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 its 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 its 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.