What causes a flame flash back on heating furnaces? ?
The other guy had some good advice: like he said if its during ignition than its due to gas building up from a delayed ignition, usually its the burners needing cleaned or possibly an ignitor out of position. If its flame roll out during operation than that would be caused by a cracked heat exchanger or lack of combustion air feeding the furnace/flame or improper draft like a plugged heat exchanger - if the furnace employs a vent motor than the last one would be quite unlikely.
If you mean why does the gas turn on but not ignite for a few seconds and then it performs a mini-explosion, it's because the pilot is either too low, it's not aimed correctly, or something is clogging the gas burner (aka gas log) near the pilot flame. That results in the gas building up for too long inside the burner chamber, and when it does finally ignite, it makes a big whump! sound. Sometimes rust from the heat exchanger can fall into the little holes in the gas burner and clog them. You can often pick them out with a thin wire. If not, the burner needs to be removed and cleaned.
Delayed ignition is a bad thing. You have dirty burners or an out of adjustment pilot. If the flame is rolling out of the front that indicates that you either have a flue problem but most likely the heat exchanger has a crack in it. You should install a carbon monoxide detector in your home as a safety precaution. That gas can kill you !
