Is a dryer vent considered an ignition source?
Only in the case of spontaneous combustion in the flue itself. So no.
What if a dryer fire occurred?
Somewhat common from lack of maintenance and all the poor type of vents that the home owners use.
Seems like the 3 foot rule would apply, but if a dryer vent is not considered a ignition source than it is what it is.
Thanks for the reply.
Then “ignition source” is the least of your problems… You could make the same argument for a kitchen window sitting above it. What if a stove fire occurred?
A dryer vent is not an ignition source. An open flame, or something that is creating a spark regularly is an ignition source.
They don’t get extremely hot.
R202 Ignition Source: A flame spark or hot surface capable of igniting flammable vapors or fumes. Such sources include appliance burners, burner ignitions and electrical switching devices
Hmmmm
Do electric HWH"s have switching devises?
(opps wrong thread) 
A dryer vent or vent termination with accumulated lint would be a potential fire hazard, but even those conditions are not ignition sources.