There was a time when furnaces had as much flexibility as boilers with respect to Btu/h input and output. Same for the air delivery side! Hard to believe, right?
Fan/limit switch. Fan/limit switches had a helical probe that was inserted between the heat exchanger sections. As the heat exchanger warmed up, the helical metal strip contained inside the slotted tube would twist to turn a dial inside the control box inside the furnace cabinet. That dial had adjustable limit stops: fan off (low temperature); fan on (higher temperature); and burner off safety overheat limit, but the fan would continue to run if the call for heating or until the fan/limit switch dial twisted back to break the lower temperature fan off setting. We could adjust both the fan-off and fan-on pointers on the face of the dial with its temperature lines. Set the lower fan off pointer a bit too low and the fan would never shut off, which led to customer complaints! The thinking was to have the fan continue running after the call for heating ended and, thereby, extract the last remaining Btu from the now cooling heat exchanger. Conversely, we could also raise the upper fan on setting if we needed to add more Btu/h input. Here’s how:
