Lets first review how most common chimneys are built. You have a masonry exterior and inside for a lining you have clay flue tiles that are stacked one on top of another. This is called a flue, some chimneys have a single flue and some have more then one flue but NFPA code states you should only vent one appliance per a flue.
Because the flue tiles are stacked one on top of the other they are usually sealed with mortar. Your chimney’s job is to take the noxious gasses created during the combustion process from your appliance outside of your home. These gasses can contain creosote/soot, carbon monoxide and other corrosive chemicals and over time they will break down the seal as well as the flue tiles. These gaps between the flue tiles will now allow gasses to seep into your home.
There are several reasons why your clay flue tiles can become damaged over time;
- seismic events
- chimney fire
- thermal expansion
- water damage
- manufacturer defects
- home settling
When your appliance is now using this flue the thermal expansion can turn a hairline into a quarter inch gap allowing these harmful gasses to escape into your home.
Another very common problem is spalling caused by direct exposure to constant moisture and the corrosive chemicals the flue is responsible for venting. This is another reason it is important to have your flues swept and inspected as recommended [link to how often they should get sweeps]*. Spalling is when the flue tiles flakeor delaminate. Once this begins to happen not only does your chimney stop working efficiently but your start to introduce threat risks to your family’s health.
The gasses that your chimney is evacuating from your home are poisonous. Along with the poisonous gasses creosote and soot can build up outside of the flue tiles and into the home. This can cause staining and destroy the inside of a home, not to mention they are carcinogens and highly flammable.