Working with Large Models

It is possible to create very large complex building models using DesignBuilder but you should consider what you're trying to achieve by your modelling exercise before diving in and including every detail of the building design. Otherwise you may find you have created a beautiful model which is impractical to simulate because it takes too long or causes the simulation engine to fail. You can should consider the following points.

 

EnergyPlus simulations are slowed down by:

 

  1. Lots of windows
  2. Lots of zones
  3. Lots of windows per zone
  4. Lots of surfaces
  5. VAV Compact HVAC descriptions
  6. Radiant heating systems (Simple HVAC)
  7. Calculated natural ventilation using AIRNET
  8. Calculating solar reflections
  9. Reading hourly or sub-hourly results for extended simulation periods
  10. Not enough computer memory (RAM)
  11. Lots of output

 

You can speed up simulations by using these techniques:

 

  1. Reduce the number of windows in the model by increasing the Window spacing. Shading issues aside, modelling a small number of large Windows has the same effect as a large number of small Windows provided you get the frame area right.
  2. Use the Lump similar windows on surfaceoption to speed up simulations with many similar windows per surface. Note that switching this option on has no effect for zones using Daylight lighting control or if the Full interior and exterior solar distribution option is selected.
  3. Only create zones to model areas of the building with specific environmental conditions, and HVAC systems or internal gains schedules.  A common mistake made by the 'beginners' is to model each room as a separate zone.  This is generally not necessary because most rooms will have very similar operating conditions.  In this case you may want to use one of the DesignBuilder Merging features to reduce the number of zones.
  4. When you first create a building model you may want to carry out simulations before partitioning the blocks into zones.  If your unpartitioned blocks are very large there could be an enormous number of windows in the single zone making up a block and this can slow down EnergyPlus simulations especially if carrying out daylighting analysis.  In this case you should try to reduce the number of windows in the model as recommended above.
  5. Try to use the smallest number of blocks possible to keep the number of surfaces down.
  6. Exclude semi-exterior unconditioned zones from the model and replace them with an R-Value.
  7. Use Simple, (convective heating) HVAC descriptions.
  8. Use the Scheduled Natural ventilation option.
  9. If you need to use Calculated natural ventilation in your simulations then you should aim to minimise the number of cracks and openings involved in the airflow calculations.  One way to do this is to switch off infiltration calculations. Normally DesignBuilder includes a single crack in each surface in the simulation to simulate infiltration. If you have other openings such as vents, windows, doors etc in a particular block/zone you can switch off infiltration. This approach can be used in roof blocks or calculations such as cooling design where infiltration is relatively insignificant.
  10. Switching off calculation of solar reflections in the Model options dialog
  11. Restricting hourly and sub-hourly results to short simulations for investigation of detailed building operation.  You should not attempt to generate hourly or sub-hourly results for a whole year unless your model is very simple.
  12. If you are interested in analysing a single zone within a much larger model, you could consider using the single zone option to carry out simulations just on the zone, excluding consideration of the rest of the building.
  13. Fit as much memory (RAM) to your computer as possible (2-4 GB ideally). Paging memory to disk during simulations of large models can cause EnergyPlus to grind to a halt.
  14. Only request the output you really need. This will speed up simulation initialisation and reading of output (especially hourly and sub-hourly) at the end. If you don't need surface data (Walls, Windows etc. heat transfer) switch this off in the Output Options tab of Simulation options dialog. If you are only looking at hourly results switch off monthly and daily. You will get noticeably faster initialisation.