The burn marks are caused by flames shooting out of the access opening to the burner compartment. It usually happens only briefly and intermittently, but can be a fire hazard and will eventually damage the internal components of the water heater. 

    The defect is called “flame rollout,” or sometimes referred to as “flashback,” and here’s a list of possible causes: 

•  Delayed ignition after gas valve opens
•  Over-firing of the burner
•  Corrosion damage or corrosion debris in burner compartment
•  Obstructed flue or downdraft
•  Negative air pressure in room
•  Lack of combustion supply air

    It can be difficult to determine if flame rollout is an ongoing problem or a previous event, although particularly dark and large marks would tend to indicate a long-term issue. Either way, we recommend immediate service by a plumber or gas appliance professional.

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Here’s links to a collection of more blog posts about WATER HEATERS:

• Are water heaters required to be raised off the floor? 

• Can a Temperature and Pressure Relief (TPR or T&P) valve be mounted to discharge horizontally?

• Where are gas water heaters not allowed to be installed?  

• Is the minimum size water heater inlet pipe 1/2" or 3/4" according to the building code?

• Can you use a light switch for a water heater disconnect? 

• How can I tell if a water heater is HUD-approved for mobile/manufactured homes?

• Can you wire a 240-volt water heater with 120 volts? 

• Is it alright to have a shut-off valve on both the hot and cold water pipes at a water heater?  

• What is the minimum clearance to doors and windows for an outdoor tankless gas water heater? 

• What is required clearance for access and working space in front of an electric water heater? 

• What is the purpose of a thermostatic mixing valve above a water heater? 

• Does a tankless gas water heater still work with no electricity during a power outage?  

•  Why do water heaters have a sacrificial anode? 

• What is the difference between a manufactured/mobile home water heater and a regular water heater? 

• Why is there water in my water heater drain pan?

• What does it mean when a water heater Temperature/Pressure Relief (TPR) discharge pipe is "trapped"? 

• Can I leave a gas water heater in place when remodeling a garage into a family room or bedroom? 

• Where do I find the water heater in a mobile home?

• Does a tankless water heater in an attic require a drain pan? 

• What is an FVIR water heater?

• What is a heat pump water heater?

• What is a dielectric union?

• What's that powdery crust on the pipe connections at the water heater? 

• What are the most common installation mistakes with water heater replacement?

   Visit our WATER HEATERS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.