A device (10) for monitoring selected operating conditions of combustion within a solid fossil fuel burning furnace having a discharge flue includes a circuit (12) for monitoring the instantaneous temperature in the flue and providing a signal proportional thereto, and a visual status indicator (35) having a plurality of light emitting diodes (40-44). The number of light emitting diodes (40-44) emitting illumination at a given time is indicative of the selected operating conditions of combustion within the furnace.
