james wrote:
I'm using a $20 household fan 24/7 to ventilate a damp crawl space by
sealing it against one of the crawl space vent (i.e. becomes an
exhaust fan).

Eventually this fan is going to fail, perhaps the bearings wear out
and then the fan stop spinning.

Should I worry about a fire, or do all UL listed fan have some sort of
overheat protection?

Is there some sort of fan that is designed to run 24/7 and is well
protected against overheating?

See if you can find a power strip with a 5-amp or so circuit breaker built
in.