A furnace burning smell that reminds you of melting plastic or hot electrical wiring should never be ignored. It usually means something inside the furnace is overheating, such as the blower motor, a capacitor, the control board or the wiring. It can also mean a plastic object has fallen near the burners. Turn the furnace off at the thermostat and at the furnace switch, then book a repair. Calgary's Purcell Heating & Air explains why this is the smell technicians take most seriously and what happens next.