As tempting as it may be to end your holidays next to a warm, inviting fire, don’t assume that your chimney is clean because it hasn’t been used since last Christmas.
Make sure the flu opens and shuts properly. A closed flu or one that slips closed will fill your house with smoke. Even small amount of chimney buildup can allow carbon monoxide to enter the house.
Keep a good carbon monoxide detector close to the fireplace. Better an obnoxious alarm sounds than not if there is carbon monoxide in the air.
If everyone feels sleepy suddenly, or gets a headache get everyone out of doors immediately and call 911.
It may not be the best idea to act as your own chimney sweep. Hire someone who is comfortable around roofs, ladders and chimneys.