The first burst of flame will shoot up the 4 chimney brush and clear out the cold air, to make room for the smoke from your wood fire. Check the damper to ensure that it is free of dirt and soot, and that it is open enough when you start your brand, so it allows the smoke to rise through the chimney.
This is when the fireplace has not been used for some time and the cold air from outside the home fan in the chimney and keeps warm air rises. This condition is usually temporary and clears itself when a fire is started and the first heat blast forces the cold air out. One trick to force a chimney to cope with the cold air out is to set fire to a band of rolled newspapers and keep it up the chimney for a moment while it burns so light your fire.
Location of the fire grate is also important so your stove is to get enough air from the outside of the stove to burn clean and not simmer. Keep it focused and toward the back. Using dry logs, which wet logs smoked more than dry wood. Keep windows closed when the stove action to prevent drafts, drawing air from the oven instead of passing it up the 4 chimney brush