HVAC model Options

Data tab on Model Options dialog.


HVAC model detail

HVAC systems can be defined in DesignBuilder using 3 different levels of detail:


See Simple HVAC Tutorial

Detailed HVAC Activity data

This option controls how zone temperature and humidity setpoints as well as ventilation requirement and hot water consumption data is defined in Detailed HVAC. Two options are available:




The tables below shows specifically where each item of Activity-related HVAC data is entered for the 2 options.

Run free (disable detailed HVAC system)

This option is provided as a quick easy way to run simulations without the HVAC system operating without the need to remove the system from the model.

1-Simple HVAC Data

Activity Tab

Setpoints, Fresh Air Rates, DHW Consumption


Setpoint timing / Operation

Heating setpoint and setback temperatures

Heating operation schedule
Cooling setpoint and setback temperatures Cooling operation schedule
Humidification setpoint DHW operation schedule
Dehumidification setpoint


Minimum fresh air per person  
Minimum fresh air per floor area


DHW consumption rate per floor area  

2-Detailed HVAC Data

HVAC Zone Dialog Water Outlet Dialog Heated Floor Dialog Chilled Ceiling Dialog

Heating setpoint schedules

Flow rate fraction schedule

Heating setpoint schedules

Cooling setpoint schedules

Cooling setpoint schedules

Peak flow rate    

Humidifying setpoint schedules




Dehumidifying setpoint schedules


Minimum fresh air per person


Minimum fresh air per floor area



Note: The tables above summarise the location of the various setpoints, fresh air, schedules etc for the 2 Detailed HVAC activity data options. For heating and cooling design calculations the humidity and temperature setpoints and ventilation requirement data comes from the Activity and HVAC tabs as described in the 1-Simple HVAC table, regardless of the Detailed HVAC Activity Data setting.

Specify Simple/Design HVAC details

When using Simple HVAC, you can choose whether to specify some basic details on the system. If you activate this option then you will be able to specify:



If you switch this option off then any heating or cooling is assumed to be entirely convective.

This option might be switched off for early design stage architectural use to simplify the HVAC data entry.


This option is additionally available when using Detailed HVAC, in which case it has a similar effect to that for Simple HVAC, but in this case it applies to data on the HVAC tab that is only used for Heating and Cooling design calculations. When using Detailed HVAC you should only uncheck this option if you do not plan to use Heating and Cooling design calculations.

Auxiliary energy calculations - Simple HVAC

When using Simple HVAC and you choose to specify Simple HVAC details (above), there are 3 options for modelling fans, pumps and other auxiliary energy associated with the HVAC system:


Mechanical ventilation method - Simple HVAC

This option defines how mechanical ventilation is treated for Simple HVAC Simulation and Cooling design calculations. Options are:


HVAC Sizing (Autosizing)

When using Simple HVAC, DesignBuilder can automatically calculate heating and cooling capacity in each zone based on the output from the Heating and Cooling design calculations. The Plant sizing model option provides control over how heating and cooling capacities are set for Simulations. Select from:



These HVAC sizing options are described below.

Note: In Heating and Cooling design calculations, heating and cooling loads are always met and so this option does not apply - it only applies to Simulations.


The effect of this option depends on the HVAC model option. When using Simple HVAC, equipment is modelled as having unlimited capacity allowing it to meet any heating or cooling demand and setpoint temperatures are always maintained. This option can cause extremely large peaks in heating and cooling delivery to zones and will tend to over-predict heating and cooling energy and under predict discomfort hours. When using Detailed HVAC this option is equivalent to 3-Autosize and the EnergyPlus HVAC autosizing mechanism is used to size the components and flow rates to ensure that the required comfort conditions are met.


No heating and cooling sizing calculations are required in this case.


Heating and cooling capacity Model data must be entered manually. Choose this option if you have specific heating and cooling capacity data you wish to use and do not want it to be overwritten when model data or model options change.


Autosize is the default option where heating and cooling capacities are always calculated prior to each simulation and the heating and cooling capacity model data is not displayed. When using Simple HVAC the heating and cooling design calculations will be run as required before the simulation to calculate the required sizes. The autosizing calculation method is defined through the Simple HVAC autosizing method model option described below. When using Detailed HVAC any autosized components and flow rates are sized using the EnergyPlus autosizing mechanism to ensure that the required comfort conditions are met. In this case heating and cooling autosizing calculations are done internally within EnergyPlus and no extra autosizing calculations are carried out by DesignBuilder.

Autosize when not set

In this case heating and cooling capacities are autosized and loaded into model data following heating and cooling design simulations.  If Capacity model data is not present before a simulation, it is generated by automatically by carrying out heating and cooling design calculations and is written into the HVAC model data.         


You should be aware that when using the 4-Autosize when not set option, the heating/cooling zone capacity model data is not treated the same way as other calculated results. In general, calculated results are deleted when any edit is made to the building model, but this is not the case for zone capacity model data.  This is only updated when:


  1. There is no heating/cooling capacity data (the value is either blank or zero when heating/cooling is switched on) and a simulation is requested.  In this case the appropriate design calculations are carried out to fill in the capacity data, before the simulation.
  2. The appropriate design calculation is carried out by clicking on the Heating/Cooling design tab.
  3. Model Options change. In this case zone HVAC capacity is deleted.


If you always want the heating capacity to be kept up-to-date with your model and do not have any specific capacity sizes you wish to use, you should consider using the 3-Autosize option.  This will cause autosizing to be carried out prior to every simulation.


You can change heating capacities by hand if you wish but you should bear in mind that, if Model options change, all heating and cooling capacities are reset to zero. Set the option to 2-Manual to avoid this happening.

Simple HVAC autosizing method

When using Simple HVAC with the 3-Autosize option, there are 2 ways to carry out HVAC autosizing calculations:



The 1-EnergyPlus option is recommended as the fastest way to carry out autosizing calculations. With this option, simulation times tend to be much shorter, particularly for short simulation periods. However, one advantage of the 2-DesignBuilder alternative option is that it provides the results of the 2 autosizing simulations on the Heating design and Cooling design screens. These results will be consistent with the plant sizes used in the simulation.


With the 1-EnergyPlus option the autosizing simulation outputs are generated in the EnergyPlus autosizing spreadsheet output files: