Specifications

20
The gure below shows the effective area of different
ue shapes:
The effective area can be calculated by rst
determining the hydraulic diameter. More on this
subject can be found in Section 5.1.1.
4.1.2 The heating appliance
Designing a gravity venting system is relatively
simple, but depends on the appliance in use.
4.1.2.1 Pre-fabricated replace for wood or gas with
integrated chimney.
This has become a very popular type of replace in
new construction and it can be gas or wood red.
The chimney has already been sized for the replace
and all that can vary is the chimney height – at least
within some mandated minimum and maximum
heights. One 30° offset is typically allowed.
The chimney size selection is often based on the
volume of the rebox, and there is no scientic
selection method behind it. The method used is often
like “a 2 cu.ft. rebox requires a 6 in chimney, a 3
cu.ft. rebox requires an 8 in chimney” and so forth.
If an installation does fall within the recommendations
given by the manufacturer then there is not much
assistance available. The manufacturers may not be
‘experts’ in venting issues and may state that using
a mechanical draft system will void their warranty or
the product listing.
If the appliance’s installation manual demands
that the installer follows NFPA211 and/or NFPA54
then that entire standard text must be followed. If
the standard allows mechanical venting, and the
installation manual does not clearly prohibit this,
then the manufacturer cannot deny such installation
neither can he reject warranty coverage.
4.1.2.2 Pre-fabricated replace for wood or gas
This type does not include an integrated chimney.
The chimney must be supplied by the installer.
However, the replace manufacturer must provide
sizing and type recommendations for the installer and
the recommendations are very much like what you
nd for the replaces that integrate the chimney.
4.1.2.3 Pre-fabricated wood or coal stoves
Stoves are supplied without a chimney. Again the
chimney must be supplied by the installer, unless an
existing chimney is used for the venting.
The manufacturers’ recommendations are limited
to type of chimney, diameter, maximum number of
elbows and height restrictions.
4.1.2.4 Custom-made heating appliances
Custom-made heating appliances, such as masonry
replaces that are wood or gas red, are more
challenging as there is no manufacturer to provide
any sizing recommendations. The sizing must be
determined by the designer or the installer.
The sizing can be performed in a number of ways
depending on the application.
In all open-red heating appliances a minimum frontal
inlet velocity of 0.8 feet per second in conjunction
with a chimney gas temperature of at least 300-
500°F above ambient, should control smoking in a
well-constructed conventional masonry replace.
The chart on page 24 shows replace and chimney
dimensions for the specic conditions of circular ues
at 0.8 feet per second frontal velocity. This chart
readily accounts for maximum frontal opening for a
given chimney, as well as for chimney size and height
with a predetermined opening. The gure assumes
no wind or air supply difculties.
It may also be adequate to use the 1:10 rule where
the cross-sectional area of the chimney ue must be
min. 10% of the replace opening area.
Other applications may include replaces with
more than one side or opening, horizontal runs or a
number of different elbows, tee’s and more. For these
an ‘approved engineering method’ must be used
such as prescribed in the ASHRAE Handbook in the
Equipment Volume, Chapter 30 for “Chimneys, Gas
Vent, and Fireplace Systems”.
The design should follow these steps:
Indoor-outdoor pressure differences caused by
winds, building stack effect, and operation of forced-
air heating systems or mechanical ventilation affect
the operation of a replace. Thus, smoking during
start up can be caused by many factors unrelated
to the chimney. Often, in new homes (especially in
high-rise multiple-family construction), replaces
Determine the
volume of
combustion
products
Use selection table
to select
proper fan size
Select size of
chimney opening and
make sure there
are no more than
2 x 90° elbows
and the chimney
does not exceed
30 feet