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Chimney Downdrafts
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Chimney Downdrafts

Ideally, air from a chimney naturally flows from the fireplace (or furnace) up through the chimney and out the top. This flow, called Chimney Draft, is what carries the exhaust and smoke through the chimney and out of the house. A Downdraft condition can exist in a chimney where the natural flow is inverted and travels down the chimney and out into the house. In this scenario, smoke (or a smoky smell) will accumulate inside the building. 

This chimney is too short for the main roofline
The fan at the top solved the draft problem...
The most common cause of a chimney downdraft is related to the chimney being shorter than parts of the house that its attached to. Building codes require a chimney to stand taller than anything else on the building within 10 feet. If the house has a higher roofline (more than 10 feet away) the draft will almost always be inverted in the chimney on the shorter part of the building. 
Competing Chimney Heights
Competng Chimneys
The chimney on the right will lose every time...

When there is more than one chimney on a house, the taller chimney can reverse (invert) the draft on the shorter fireplace.