Metal duct work under fireplaces in crawl space leading to crawl space vents.

[Michael Potter]

I was under a house the other day and noticed that under both of the wood burning fireplaces there was ducting which as run to the crawl space vent from the base of the fireplace area.

What would the purpose of this be?

[Raymond Wand]

Combustion air intake for fireplace?

[Bill Chinook]

Depends on how many bends in the ductwork, but it could be an attempt at easy removal of ash.

[Mark Reinmiller]

Many manufactured fireplaces have combustion air ducts. They typically go through the wall of the enclosure, since that is typically on an exterior wall (at least around my area). If the fireplaces are on interior walls the ductwork would make sense.