Smoke can enter the house because of poor chimney draft. This draft issue happens because of several reasons:
The chimney draft is affected if the dimensions of the chimney flu are small in comparison with those of the fireplace.
If your chimney doesn’t stick out the required 2 feet over the roof, the proper draft will not be achieved.
Having other mechanical systems on you roof can affect the chimney draft, creating what is called a reverse draft, sending the smoke back down the chimney. Exhaust systems, fans and dryers can cause this reverse draft.
Cold air tends to go downward rather than rise, so if the outside temperature is cold, this air will tend to travel down the chimney.
The deposit of debris and creosote will block the air from rising up the chimney. It is simple to fix this issue by cleaning the chimney flue.