Elite chimney sweep Maesteg and Porthcawl. Beware birds nests in your chimneys. Here is a link to a nest I removed last week from a chimney in Southerndown. Extremely large and very dangerous if the owners had tried to light the fire
https://www.youtube.com/watch?v=KMuPY0lkyFc&feature=em-upload_owner Check this out.
If you are worried about the risks of Carbon Monoxide Poisoning, you need to look at this video and the following article.
Hope this helps you.