How does the use of a fireplace typically affect a building's pressure differentials?

The use of a fireplace typically reduces interior building pressure. When a fireplace is used, it draws air from the interior of the building for combustion. This creates a negative pressure situation within the home because the fireplace exhausts air up the chimney, simultaneously pulling air out of the home. As a result, there is less air pressure inside the building compared to the outside, leading to a net reduction in interior building pressure.

This negative pressure can create several implications for indoor air quality and safety, as it may draw in unconditioned outside air through other openings such as windows, doors, or leaks in the building envelope. Additionally, this situation can affect other combustion appliances in the home, potentially leading to backdrafting, where combustion gases from those appliances can enter living spaces rather than venting outside properly. Understanding this dynamic is crucial for managed ventilation in homes using fireplaces, especially in terms of maintaining healthy pressure balance and indoor air quality.