What is the purpose of a "pressure switch" in a furnace or air handler?

• A. To detect duct/negative pressure conditions and shut down equipment if unsafe.
• B. To regulate gas pressure
• C. To measure refrigerant pressure
• D. To change fan speed automatically

Answer: (A)

The purpose of the pressure switch is to ensure safe venting and proper draft before and during furnace operation. It monitors the draft inside the venting path (negative pressure created by the inducer or blower) and only allows the furnace to ignite or stay running when the proper pressure is detected. If the venting is blocked, or the draft is insufficient due to a clogged flue, a failed inducer, or other venting issues, the switch opens and shuts the furnace down. This prevents dangerous conditions like combustion gases, including carbon monoxide, from leaking into the living space.

This function is not about regulating gas pressure, measuring refrigerant pressure, or changing fan speeds.