HVAC model Options

Data tab on Model Options dialog.

HVAC model detail

HVAC systems can be defined in DesignBuilder using a choice of 2 different levels of detail:

• Simple HVAC where the heating/cooling system is simulated using the basic EnergyPlus ZoneHVAC:IdealLoadsAirSystem method. This supplies hot/cold air to meet heating and cooling loads. See topics on Heating and Cooling for more on defining the condition of the supply air. Mechanical ventilation loads are calculated locally for each zone. With Simple HVAC, fuel energy consumption for heat generation (typically a boiler or heat pump) and chiller is calculated from zone heating and cooling loads as a post-process using simple seasonal efficiency factors.

See also the Simple HVAC Data tutorial

• Detailed HVAC where the HVAC system is modelled in full detail using EnergyPlus air and water-side components linked together on a schematic layout drawing (e.g. below). In this case HVAC data is accessed by clicking on the <HVAC System> navigator node. This method requires more effort in setting up the HVAC model description and a greater knowledge of HVAC systems than the Simple HVAC option.
  
  

Detailed HVAC Activity data

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

• 1-Simple HVAC Data where zone temperature and humidity setpoints, ventilation requirements and hot water consumption rates for simulations are taken from the Activity and HVAC model data tabs. The advantage of this option is that the Activity-related HVAC data will typically already be set up based on the activity selection and so a) there is less work involved in setting up the setpoints etc and b) the simulation outputs will be more comparable with any previous Simple and Detailed HVAC simulations carried out. The downside is some loss of flexibility in setpoints (only 2 temperature setpoints per zone can be used - Setpoint and Setback temperatures).

• 2-Detailed HVAC Data where the activity-related HVAC data is entered on Detailed HVAC dialogs. This option provides increased flexibility but at the expense of extra work in setting up the setpoints etc, especially for large models. For example it is possible to have a range of different heating and cooling setpoint values set up using schedules for different times of the day or year. It is also possible to apply different setpoints for Chilled ceilings and other cooling equipment and likewise for Heated floors.

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

1-Simple HVAC Data

2-Detailed HVAC Data

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.

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.

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:

• System type (convective or convective/radiant).
• Supply air temperatures for simulation.
• HVAC auxiliary data (below).

• Mechanical ventilation method.

• Humidity control, heat recovery and economiser (below).

This option might be switched off for early design stage architectural use to simplify the HVAC data entry. If you switch this option off then any heating or cooling is assumed to be entirely convective.

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:

• 0-None - Auxiliary energy is not calculated. This option is typically used at early design stages where HVAC modelling may not be not required.
• 1-NCM - This option models auxiliary energy as a fixed annual energy consumption per floor area, with default values stored in the HVAC templates coming from the UK NCM.
  
• 2-Separate fans and pumps - This option allows you to use the EnergyPlus fan energy consumption calculation and a basic W/m2 + schedule approach to modelling pumps and control equipment.

Mechanical ventilation method - Simple HVAC

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

• 1-Room ventilation - Mechanical ventilation is modelled using EnergyPlus ZoneVentilation:DesignFlowRate data separate from the main HVAC system. With this option the fan energy consumption is included and the effect of heat pick up from fans in included in the zone heat balance. No humidity control, economiser or heat recovery is possible with this option. Radiant/convective heating is possible with this option. Mechanical ventilation outdoor air loads are displayed separately from HVAC heating and cooling loads in heat balance results.
• 2-Ideal loads - Mechanical ventilation is modelled using the EnergyPlus ZoneHVAC:IdealLoadsAirSystem data. In this case there are options to include the effects of heat recovery, economiser, humidification and dehumidification. Fan energy and pickup is not included in the simulation. Select this option if humidity control is required in Cooling design calculations. Radiant/convective heating is not possible with this option. Mechanical ventilation outdoor air loads are displayed combined with HVAC heating and cooling loads in heat balance results.

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 HVAC sizing model option provides control over how heating and cooling capacities are defined. Select from:

• 1-Adequate
• 2-Manual
• 3-Autosize
• 4-Autosize when not set

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.

Adequate

When this option is set, HVAC equipment is modelled as having unlimited capacity allowing it to meet any heating or cooling demand ensuring that setpoint temperatures are always maintained. This option can cause extremely large peaks in heating and cooling delivery to zones and will tend to significantly over-predict heating and cooling energy and under predict discomfort hours.

Manual

Heating and cooling capacity Model data must be entered manually on the HVAC Model data tab. 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

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 (within the same EnergyPlus run) to calculate the required sizes.

The autosizing calculation method is defined through the Simple HVAC autosizing method model option described below.

Autosize when not set

In this case heating and cooling capacities are autosized and loaded into model data following DesignBuilder Heating and Cooling design simulations.  If heating/cooling 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. It 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 normally use the default 3-Autosize option. This will cause EnergyPlus autosizing to be carried out prior to every simulation.

Tip: When using 4-Autosize when not set, you can manually change heating and cooling capacity settings on the HVAC tab if you wish but you should bear in mind that, if Model options change, all heating and cooling capacities are reset to zero. To avoid this happening, once you have established all the required heating and cooling capacity settings, set HVAC Sizing to 2-Manual.

Simple HVAC autosize method

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

• 1-EnergyPlus where EnergyPlus autosizing pre-simulations are carried out as part of the main simulation using the EnergyPlus built-in autosizing function, in the same way as autosizing for Detailed HVAC. This is the default option.
• 2-DesignBuilder, where Heating design and Cooling design calculations are run first to calculate the size of heating and cooling plant to use in the main simulation. This option involves 3 separate simulations of the building: 2 for the autosizing design calculations plus 1 for the main simulation.

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:

• epluszsz.csv for zone outputs and
• eplusssz.csv for system outputs.

Some minor differences in zone sizing results between the 2 methods are likely as explained below.

Differences between 1-EnergyPlus and 2-DesignBuilder Autosizing methods

 

In general the autosized heating and cooling sizes between the 2 methods should be close, however because the simulation methods are different some differences may be observed due to one or more of:

 

1. The Cooling design calculations used in the 2-DesignBuilder method are run using 2 timesteps per hour whereas the cooling autosizing simulations are run using the number of timesteps selected for the simulation. The timestep difference doesn't arise for heating design where the inside and outside conditions are static.
2. The WinterDesignDay settings control the operation for the heating autosizing simulations, whereas for Heating design calculations, fixed values are always used and there is less scope for obtaining "surprising" results. E.g. there is no way to include internal gains from occupants and equipment, whereas it is possible to include these gains (deliberately or inadvertently) in heating autosizing simulations through compact schedule settings.
3. When the Calculated natural ventilation model option is selected this is used in autosizing simulations (if the relevant schedules allow it) whereas for Cooling design calculations the scheduled natural ventilation settings are used (if natural ventilation is enabled).
4. Heat recovery can be activated or not for Cooling design calculations whereas it is not included when sizing zone loads in autosizing simulations.