simulation, however, often absorbed by the equipment selection. The coil capacity input for a load selection does not generally account for fan heat as it varies per system selection, thus disabling fan heat will provide result in coil capacity outputs most generally used to input into system selection software. For the equivalent zone-fan option see the Zone preferences section.
Tick this box to adjust ventilation airflow for the density of outside air at the heating and cooling design conditions during the ASHRAE Heating and Cooling System Loads and Sizing runs. This is an override: it is applied only during System Sizing and does not change the authored system. Unticked by default. When the box is unticked, adjustment factors of 1.0 are used, giving results identical to the current baseline.
Separate adjustment factors are derived for the heating and cooling design conditions, each as the ratio of the derived air density to
the standard air density.
The Reference air density in the Site Data section of the ApLocate dialog is always used as the standard air density in the denominator. The derived air density in the numerator is calculated from the design weather data on the ApLocate Design Weather Data tab, using the same method already used elsewhere in the VE. For heating, a design outdoor relative humidity of 50 % is assumed, as only the dry-bulb temperature is given; for cooling, the design outdoor relative humidity is derived from the dry-bulb and wet-bulb temperatures.
Where the Derived radio button is selected in the Site Data section of the ApLocate dialog, and only then, any value in the Elevation box is also accounted for in the derived air density, so that the adjustment factor is consistent with the reference air density Apache uses throughout the model.
The factor is applied to the outside air path appropriate to the system. For a mixing system, which has an outside air damper set, it is applied to the outside air damper Minimum Flow. For a dedicated outside air system, which has no outside air damper set, it is applied to the zone-level ventilation airflow controllers. Whether a system is treated as a dedicated outside air system is determined by the ‘100% outside air system’ checkbox on the System Parameters tab of the System Parameters dialog.
Room and zone loads calculations and normal dynamic simulation are unchanged. With the option enabled, sizing more accurately represents actual loads by using corrected ventilation airflow volumes. For the underlying method see the ApLocate documentation.
Tick this box to disable a Cooling Thermostat Setback assigned in the room data for the sizing calculation. Unticked by default. This may be useful when the engineer is unsure of a setback period. However, be cautious when sizing with this preference enabled as the instant change of a room setback sometime causes a peak load due to the instant change in dT at the time of going from setback to setpoint; this is more common for sizing calculations for heating but sometimes occurs in cooling for buildings with high thermal mass.
Tick this box to disable a Heating Thermostat Setback assigned in the room data for the sizing calculation. Unticked by default. This may be useful when the engineer is unsure of a setback period. However, be cautious when sizing with this preference enabled as the instant change of a room setback sometime causes a peak load due to the instant change in dT at the time of going from setback to setpoint; generally peak heating occurs during a morning warm-up period, though ASHRAE heating load calculation is not an hourly calculation so this often goes unrealized and the reason ASHRAE recommends a 25% heating oversizing factor.
Tick this box to set the Motor airstream heat pickup factor to 0.00% for all fans located within a multiplex, for the sizing calculation. Your input value is kept for dynamic simulation. This is an override: it is applied only during System Sizing and does not change the authored system. Unticked by default. At times, engineers may want to disable the system fan motor heat and leave the zone fan motor heat as enabled to ensure that zone heat which is often reabsorbed by the return air is accounted for at the system. For a multiple zone air handling unit the zone level fan heat may be considered a load on the system depending on the circumstance and the engineer's preference.
Tick this box to size every advanced cooling coil on a chilled water loop against the loop’s warmest reset supply water temperature during the sizing calculation. The supply water temperature is unchanged for dynamic simulation. Unticked by default.
Tick this box to constrain the capacity of every chilled water loop to the sum of the capacities of the cooling equipment that operates in the maximum part-load range for that loop. The loop then carries no spare capacity for equipment modelled as operating only in lesser load ranges. Unticked by default. To make an exception for a single loop, disable the preferences and edit that loop directly.
Tick this box to set the chilled water supply temperature reset type to No reset for both the primary and secondary circuits during the sizing calculation. Your setting is kept for dynamic simulation. Unticked by default.
Tick this box to size every advanced heating coil on a hot water loop against the loop’s coolest reset supply water temperature during the sizing calculation. The supply water temperature is unchanged for dynamic simulation. Unticked by default.
Tick this box to constrain the capacity of every hot water loop to the sum of the capacities of the heating equipment that operates in the maximum part-load range for that loop. The loop then carries no spare capacity for equipment modelled as operating only in lesser load ranges. Unticked by default. To make an exception for a single loop, disable the preferences and edit that loop directly.
Tick this box to set the supply temperature setpoint to Constant for all hot water loops, primary and secondary, during the sizing calculation. Your input value is kept for dynamic simulation. Unticked by default.
Tick this box to set the Heat Transfer Loop heating supply water temperature setpoint to Constant during the sizing calculation. Your input value is kept for dynamic simulation. Unticked by default.
Tick this box to set the Heat Transfer Loop cooling supply water temperature setpoint to Constant during the sizing calculation. Your input value is kept for dynamic simulation. Unticked by default.
Tick this box to apply the factors on this tab. When ticked, the value you enter for each factor overrides and populates the corresponding local factor, and those local factors become uneditable and are shown as black text on a grey background matching the values entered here. When unticked (the default) the local factors return to their normal editable state; any values previously applied as global overrides persist as local factors until you edit them. Factors are entered as percentages above the stated load or airflow.
The oversizing applied to the space peak cooling load. This oversizing factor directly influences the airflow calculated for each space.
The oversizing applied to the space peak heating load. This oversizing factor directly influences the airflow calculated for each space.
The oversizing applied to the coincident peak cooling coil load. Populates the corresponding factor in the Advanced Cooling Coil and
Simple Cooling Coil dialogs.
The oversizing applied to the coincident peak heating coil load. Populates the corresponding factor in the Advanced Heating Coil
and Simple Heating Coil dialogs.
The oversizing applied to the fan peak airflow. Care should be taken not to cause a duplicate oversizing factor between this override and the % above space peak load overrides.
The oversizing applied to the chilled water loop coincident peak load.
The oversizing applied to the hot water loop coincident peak load.
The oversizing applied to the heat transfer loop coincident peak load.
When the preferences are enabled, those labeled as a global preference and the oversizing factors are applied to a system as it is loaded or imported, which can change settings you have previously made. To keep this visible, you are asked to confirm before those changes are made.
When you open an existing HVAC system (an ASP file), or import a prototype or saved system through the HVAC Library Import dialog into the open ASP file, and the preferences are enabled, a message asks whether you wish to continue and apply the preferences to the model, or continue and disable the preferences.
The prompt is not shown every time. When applying the current preferences would not change the system – because it already matches them – the notification is suppressed, nothing is written to the ASP file, and the enabled state of the preferences is retained so they still apply to the next system you load. A system with no relevant system link, saved and reloaded with the bypass preferences enabled, is treated the same way: the preferences are not reapplied and no notification is shown.
Those preferences labeled as System Sizing Overrides are applied only when the System Sizing run starts, and this is temporary. They do not change .asp file. The enabled state of the preferences is indicated in the ASHRAE Loads dialog beside the HVAC Sizing Preferences button; if you need an exception to your normal overrides for one run, disable the preferences and run System Sizing again.
HVAC Sizing Preferences and oversizing factors are never applied to rule-based compliance systems or to locked systems. This holds whether the preferences would otherwise be applied on opening the system or as overrides during the System Sizing run: opening such a system changes none of its true preferences or oversizing factors, and its sizing results are not altered by any override.
While a rule-based compliance system is open in ApacheHVAC, the Enable checkboxes on both tabs of the HVAC Sizing Preferences dialog are disabled. They revert to their previous ticked or unticked state once the compliance system is no longer open.
Rule-based compliance systems are those generated by the Compliance tools that use ApacheHVAC, including the following:
Note that the Title 24 Proposed model must use a Title 24 compliant HVAC system from the HVAC wizard, unlike the ASHRAE 90.1 PRM Proposed model.