The pressure switch is critical for ensuring safe operation in a furnace system. It serves as a safety device that confirms whether the furnace venting is clear and functioning properly. The operation of the pressure switch often relies on 24 volts to function correctly. This low-voltage system is designed to control and monitor various components of the furnace, including the ignition sequence and the overall operational safety. If the pressure switch does not operate correctly due to a malfunction or lack of voltage, the furnace will not ignite, as it relies on this confirmation to ensure safe venting before allowing gas to flow.
In contrast, while the transformer indeed functions within the low-voltage system and converts higher voltage to 24 volts, it is not directly tested for operation as a load like the pressure switch is. The blower motor generally operates on higher voltage, typically 120 volts, and the limit switch is a safety device that monitors the furnace's temperature but does not operate on 24 volts directly. Thus, the pressure switch stands out as the component that specifically requires checking for 24-volt operation.