What problem can whole-house fans cause in winter?

The correct answer highlights a significant issue that can arise when using whole-house fans during the winter months. Whole-house fans operate by pulling air from the home and exhausting it through the attic, which can create a negative pressure in the living spaces. When the air is drawn from the home, the conditioned air—whether it’s heated or cooled—can escape into the attic. This not only results in a loss of warmth during winter but can also lead to increased energy costs as the heating system works harder to replace the lost warm air.

In contrast to the other issues presented in the choices, this situation specifically addresses the impact of using whole-house fans in a season where maintaining indoor temperatures is critical. Increased humidity indoors is generally more of a concern during warmer months, while concerns about freezing or operational efficiency are not as immediate as the critical issue of warm air loss.