A back drafting alarm assembly is designed to detect back drafting in combustion fired appliances. The assembly includes a temperature sensor unit that is attached to the diverter bonnet of a water heater or furnace to measure the temperature at all times. The sensor is connected to an alarm assembly that sounds an alarm when the temperature of the bonnet reaches or exceeds 130 degrees F. for a predetermined time period. The preferred time period is three (3) minutes. The alarm assembly also includes a reset button to shut off the alarm and a test button to make certain that the device functions when installed. A microprocessor provides the additional capability to record the total number of excursion events over a longer monitoring period. The alarm operates on direct current from a battery, or standard 110 volts AC with a suitable AC/DC transformer. A battery backup feature provides uninterrupted power when using 110 volts AC as a power source. Back drafting is undesirable due to entry of carbon monoxide, produced by combustion, into the dwelling, resulting in dangerous or deadly conditions within the dwelling.