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VAV Terminal Units

VAV

Heating

VAV Cooling Sizing

Calculation of DesignBuil

HVAC System Type

Outside Air

Plenums

HVAC Templates - Syste

0 Cooling

HVAC data

2 Heat Recovery

3 Working with Large Model

4 HVAC Control Zones

5 Cooling systems sizing

Working Wit h Large Models

It is possible to create very large, complex building models using DesignBuilder but you sho

consider what you're trying to achieve by your modelling exercise before diving in and includ

every detail of the building design. Otherwise you may find you have created a beautiful mod

which is impractical to simulate because it takes too long or causes the simulation engine to

You can should consider the following points.

EnergyPlus simulations are sloweddown 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 and Compact HVAC)

7. Calculated natural ventilation

using AIRNET

8. Calculating solar reflections9. Reading hourly or sub-hourly

results for extended simulation

periods

10. Not enough computer memory

(RAM)

11. Lots of output (quantity of data

and number of reports)

12. Lots of timesteps per hour

You can speed up modelling, and in particular the drawing and deleting of partitions in large

models, by switching off Automatic block zoning option.

You can speed up simulations by using these techniques:

1. Reduce the number of windows in the model by increasing the Window spacing. Sha

issues aside, modelling a small number of large Windows has the same effect as a la

number of small Windows provided you get the frame area right.

2. Avoid unnecessary wiggles and indents in block perimeters and partitions - keep it as

simple as possible. The larger the model the more important it is to follow this advice

Where an indent in the perimeter gives shading then you could instead model this us

a single flat surface without indent and use local shading devices with sidefins to mod

local window shading.

3. Use the Lump similar windows on surface option to speed up simulations with many

similar windows per surface. Note that switching this option on has no effect for zoneusing Daylight lighting control or if the Full interior and exterior solar distribution optio

selected.

4. Wherever possible lump similar adjacent zones together. 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 eac

room as a separate zone. This is often not necessary for building energy simulations

because many rooms will have reasonably similar operating conditions. In this case

may want to use one of the DesignBuilder Merging features to reduce the number of

zones.

5. If there are multiple identical zones then you can use the Zone multiplier to reduce the

number of zones processed.

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6. When you first create a building model you may want to carry out simulations before

partitioning the blocks into zones. If your un-partitioned blocks are very large there co

be an enormous number of windows in the single zone making up a block and this ca

slow down EnergyPlus simulations especially if carrying out daylighting analysis. In t

case you should try to reduce the number of windows in the model as recommended

above.

7. Try to use the smallest number of blocks possible to keep the number of surfaces dow

8. Exclude semi-exterior unconditioned zones from the model and replace them with an

Value.

9. Use Simple, (convective heating) HVAC descriptions.

10. Use the Scheduled Natural ventilation option.11. If you need to use Calculatednatural ventilation in your simulations then you should a

to minimise the number of cracks and openings involved in the airflow calculations. O

way to do this is to switch off infiltration calculations. Normally DesignBuilder includes

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

infiltration. This approach can be used in roof blocks or calculations such as cooling

design where infiltration is relatively insignificant.

12. Switching off calculation of solar reflections in the Model options dialog

13. Restricting hourly and sub-hourly results to short simulations for investigation of detai

building operation. You should not attempt to generate hourly or sub-hourly results fo

long periods (such as a whole year) unless your model is simple.

14. 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, excludconsideration of the rest of the building.

15. If you want to model a only a group of zones then you can exclude unnecessary zone

unchecking the Include zone model option.

16. 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.

17. Only request the output you really need. This will speed up simulation initialisation an

reading of output (especially hourly and sub-hourly) at the end. If you are only looking

hourly results then you can get noticeably faster initialisation and simulations by switc

off monthly and daily results. For example, simulation times for a test winter week

simulation on a real model were:

1. - With hourly, daily, monthly outputs: 39 m in

2. - Only hourly outputs: 14 m in

3. - Only hourly outputs and checking 'Internal surface as adiabatic': 12 m in

18. If you don't need surface data (Walls, Windows etc. heat transfer) switch the Surface

transfer incl solaroutput option off.

19. Note that if you request large volumes of data this can result is a program crash whe

results are being loaded into DesignBuilder.

20. It is sometimes possible to use less timesteps per hour to speed up simulations. Som

models can give reliable results using 2 timesteps per hour, however when complex

Compact HVAC systems are being modelled, 4, 6 or even 10 timesteps may be requ

If you aren't sure, compare the hourly results you obtain when using a range of times

per hour values and choose the minimum value that gives results equivalent to the 6

10 timesteps case.21. If all else fails then you can model very large buildings by splitting them into smaller u

and summing results outside DesignBuilder.
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