Chapter 2
Chimney Venting
Figure 1
12 National Oilheat Research Alliance
Here’s how chimney venting works: Since
hot combustion gases weigh less than room
air or outdoor air, they tend to rise. During the
combustion process, heated gases expand and
rise through the heating appliance to the flue
pipe. They then travel up the chimney, creat-ing
negative pressure or suction, also known as
“negative draft”, before being released to the
outside atmosphere.
All venting systems rely on draft. Draft is a
pressure difference that causes gases or air to
flow through a chimney, vent, flue, or appli-ance.
In practical terms, draft is a force that “pulls”
or “sucks” the exhaust gases out of the heat-
Chimney venting was required for early
Oilheat equipment to remove the flue gases
from the building and to draw in combustion
air. It is still the most common type of venting
for Oilheat systems in use today.
Chimneys can be “lined” or “unlined.”
(Figure 1). A lined chimney is typically made
of brick and mortar on the outside with an
inside liner made of clay or stainless steel. The
liner protects the chimney walls from heat and
corrosion.
Unlined chimneys provide no means of pro-tection
for the chimney walls, are not permitted
under current NFPA regulations and are not
recommended for Oilheat systems.
Lined Chimney Unlined Chimney