Airflow Windows

Openings tab in model data under External Glazing header

Airflow windows can be modelled by checking the Airflow windows option on the Openings tab. This option can be used to specify the control mechanism for windows in which forced air flows in the gap between adjacent layers of glass. Such windows are called “airflow windows.” They are also known as “heat-extract windows” or “climate windows.”

A common application is to reduce the zone load by exhausting indoor air through the window. In the cooling season this picks up and expels some of the solar heat absorbed by the window glass (and by the between-glass shade or blind, if present). In the heating season this warms the window, reducing the heat loss from the window. A side benefit is increased thermal comfort. This is because the inside surface of the window will generally be cooler in summer and warmer in winter.

Limitations

• Windows must have two or three glass layers, i.e., double or triple-glazing. For triple-glazing the airflow is assumed to be between the two inner glass layers.
• An error will result if the gas in the airflow gap is other than air.
• If an airflow window has a between-glass shade or blind, the gas in the gap on either side of the shade or blind must be air.

The surface output variable Surface Window Gap Convective Heat Transfer Rate gives the heat picked up (or lost) by the gap airflow.

Gap airflow configurations for airflow windows:

(a) Air exhaust window: Airflow Source = 1-Indoor Air, Airflow Destination = 2-Outdoor air;

(b) Indoor air curtain window: Airflow Source = 1-Indoor air, Airflow Destination = 1-Indoor air;

(c) Air supply window: Airflow Source = 2-Outdoor air, Airflow Destination = 1-Indoor air;

(d) Outdoor air curtain window: Airflow Source = 2-Outdoor air, Airflow Destination = 2-Outdoor air;

(e) Airflow to Return Air: Airflow Source = 1-Indoor air, Airflow Destination = 3-Return air.

Based on “Active facades” Version no. 1, Belgian Building Research Institute, June 2002.

Note: Although Airflow control windows are defined under the External Glazing header (which normally holds data for wall glazing only) Airflow control window settings apply to all windows, including rooflights.

Airflow control

Check this option to include airflow control for external windows.

Airflow source

The source of the gap airflow. The choices are:

• 1-Indoor air - Indoor air from the window’s zone is passed through the window.
• 2-Outdoor air - Outdoor air is passed through the window.

Airflow Destination

This is where the gap air goes after passing through the window. The choices are:

• 1-Indoor air - the gap air goes to the indoor air of the window’s zone.
• 2-Outdoor air - the gap air goes to the outside air.
• 3-Return-air- the gap air goes to the return air for the window’s zone. This choice is allowed only if Airflow Source = 1-Indoor air. If the return air flow is zero, the gap air goes to the indoor air of the window’s zone. If the sum of the gap airflow for all of the windows in a zone with Airflow destination = 3-Return air exceeds the return airflow, then the difference between this sum and the return airflow goes to the indoor air.

The diagram above shows the allowed combinations of Airflow source and Airflow destination. The allowed combinations of Airflow source and Airflow destination are:

• IndoorAir to OutdoorAir
• IndoorAir to IndoorAir
• IndoorAir to ReturnAir
• OutdoorAir to IndoorAir
• OutdoorAir to OutdoorAir

Maximum flow rate

The maximum value of the airflow (in m3/s per m of glazing width or ft3/min-ft). The value is typically 0.006 to 0.009 m3/s-m (4 to 6 ft3/min-ft). The airflow can be modulated through the Airflow multiplier schedule (below).

The fan energy used to move the air through the gap is generally very small and so is ignored.

Airflow multiplier schedule

The name of a schedule with values between 0.0 and 1.0. The timestep value of the airflow is Maximum flow rate times the schedule value.