When I lived in Pennsylvania, my co-worker, Gary laughed at the fact I worked as a chimney sweep. He believed that anyone could do such an easy task and no one in their right mind would hire one.
When the weather turned cold in late October of 2007, Gary began complaining about headaches. He told me his entire family was sick. His wife mentioned she was always tired, though she attributed her fatigue to her pregnancy.
After hearing the same complaint for four days, I asked him when was the last time he had the furnace flue cleaned. He was sure the oil company took care of it. The technician had just inspected the unit, after all.
I suggested he let me take a look if only to eliminate my suspicions. When I went up on the roof of his house, it didn't take long to spot the problem. A bird nest blocked at least 85% of the 8x12 chimney flue. An inspection inside revealed an accumulation of soot and debris bound to choke the thimble, the area where the furnace pipe connects into the chimney flue. Over time, oil residue reacts with water, creating an acid that accelerates the deterioration of clay flues. 
After removing the blockage, the bird nest, and sweeping the flue, I found a large piece of flue tile missing where his bedroom wall met the chimney. Gary may still be losing sleep over what might have happened had we not spotted the problems. Today, his new stainless steel liner and air-tight system is protected by a reliable cap to keep the critters out, ensuring the safety of his home.
Carbon Monoxide is a colorless, odorless, and tasteless gas that's lighter than air. If not properly vented, the fumes can leak through the tiniest masonry flaw and threaten the safety of your home by contaminating the air you breathe.
At minimum, install Carbon Monoxide detectors in your home and make sure they're in perfect working order. The next step up is to employ a chimney service at the beginning of the season to inspect and take care of any hazardous issues.
Undetected blockages are more common than anyone knows. During warmer months, dormant, uncapped flues, attract raccoons, squirrels, bats, bees, and birds. Their nests are effective plugs that will create a dangerous situation the first time the unit runs. Don't wait.
Lightheadedness, confusion, headaches, vertigo, and even flu-like symptoms may be linked to a Carbon Monoxide leak somewhere in your house. More often than not, it may be coming from the furnace flue, particularly less efficient oil units venting through unlined chimneys in older homes.
A higher exposure to low levels of Carbon Monoxide can lead to depression and memory loss. Higher levels will lead to hypoxia, a condition which reduces the blood's capacity to carry Oxygen, and eventually death.
Fortunately, these risks can be lessened dramatically with regular maintenance of not only the unit, but the flue as well. A conscientious chimney company will evaluate the condition of the flue after a thorough inspection, and alert the homeowner of any issues.
Note from the author: I personally detest articles that use fear tactics, but Gary's experience hit close to home. At the time, his wife was pregnant with their second child, and their baby was only two. I hate to think what would've happened.
As it was, keeping their bedroom door open to be able to listen to their child and running a ceiling fan, may have saved their lives.
