The DFE 135/190 fixed temperature heat detector is suited to alarm in the presence of slowly rising temperatures. The construction of this model incorporates an oversized heat collector protected from damage by the built-in, durable plastic guard. The DFE135/190 fixed temperature heat detector used the proven snap-disc principle of operation. This bi-metal disc deflects when temperature reaches a predetermined value. The disc deflection then causes a push-rod to close the internal contact resulting in an alarm condition. The bi-metal disc returns to its normal shape when the heat subsides causing internal contacts to return to their normally open position. A standby condition is restored.