Under what circumstance might a flow switch trigger in a hydronic heating system?

In a hydronic heating system, a flow switch is designed to monitor the flow of water throughout the system. It typically serves as a safety device to ensure that water is circulating properly. When there is no flow of water, the flow switch is activated to signal a problem, such as a pump failure or blockage in the system. This condition is critical because proper water flow is necessary to transfer heat effectively. If the system fails to circulate water, it could lead to overheating or damage to equipment, so the activation of the flow switch in this case serves to protect the system and initiate necessary safety protocols.

The other scenarios presented do not directly relate to the primary function of the flow switch, which is to detect flow status rather than temperature, pressure, or the velocity of flow.