Yes, it was a limit switch exactly like one shown on link in C2F's reply above. So what are possibilities that could trip this switch? crack in heat exchanger is mentioned...would electrical power outages cause it? It has been sitting dormant for years and always worked before this last start up.
Heat exchangers fail from: Age, extremely moisture, corrosive chemical in the air, and overheating. Think of the exchanger like a cars radiator coil. Inside is the hot exhaust gas (car is glycol) and outside is the air being conditioned (car is air from outside).
The return air blows over the hot heat exchanger removing heat from the exchanger, cooling the hot exhaust gas and warming the air being conditioned.
All of the tubes on the modine that the fan blows across is the exchanger.
If seen relatively new exchangers get destroyed because the blower motor failed and the unit kept tripping on high limit. the customer resetting the unit. A high limit that is of automatic reset closes relatively fast compared to the heat exchanger. The exchanger just hotter and hotter and hotter, eventually it gives.
With that said, high limits on residential equipment is usually an automatic reset with a rollout switch being a manual reset. Manual reset is the red button you pressed. A rollout switch is in place in the chance the flame continues to run when proper exhausting ceases to happen or the heat exchanger is cracked and the conditioned air is forced into the heat exchanger via the pressure of the fan forcing the exhaust gases to reverse direction blowing out toward the gas train manifold. This would be very bad. The flame will burn up all the wires, the board. If it kept running who knows how bad **** can get. People die all the time from idiots jumping out safeties.
