Calculation Options

The 'time steps per hour' is the number of times the building thermal network is solved per hour in the simulations. When the HVAC portion of the simulation begins it's solution for the current time step, it uses the basic time step as its maximum but then can reduce the time step, as necessary, to reach the solution.

The technical details of the approach are explained in the EnergyPlus 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 (e.g. Zone/Sys Air Temp).

The time step entered here is also known in the EnergyPlus documentation as the Zone Time Step.

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

Hourly data (such as outdoor conditions expressed by Design Days or Weather data) are interpolated to the Zone Time Step. This is discussed the EnergyPlus documentation under: Weather Data Hourly Interpolation.

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:

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.

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.

Notes

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.

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 Purchased Air system. In convective systems the equation used is:

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

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.

Checking results

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.

Exclude all zone mechanical ventilation

Select this option if you wish to exclude the zone mechanical ventilation load from the heating/cooling loads in all zones. The option is useful in cases where the AHU is to meet mechanical ventilation loads and these loads are to be calculated separately when sizing the AHU coil capacity.

Exclude all zone natural ventilation

This option should be selected when natural ventilation is to be excluded from the Heating/Cooling calculations in all zones.

Air Velocity (m/s)	<0.2	0.2 - 0.6	0.6 - 1.0
Radiant fraction	0.5	0.4	0.3