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HVAC model Option

Data tab on Model Options dialog.

HVAC can be defined in DesignBuilder using:

• Simple - the heating/cooling system is modelled using basic loads calculation algorithms. For convective heating systems and all cooling systems the EnergyPlus 'Purchased air' system is used to calculate heating and cooling loads. 'Purchased air' supplies hot/cold air at predefined temperature and humidity for heating and cooling. 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 Radiant/convective the heating system type the EnergyPlus 'High Temp Radiant System' system type is used to calculate heating loads.
• Compact - the heating/cooling systems are defined in DesignBuilder using Compact HVAC descriptions but modelled in detail in EnergyPlus.  

In both cases heat generator (boiler) and chiller fuel energy consumption is calculated from loads as a post-process using efficiency factors.

HVAC + natural ventilation operate with occupancy

You can force all HVAC systems to have the same operation schedule as occupancy by checking the 'HVAC + natural ventilation operate with occupancy' option under 'Timing'.  

Plant Sizing- Autosizing

DesignBuilder can be used to 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 - The effect of this option depends on the HVAC model option. When using Simple HVAC, equipment is modelled as having adequately large capacity 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.
• 2-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.
• 3-Autosize - 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.
• 4-Autosize when not set - 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.         

Note: Heating and Cooling design calculations assume that heating and cooling loads are always met and so do not use this option.

Important Note About Using '4-Autosize when not set'

You should be aware that when using the default 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.