When is it advised not to use a whole-house fan?

Using a whole-house fan is generally not advised when a combustion appliance draws air from inside the home due to the potential for backdrafting. Backdrafting occurs when the pressure within the home becomes so negative, usually because of a powerful exhaust device like a whole-house fan, that it causes combustion gases from appliances (such as furnaces, water heaters, or fireplaces) to be drawn back into the living space instead of venting safely outside. This can create dangerous conditions that lead to carbon monoxide buildup and pose significant health risks to the occupants.

Maintaining proper indoor air quality and ensuring safety is paramount, especially with appliances that rely on combustion processes. The use of a whole-house fan under these circumstances can disrupt the necessary air balance and thus create hazardous situations.

In other scenarios, while they may have implications for comfort or efficiency—like very low outdoor temperatures or high humidity—those factors do not present immediate safety risks that compromised combustion appliances do. Therefore, avoiding the use of a whole-house fan when there are combustion appliances drawing air from the home is focused on ensuring occupant safety.