National Oilheat Research Alliance 13
appliance, up the chimney and out of the
building.
There are two types of draft that affect the
operation of a standard chimney venting sys-tem:
thermal draft (sometimes called “natural
draft”) and currential draft.
Thermal draft (Figure 2) is produced by the
difference in the weight of a column of flue
gases within a chimney or vent system and a
corresponding column of air of equal dimen-sion
outside the chimney or venting system.
(Hot gases rise because they are less dense and
weigh less.)
Currential draft occurs when wind passing
across the top of a chimney creates suction and
draws gases and/or air up. Currential draft can
also cause “down drafts” and push air down the
chimney.
Three main factors control the amount of
chimney draft that is generated:
1. Chimney height—the higher the chimney,
the greater the draft;
2. The temperature of the combustion
gases—the hotter they are, the greater the draft;
3. The temperature of the air outside the
home—the colder it is outside, the greater the
draft.
How Much Draft Does
an Appliance Need?
Oilheat equipment manufac-turers
specify the amount of
draft required “over the fire”
and also what the “draft drop”
should be through the appliance.
Over the fire draft is the draft
measured at the flame observa-tion
door, just above the com-bustion
chamber. It is normally
about -.02 inches water column
(-.02" wc). “Normally” is
emphasized because the manu-facturer’s
guidelines must be
followed and there are some
oil-fired appliances that require
positive over-the-fire draft. Fig-ure
3 on following page.
Figure 2