If there is a blocked flue, which switch would open?

In the context of a blocked flue within an HVAC system, the pressure switch plays a critical role in ensuring safe operation. A blocked flue can prevent the proper exhaust of combustion gases, which poses a significant safety risk. To mitigate this hazard, the pressure switch is designed to monitor the airflow in relation to the venting system.

When the flue is obstructed, the pressure switch detects that the pressure within the system is outside of the normal operating range. As a result, the pressure switch opens, interrupting the operation of the appliance. This automatic response is a safety feature intended to prevent dangerous conditions from arising, such as carbon monoxide buildup in the space being heated.

In contrast, the other switches—limit, safety, and temperature—serve different purposes. The limit switch typically monitors temperature limits to prevent overheating and is generally activated under different circumstances. The safety switch is more of a general control mechanism that ensures safe operation but does not specifically address issues related to pressure in the flue. The temperature switch responds to the thermal conditions within the system but is not involved in monitoring flue blockage.

Thus, the function of the pressure switch in recognizing blocked flue conditions and opening to shut down the system makes it the correct choice.