Safely venting the exhaust gasses from a fuel furnace has traditionally been done with a masonry chimney. A wood furnace burns with high exhaust temperatures loaded with creosote that can deposit on the chimney liner and catch fire. Since temperatures can be in excess of 2400 degrees Fahrenheit, the only alternative to a masonry chimney is an all fuel chimney that has a stainless steel liner, an outer wall and insulation between the two.
The modern oil fired furnace has become so efficient that it’s exhaust gas temperature is low enough to vent alternatively if a masonry chimney is not available, such as a conversion. Using a small electric motor and high temperature blower, the power venter produces a draw(draft) equivalent to or stronger than the natural draft a chimney makes to remove the exhaust gasses. Double wall construction(a pipe within a pipe with an air space between them) allows tight clearances to combustibles. Safety controls such as a pressure switch that senses that the motor is drawing air and a purge timer will not allow the burner to run if the venter motor fails. Post purge keeps the venter running after the burner has shut off to cool the firebox down and remove any residual exhaust gasses. The correct draft is important to the proper burn of an oil or wood furnace. If a chimney does not have a good enough draw, a device called a draft inducer can remedy the problem. Constructed of high temperature blower driven by a small electric motor, it is mounted in the smoke pipe just before it enters the chimney. Wired in conjunction with the burner motor or to a manual switch, it can be adjusted to help the chimney do it’s job.
It was always assumed that the exhaust gasses from a gas burning appliance would not harm a masonry chimney, but time has proven that assumption wrong and dangerous. The water vapor in the exhaust gasses can condense on the clay liner, and being slightly acidic, slowly attack the liner and cause deterioration. As the liner crumbles, it falls to the bottom off the chimney , eventually blocking off the flow of exhaust gasses. With no place to go out, the gasses back up, causing the appliance to burn fuel poorly and produce carbon monoxide, a poisonous gas. To correct or prevent this problem, metal liners have been developed that will slide down the chimney and hook directly to the appliance, eliminating contact between the exhaust gasses and the clay liner.