Mystery system

[Harold hydronicnetwork.net]

This system is a forced air system that has its limit controls on the hot water line – figure that! I guess you have a hot water coil in the ductwork heating the air. Sometimes, people do control the heating fan by using a switch that starts the fan when the coil is warm enough. If the switch turns OFF the fan, then it is preventing the air from getting too hot – it is strange to control the system that way. So you need to find out more about your system.