Thanks to the improvements in technology residential water heaters are now the FVIR or (Flammable Vapor Ignition-Resistant) type. What the manufacturer has done is to enclose around the burn chamber, essentially sealing the burner access point where flammables may enter. A good indicator that you have a FVIR water heater would be a sealed burner access and a piezo-starter or igniter.
The principle is that water heaters are now designed to prevent the lower burner from igniting flammable liquids near the water heater. An example of use would be in a garage where gasoline might be stored and spilled.
There are two key elements of these water heaters. A thermal sensor, or cutoff switch (TCO) or a thermal release device (TRD). And a flame arrestor screen located at the bottom of the unit that allows combustion air to enter but does not allow flame out i case of ignition.
If and when a problem happens the water heat shuts down:
For example if the flam arrestor screen becomes clogged by lint or debris the water heater will shut down due to “”inadequate combustion” and there will be no hot water for the home. Also if the water heater is under negative pressure caused by flue venting problems or obstructions, the unit wil shut down due to “inadequate combustion”. And again there will be no hot water.
Lint from clothes dryers must be closely watched to make sure the dryer duct does not become disconnected. This area needs to be kept free of lint debris and appears to be one of the biggest issues. If you canot see the flame through the glass inspection port this is a problem and should be corrected.