Tim Daneliuk writes:
http://www.mediafire.com/view/v070ra...coalchute.jpg#

There is noting on the exterior of the home to indicate this is present.

We have those for our chimneys, one per shaft (we have a three-holer).
They're used when the chimney sweep cleans the chimney. If you have a
fireplace, there's likely an ash door in the floor of the fireplace that
leads to this, but for a furnace it's a straight shot to the sky. The
water might just be rain.

In any event, contact your local chimneysweep and have the system
cleaned and inspected; they often have remote cameras and such for these
tasks. Adding a rain shield (with screen, for critters/birds) to your
chimney may help.

Or just open the door and look. That's what it's there for.