Air Temperature Distribution

HVAC tab in model data

 

By default EnergyPlus assumes that air temperature within a zone is completely uniform (i.e. the air is fully mixed). The Air Distribution model data allows you to model stratification within a zone by setting up a temperature gradient which varies dynamically depending on:

 

How it works

Changing the Distribution mode to 2-Dynamic gradient reveals the air temperature distribution data. By default the Interpolation mode is 2-Inside-outside DeltaT. This means that the temperature gradient within the space is modelled as varying according to the inside-outside temperature difference. The Upper conditions define the most extreme temperature gradient, typically under design conditions. In the screenshot above right, the upper temperature difference is 10°C and the corresponding temperature gradient is 4°C/m. This means that when the temperature difference between inside and outside if 10°C or more the internal temperature gradient is 4°C/m. The Lower conditions data shows that when the temperature difference between inside and outside is zero there is no temperature gradient, i.e. the air is fully mixed. EnergyPlus calculates the temperature gradient for other inside-outside temperature differences by linear interpolation.

Operation schedule

The operation schedule defines the times when air temperature distribution is to be calculated. When the value of the schedule is 1, the distribution calculations take place, when the value is 0, the zone air temperature is fully mixed.

Thermostat  height

Thermostat height specifies the distance above the floor where the thermostat for HVAC control is situated. This height is used by the model to determine the thermostat temperature relative to the mean air temperature by applying the gradient.

Return air height

Return air height specifies the distance above the floor where the air leaves the zone and returns to the air system. This height is used by the model to determine the return air temperature relative to the mean air temperature by applying the gradient.

Range of applicability

The air temperature distribution within the zone is used as follows:

 

  1. To calculate boundary conditions for conduction through surfaces (walls, roofs, glazing etc).
  2. To include the effects of extracting relatively warmer air at the ceiling level when using Compact HVAC.
  3. To include the vertical position of the thermostat sensor for HVAC control.

Limitations

The air temperature distribution mechanism does not include:

 

  1. The effects of extracting relatively warmer air at the ceiling level when using Scheduled and Calculated natural ventilation.
  2. The vertical position of the sensor for ventilation control.