WeatherWhys for February 17,2016 from accuweather.com

A downdraft in your chimney can make it difficult to start a fire. The downdraft is caused by a difference in pressure. Outside, the air pressure is relatively higher than inside your home because the air is colder. Since air always flows from higher to lower pressure, the air will come down the flue and into your home. To offset this, open a window for a while. As colder air enters the home, it will reduce the difference in pressure. The downdraft will subside and you will have less hassle building a fire.