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:

• Simple HVAC - the heating/cooling system is modelled using basic loads calculation algorithms. For convective heating systems and all cooling systems the EnergyPlus ZoneHVAC:IdealLoadsAirSystem system is used to calculate heating and cooling loads. 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. When using the Radiant/convective heating system type the EnergyPlus ZoneHVAC:HighTemperatureRadiant system type is used to calculate heating loads. With Simple HVAC, heat generator (typically a boiler or heat pump) and chiller fuel energy consumption is calculated from zone heating and cooling loads as a post-process using efficiency factors.

See Simple HVAC Tutorial

• Compact HVAC - the heating/cooling systems are defined in DesignBuilder using moderately basic HVAC descriptions which are expanded into detailed HVAC definitions and modelled in EnergyPlus including boiler, chiller and fan part-load characteristics.
  
  
• Detailed HVAC - the HVAC system is modelled in full detail using EnergyPlus air and water-side components linked together on a schematic layout drawing. In this case HVAC data is accessed by clicking on the <HVAC System> navigator node.

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:

• 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 correctly 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 Compact 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).
  
  

  See the Detailed HVAC Activity Data and Zone Groups Tutorial

• 2-Detailed HVAC Data where the activity-related HVAC data is entered on Detailed HVAC dialogs. This option will usually involve more work in setting up the setpoints etc, especially for large models, but there is increased flexibility. 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 shows 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.

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).

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:

• 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 and Compact 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:

• 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

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 Compact HVAC this option is equivalent to 3-Autosize and the Compact 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.

Manual

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.

Manual Capacity - Compact HVAC

If you would like DesignBuilder to calculate the heating and cooling sizes and also maximum supply flow if you are using Compact HVAC you should follow this procedure:

1. Switch to the 3-Autosize option.
2. Run heating and cooling design calculations.
3. If you are using Compact HVAC also run a simulation.
4. Switch to the 2-Manual option.
5. You should now see the heating and cooling sizes shown for each zone in the HVAC model data.
6. If you are using Compact HVAC then you can also copy the Maximum supply flow in m3/s from the epluszsz.csv file. This can be found in the EnergyPlus folder straight after a Compact HVAC system simulation. The Maximum supply flow data is the bottom row labelled 'Peak Vol Flow (m3/s)'.

If you are using the Manual sizing option together with Compact HVAC and wish to use different cooling sizes to those obtained from looking at the output of previous EnergyPlus autosizing calculations (e.g . from eplusout.eio) you will need to ensure that the maximum supply flow is changed accordingly. If you increase the maximum cooling capacity but don't increase the Maximum supply flow rate (or even leave it as Auto) you will get "Rated air volume flow rate per watt of rated total cooling capacity is out of range" errors in the EnergyPlus simulation because the cooling capacity requested cannot be achieved with the specified air flow rate. Also due to a quirk in EnergyPlus, the Auto Maximum supply flow rate option does not work well with Manual sizing option and automatically calculated Maximum supply flow rates do not account for the 'higher than autosized' cooling capacity requested.

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 to calculate the required sizes. When using Compact HVAC the 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.

If the Autosize option is selected you can choose between 2 different autosizing methods using the Simple HVAC autosizing method option.

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:

• 1-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.
• 2-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 autosizing for Compact and Detailed HVAC. This is the default option.

The 2-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 1-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 2-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.