Calculation Options - Simulation

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:

• The solution technique used in EnergyPlus has been designed to be stable with zone timesteps of up to sixty minutes (Number Timesteps in Hour = 1). However, 60 minutes is considered a “long” timestep and it should only be used in rare occasions where there is no HVAC system, accuracy is not a concern, and short run times are critical. Such long timesteps are not recommended to use because simulation results are more accurate for shorter timesteps, of say 10 minutes or less (Number of timesteps per hour of 6 or more). Shorter zone timesteps improve the numerical solution of the Zone Heat Balance Model because they improve how models for surface temperature and zone air temperature are coupled together. Longer timesteps introduce more lag and lead to more a dampened dynamic response.
• Simulation run time increases with shorter timesteps or larger values for Number of timesteps per hour. The effect varies with the nature of the model. The user can test out different values on their particular model to understand the implications for his or her particular case. Sometimes large models with multizone HVAC and Plant systems execute nearly as fast with 15 minute timesteps as with 60 minute timesteps because fewer iterations are required in the system modelling since the prior timestep’s results are close to the final outcome of next timestep.
• The weather data files usually have 60-minute (or hourly) data. However, it does not follow that this should be used as the basis for choosing the zone timestep because:
  • EnergyPlus carefully interpolates the weather data between data points for use at shorter timesteps.
  • Many aspects of a model have time scales that differ from the that of the weather data. A goal of the modelling is to predict how the building will respond to the weather. However, the building’s response is not governed by the time scale that the weather data are available at, but rather the time scales of the dynamic performance of the thermal envelope as well as schedules for internal gains, thermostats, and equipment availability.

• If the model will also be used to calculate the cost of electricity then you should be aware that many electric utility tariffs base charges on demand windows of a specified length of time. If the choice of Number of timesteps per hour is not consistent with the demand window, then unexpected results may be obtained. For reasonable prediction of the maximum rates for electricity use for in calculating demand charges, the length of the zone timestep needs to be consistent with the tariff’s demand window. The following table lists what values are consistent with various demand windows.

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:

• 1-Air temperature - control the zone mean air temperatures to the heating and cooling setpoint temperatures specified on the Activity tab.
• 2-Operative temperature - control the room temperature using 0.5 radiant fraction. See notes of operative temperature control below if you plan to use this option.
• 3-Other- you can enter the radiant temperature control fraction.
• 4-Fanger comfort - this option is only available when using Detailed HVAC. It allows you to control zone comfort conditions using Fanger PMV heating and cooling comfort setpoints instead of the usual temperature setpoints. The PMV setpoint values are defined under the Environmental control header on the Activity tab when using the default 1-Simple HVAC Detailed HVAC Activity data option. Alternatively, when using the 2-Detailed HVAC Detailed HVAC Activity data option, time-varying PMV setpoints for heating and cooling are defined by Fanger PMV comfort schedules selected on the HVAC zone dialog .

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

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:

• The radiant fraction sensed by the room thermostat, or,
• Given a room thermostat that mainly senses air temperature, the extent of automatic modification of the setpoint temperature to ensure occupant comfort.

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.

Operative vs Air temperature control - Advanced discussion

There has been much debate amongst simulation experts over the years on the extent to which radiant effects should be included on the simulated thermostat. Most real-world  thermostats probably don't actually sense more than about 20% radiant heat transfer - mostly the temperature sensor will be sensitive to the temperature of room air nearby. So one would think that air temperature (or 20% radiant) control is the best choice. But Operative control (radiant fraction = 0.5) can be useful for calculating realistic heating and cooling energy based on published summer and winter temperature requirements for the activities in each zone. This is because HVAC systems controlled using the operative temperature continue to condition the building until comfort conditions are met (just like they are in the real building where occupants may adjust thermostats until they are comfortable). Also, the default temperature set points from the Activity templates are generally derived from sources quoting operative temperatures. With Air temperature control the room air temperature is controlled to the set point temperature, which (depending on internal radiant temperatures) may not necessarily be comfortable.

The disadvantage of Operative temperature control is that start up loads can be unrealistically high due to the lag in thermal response of the walls, floor, ceilings.  The slow temperature response of the building fabric part governs the output of the operative thermostat and hence the operation of the heating/cooling equipment. If this effect dominates it can lead to an overestimate of the required design cooling load. You should be familiar with this issue before using operative temperature control to size heating and cooling equipment. In our experience using Operative temperature control usually leads to higher peak loads in Heating and Cooling design calculations and higher heating and cooling energy consumption in Simulations.

Caution: Operative temperature control can cause EnergyPlus Error 3 when using Simple HVAC, or Cooling design calculations with Operative control in zones with strong radiant heat gains causing high radiant temperatures (e.g. uninsulated roof or zone is highly glazed). The error is caused by the fixed supply air temperature being higher than the zone air temperature required to give the operative setpoint (.e.g. 24°C). The solution may be to use Air temperature control and to manage the high radiant temperatures using solar shading/insulation as appropriate.

Air temperature control is easier to use as none of the aforementioned problems apply, but it can lead to inadequate equipment sizes peak loads in Heating and Cooling design calculations when not used with a design safety factor. This is especially true when radiant temperatures are very different from air temperatures for example in poorly insulated buildings, buildings with large unshaded glazing areas or high ventilation rates. Generally using air temperature control in Simulations of such buildings will underestimate energy consumption.

Autosizing simple HVAC convective systems with operative temperature control

Another issue to bear in mind with operative temperature control is that in simple HVAC convective systems, autosized cooling systems use a different algorithm to calculate the maximum supply airflow rate used in the EnergyPlus Ideal Loads system. In convective systems the equation used is:

DeltaT = ZoneCoolingSetPointTemperature - HVACCoolingCoilSetpointTemp (difference in temperature between zone air and supply air)

DesignSupplyAirFlowM3PerS = DesignMaxCoolingLoad / (Cp * DeltaT * AirDensity)

This equation works because the temperature of the air in the zone can safely be assumed to be the zone cooling set point and so there is fixed difference in temperature between zone air and supply air. Calculating the design supply cooling airflow rate in this way does not work for operative temperature control because the air temperatures in the space are often much lower than the zone cooling setpoint temperature and sometimes in zones having very high radiant temperatures, the air temperature in the zone approaches the supply air temperature. In other words the difference in temperature between zone air and supply air in the simulated system becomes very low and therefore very large airflow rates are required to meet cooling loads. So in order to account for this DesignBuilder assumes a Delta T of 1K in the above calculation when operative temperature control is in use.         

Tip: As a general rule you should prioritise checking building comfort levels when using Air temperature control and realistic plant operation (oversized equipment, supply temperatures very low) when using Operative temperature control.