Outside air introduction model in TRNSYS

There was a question posted on TRNSYS-USERS about the outdoor air introduction model. A way to handle this using Equation has been proposed.


This is a commonly used process, so I made an example. (I’ve changed it a bit from the suggested method.)

Example of outside air introduction using Equation

Calculation conditions

The conditions for determining the introduction of outside air are as follows.

  • Ventilation flow rate is 0.6 m3/m2h normally, 12.0 m3/m2h when introducing outside air.
  • Outside air introduction at 18°C < ambient temperature < 24°C

Equation settings

Equation is set as follows.

vent_condition = AND(GT(Tamb,18) , LT(Tamb,24) )  
vent_volume = 12.0 
flow_rate = vent_condition*vent_volume + (1- vent_condition )*0.6

The meaning of the variables is as follows.

vent_conditionDetermine the condition: 1 if the outside temperature is between 18°C and 24°C, 0 otherwise.
TambAmbient temperature, from weather data reader.
vent_volumeVentilation air flow rate when outside air is introduced [m3/m2h].
flow_rateVentilation air volume per unit area [m3/m2h].

Ventilation Settings

Set the Ventilation type in TRNBuild as follows.

  1. Select “volume flow rate related to reference floor area” to specify the ventilation rate per floor area.
  2. Click on the green arrow to open the Ventilation settings window.
  3. Select “Input”.
  4. Add a new Input with the name “FlowRate”

Finally, connect Equation and Type56 as follows.

Now the ventilation rate determined and processed in Equation will be used in Type 56.

Cooling Period

With the settings so far, the model has outdoor air introduction working throughout the year. If you want to limit it to a certain period of time, such as the cooling period, you can add more conditions.

In this example, Type 14 is used to set the conditions for the cooling period. (Red circle)

Assuming that the cooling period is from 2880h (May 1) to 6552h (Sep 30), set Type 14 as shown in the figure.

Add the variable cooling_period to Equation to get the cooling period, and change the first line as follows.

vent_condition = cooling_period * AND(GT(Tamb,18) , LT(Tamb,24) ) 
vent_volume = 12.0 
flow_rate = vent_condition*vent_volume + (1- vent_condition )*0.6 

In this example, Ventilation type is used, but the concept is the same for Infiltration type.
When using the Infiltration type, replace the ventilation air volume with the air change rate.

In addition, what if we want to introduce outdoor air only during certain times of the day, such as night ventilation or according to the schedule of the occupants?

The concept is the same as for the cooling period, and can be handled by incorporating a schedule that repeats over a 24-hour period.
Add another Type14 to define the daily schedule, and modify the first line of Equation to look like the following.

In this example, the schedule of the person in the room is received in the variable “occupant” and incorporated into the expression.

vent_condition = cooling_period * occupant * AND(GT(Tamb,18) , LT(Tamb,24) ) 
vent_volume = 12.0 
flow_rate = vent_condition*vent_volume + (1- vent_condition )*0.6

To download the sample, click on the link.

Test Environment

It has been confirmed that the software works in the following environments.

Windows10 Pro(64bit, 20H2)


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