The flame arrestor is factory built into the bottom of the gas tank-type water heater, and is part of the Flammable Vapor Ignition Resistant system or FVIR, together with the LDO screen and combustion chamber TCO (thermal cut off).
The FVIR is the system that is installed in all new models, and it protects the users and surrounding from accidental ignition of flammable vapors (coming from gasoline, paint, and solvents) outside the gas combustion chamber.
The flame arrestor is made of the fireproof material and is used to block the flame from escaping the gas combustion chamber. When looking from the outside the heater, it is accessible through the air intake opening and LDO screen, which keeps the lint, dirt, and oil away.
In order to prevent any accidence due to clogging the elements, preventive maintenance should be performed regularly, and that includes inspection and cleaning.
Use the non-metallic brush and vacuum cleaner for cleaning the bottom part of the flame arrestor and LDO screen (you might need a mirror and flashlight). In order to check the top part, you have to remove the inner door and burner assembly. If there are damages on the flame arrestor or there was a flammable vapor ignition, the water heater must be replaced.