Specifications

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of normal design cannot cope with mechanically
induced reverse ow or shortages of combustion air.
In such circumstances, a replace should include
a mechanical draft system of sufcient capacity to
overcome other mechanized air-consuming systems.
4.2 Mechanical Venting Systems
Designing and sizing of a mechanical draft system
must be performed using an “approved engineering
method” such as prescribed in the ASHRAE
Handbook in the Equipment Volume, Chapter 30 for
“Chimneys, Gas Vent, and Fireplace Systems”.
4.2.1 Location of a Mechanical Draft System
A mechanical draft system can be installed at the
termination point or in-line between the appliance
and the termination point.
In-line mechanical draft systems can only be used
when the ue or chimney is airtight downstream
from its location. The table on page 20 shows which
chimney types can use mechanical draft systems and
where they should be located.
4.2.2 Installation at Termination Point
When used with a pre-fabricated chimney system,
the mechanical draft system should be installed on
a special chimney adapter that gives proper support
of the additional weight. It is usually not necessary to
provide special support for the system as long as the
chimney is terminated within 1-5 feet from the roof
support.
If the chimney is a type that requires ventilation
between the chimney walls outside the ue,
special precaution must be taken not to block the
passageways. This can be accomplished by using an
adapter that provides an opening of 1 inch between
the termination of the chimney and the bottom of the
fan (adapter).
4.2.3 Installation on Masonry Chimneys
In most cases, a mechanical draft system can be
installed directly on the chimney where the ue
terminates. The system is placed directly over the
ue in lieu of a chimney cap.
Aesthetics:
From an aesthetical point of view, the installation of
a mechanical draft system is no different from the
installation of a chimney cap.
However, sometimes a building design does not
allow a visible chimney cap or fan. In those situations
it may be possible to recess the fan into the cavity of
the chimney. The fan must have adequate ventilation
available, so it cannot be completely enclosed. A 1-2
inch space or clearance around the sides of the fan is
usually acceptable and the discharge can be through
weep-holes on the side of the chimney or straight up.
There should not be any restrictions.
4.2.4 Chimney and Flue Sizing
Sizing of chimney and ue is as important as for
gravity venting.
Measures should be taken to assure that the ue
size is adequate to allow for a velocity that does not
exceed 1,500-1,800 feet per minute. A higher velocity
will lead to ow noise - especially in steel chimneys.
On the other hand, the velocity should never fall
below 300 feet per minute as this may lead to
excessive build-up of creosote or condensate.
“Oversized ues” can also make it more difcult to
install the mechanical draft system. Special adapters
may be needed to prevent the mechanical draft
system from falling into the ue.