Who hasn’t been there when it’s a scorching hot summer day, standing in front of a single box fan, trying to determine whether it’s actually cooling off the room? The old adage goes like this: when it’s stifling, set up the fan facing the window, and “blow” the heat out of the room. Doesn’t that sound simple enough? You are actually pushing the heat out of the room. Except, if you have done it and are drenched after ten minutes, the physics involved aren’t as straightforward as they seem.Air movement is not only about air itself but about the management of pressure and temperature zones. The principle of directing an airflow outwards is widely acknowledged, but its effectiveness heavily relies upon the external environment. Knowing how to manage these invisible streams of air will help create a cosy atmosphere in your bedroom without even using your thermostat.Negative pressure and “Cup effect”In the attempt to direct a fan outwards, one creates a sort of vacuum or, rather, negative pressure. When exhausting indoor air, the room is forced to draw air in again, which becomes effective when there is another window opened in a different area of the building that offers more shade.However, there is a catch. In a study titledAn Improved FFR Design with a Ventilation Fan: CFD Simulation and Validation, researchers used computational fluid dynamics to show how fan orientation changes the way air “hugs” a space. They found that fans blowing inward tend to create a cup-shaped airflow pattern. This pattern is often more effective at circulating air throughout the entire volume of a room, rather than just pulling a narrow stream of air toward a window.With fans blowing from the interior, the wind movement helps with heat transfer and speeds up the evaporation process in order to cool you down. On the other hand, if the fan blows outwards, it will exhaust some of the hot air, but you do not get that breeze or the “wind chill” effect that makes a warm day bearable.Strategies for dealing with warm, humid conditionsThis technique is bound to fail in humid environments, where replacing indoor air with outdoor air is not always enough. In cases where the temperature outside is, for example, 35°C, bringing in fresh air to replace the 32°C air indoors is futile.Research featured in the paper Airflow utilization in buildings in hot and humid areas of China highlights that in these specific environments, the goal should be “convective cooling.” This means you need high-velocity air moving directly over your skin to encourage evaporation. In these cases, having the fan blow inward, directly at the occupants, is almost always the superior choice. The study suggests that focusing on the “perceived” temperature through air movement is often more effective than trying to lower the actual thermometer reading of the room.
For humid conditions, high-velocity air directly on skin is key, and strategic fan placement utilizing the “stack effect” can enhance DIY ventilation.
Timing is essential for optimal DIY ventilation. During the heat of the day, use a fan to draw air into the building through your coldest and shadiest window opening. As soon as evening falls and temperatures outdoors fall below those indoors, it’s time to try cross ventilation. One fan draws cool air into the home through one end of the house while another exhausts warm air out the other end of the house for an instant high-speed tunnel effect to eliminate all day heat in minutes.Increasing your DIY ventilation effectIf you want to make your fans more effective, you should plan your route of airflow in advance. You lose effectiveness if there is dust build-up on the blades of the fan or on the window screen. Additionally, it’s best to place the intake fan where it is shielded by trees or an overhang to prevent drawing in any more heat from the outside environment.You could utilise your window height to the fullest potential. The hot air rises, so install a vent fan at the higher window and open a slightly lower window on the other end of your house. This stack effect will help the buoyant air push itself upwards to be exhausted first. Instead of thinking about your house as just another structure, imagine it as something that has to breathe in summer!
