Specifications

3
1.1 Designing Venting Systems
Open-red heating appliances, like replaces, with
natural draft chimneys, follow the same gravity uid
ow laws as gas vents and thermal ow ventilation
systems. All thermal or buoyant energy is converted
into ow, and no draft exists over the re or the
appliance inlet.
Up to some limiting value, mass ow of hot ue
gases through a vertical pipe is a function of the rate
of heat release, the chimney area, height and system
pressure loss coefcient. An open-red heating
appliance may be considered as a gravity duct inlet
tting with a characteristic entrance-loss coefcient
and an internal heat source. The heating appliance
functions properly (does not smoke) when adequate
intake or face velocity across those critical portions
of the frontal opening nullies external drafts and
internal convection effects.
A mean ow velocity into a heating appliance’s
frontal opening is nearly constant from 300°F gas
temperature rise up to any higher temperature. Local
velocities vary within the opening, depending on its
design, because the air enters horizontally along
the hearth and then draws into the re and upward,
clinging to the back wall as shown below:
A recirculating eddy forms just inside the upper half
of the opening, induced by the high velocity of ow
along the back. Restrictions or poor construction in
the throat area between the lintel and the damper
also increases the eddy. Because the eddy moves
smoke out of the zone of maximum velocity,
the tendency of this smoke to escape must be
counteracted by some minimum inward air movement
over the entire front of the replace, particularly
under the lintel.
1.2 Gravity vs. Mechanical Venting
Heating appliances can be vented by gravity or by
mechanical means. “Gravity vented” is also called
“naturally vented”.
Gravity venting uses the difference in gravity of cold
vs. warm air to create draft for the heating appliance.
Gravity venting is very sensitive to the temperature
of the products of combustion, the ambient/outdoor
temperature, the barometric pressure and wind. It
is difcult, if not impossible, to adjust the draft to
changing conditions.
Mechanical venting uses a fan, blower or similar to
create proper draft. It is not sensitive to the same
issues as gravity venting, and it is possible to adjust
the draft to meet changing conditions.
Gravity venting is by far the most popular method of
venting a heating appliance. However, mechanical
venting is becoming more popular, and for some
heating appliances it has become an integrated part
essential for maintaining proper draft and combustion
and reducing emissions.
Many believe they know how gravity venting
works, but very few can explain how a chimney
works and why. Few really know how a mechanical
venting system works, but many think they do.
Chimney size, location and weather variations can
have a large impact on a gravity vented chimney
system. Misconceptions and misapplications
cause these systems not to function as they are
expected to. There are also many misconceptions
about mechanical venting. While there is fear that
installing a mechanically vented system will create
a hazardous situation, mechanical venting is a safe
and efcient way to vent a heating appliance.
The denition of a Mechanical Draft System can
be found in NFPA 97, Standard Glossary of Terms
Relating to Chimneys, Vents, and Heat-Producing
Appliances:
“Equipment installed in and made part of a duct,
chimney, or vent, that provides an induced draft”
Forced Draft: “Where a fan is located so as to push
the ue gases through the chimney or vent”
Induced Draft: “Where a fan is located so as to pull
the ue gases through the chimney or vent”
Obviously, a mechanical draft system cannot be used
in any stack or chimney exposed to habitable space
if it pressurizes the chimney or vent. Products of
combustion will be forced out through cracks in the
chimney, and this will create a potentially hazardous
or dangerous condition, due to the presence of
carbon monoxide.
On the other hand, a mechanical draft system
installed at the termination point can be used on any
type of chimney because it maintains a negative
pressure in the entire system.
1. Venting in General