Ventilation fans do not lower air temperature—they move air and can provide comfort through evaporative cooling on skin when indoor temperatures are below approximately 35°C. Above that threshold, fans risk increasing dehydration and heat stress, particularly for vulnerable populations.

Extractor fans (kitchen hoods and bathroom fans) are genuinely helpful when used at the source of heat and moisture. Running a kitchen extractor while cooking or a bathroom fan during and after showers removes localized heat and humidity before they spread throughout the home, preventing appliances from adding to overall thermal load. However, extractor fans work by expelling indoor air, which creates negative pressure that draws replacement air in through gaps, windows, or vents. During the hottest part of the day, when outdoor air is significantly hotter than indoors, this replacement air can counteract cooling benefits. Therefore, use extractor fans briefly and targeted to specific heat sources during peak heat, then switch them off.

Whole-room or box fans become effective in the evening or night when outdoor temperature drops below indoor temperature. Positioned in windows, they can actively flush accumulated daytime heat out or draw cooler night air in, especially when combined with open windows on opposite sides to create cross-ventilation. In multi-story homes, running an upstairs extractor fan while opening downstairs windows leverages the stack effect, pulling cooler air up through the house and exhausting warm air from above.

The core principle is timing and temperature differential: use ventilation fans only when they remove a specific heat or humidity source, or when outside air is demonstrably cooler than inside. During peak heat with closed windows and doors, avoid fans that would pull hot outdoor air into your home.