This video shows the proper operation of the burners in a high efficiency furnace. You can see the flames being drawn directly into each of the heat exchanger's tubes. During the past decade, there have been several service bulletins from some of the major manufacturers regarding failed heat exchangers on the high efficiency furnaces.
When the heat exchanger fails, 'flame disturbance' will be visible. What you will see ranges from the burner flames being slightly off-centre from the heat exchanger tubes, to as drastic as the flames being blown backwards out of the tubes.
If you notice these signs, have a furnace service company inspect the unit as soon as possible. If the heat exchanger has failed, the required furnace repair will be the replacement of the heat exchanger. If your furnace is still under warranty, some or all of the cost will be covered by the manufacturer. You may still have some or most of the cost covered even if your furnace is out of the warranty period if the manufacturer has released some type of replacement advisory to the factory authorized warranty centre. Be aware that a non-qualified service firm may not be able to perform the needed repairs at the reduced cost.