Simulation Calculation Options - Calculation Options

Simulation tab on Model Options dialog and Options tab on Simulation Options dialog.


Number of time steps per hour

The Timestep object specifies the "basic" timestep for the simulation. The value entered here is also known as the Zone Timestep. This is used in the Zone Heat Balance Model calculation as the driving timestep for heat transfer and load calculations. The value entered here is the number of timesteps to use within an hour. Longer length timesteps have lower values for Number of timesteps per hour. For example a value of 6 entered here directs the program to use a zone timestep of 10 minutes and a value of 60 means a 1 minute timestep.


The user’s choice for Number of timesteps per hour must be evenly divisible into 60 and the allowable choices are 1, 2, 4, 6, 10, 12, 30, and 60.


The choice made for this field has important implications for modelling accuracy and the overall time it takes to run a simulation. Here are some considerations when choosing a value:



Demand Window Applicable Number of timesteps per hour
Quarter Hour 4, 12, or 60
Half Hour 2, 4, 6, 10, 12, 30, or 60
Full Hour, Day, Week Any


There is also second type of timestep inside EnergyPlus that is known as the System or HVAC timestep. This is a variable-length timestep that governs the driving timestep for HVAC and Plant system modelling. The user cannot directly control the system timestep (except by use of the Convergence limits data). When the HVAC portion of the simulation begins its solution for the current zone timestep, it uses the zone timestep as its maximum length but then can reduce the timestep, as necessary, to improve the solution. The technical details of the approach are explained in the Engineering Documentation under "Integrated Solution Manager".


Advanced EnergyPlus users can obtain and view data at intervals of the HVAC time step used if they select the 'detailed' option on an HVAC report variable when working directly with IDF data.


Though many buildings can be successfully simulated with 1 or 2 time steps per hour, EnergyPlus suggest a minimum of 4 for non-HVAC simulations and 6 for simulations with HVAC.


20 Timesteps per hour is the minimum when using the Finite difference solution method.

Green roof simulations may also require more timesteps.

Note 1: In general, increasing the number of time steps improves accuracy but slows the simulation (and generates more data if output is requested at the 'sub-hourly' interval).

Note 2: When using 1 time steps per hour you will not be able to access Temperature distribution results

Temperature control

Heating and cooling systems control internal temperatures to meet the setpoint temperatures specified on the Activity tab. These setpoint temperatures can be interpreted as air, operative or some other radiant fraction and DesignBuilder provides corresponding options to allow HVAC systems to be controlled by:



When using the 3-Other option the radiant fraction should be less than 0.9 and the minimum is 0.0. A value of 0.0 is the same as controlling on only zone air temperature. If air velocities are higher than 0.2 m/s, then lower values for radiative fraction might apply. Niu and Burnett (1998) cite International Standard ISO 77300 in recommending the values for this fraction listed in the following table.


Radiative Fraction vs Air Velocity

Air Velocity (m/s)


0.2 - 0.6

0.6 - 1.0

Radiant fraction




Note: When the 2-DesignBuilder Simple HVAC autosize method is selected, the temperature control settings used in heating design and cooling design will also apply in simulation autosizing calculations for the winter and summer design days respectively.

Reference: J. Niu and J. Burnett. 1998. Integrating Radiant/Operative Temperature Controls into Building Energy Simulations. ASHRAE Transactions Vol. 104. Part 2. page 210. ASHRAE. Atlanta, GA.


You can think of the Temperature control option as:


Note: This option is overridden when using radiant heating systems by the radiant heating system control setting.

Note: This option does not affect natural and mechanical ventilation setpoints - these always use air temperature set points.