on the Equipment List/Submittal Data in the boiler
manual. The boilers will also be provided with a
label indicating that they have been prepared for
high altitude. If a boiler is to be installed at an
altitude other than that for which it was factory
built, orifices must be replaced to properly adjust
the gas input. Consult the factory or the local
Bryan Representative for the proper parts. For
the purpose of vent system sizing, assume full
input and determine sizing as if at sea level. The
derate factor of 4% per 1000 feet above sea
level accounts for the increased volume per
Btu/hr of flue products at high altitude.
INDUCED DRAFT FANS
Occasionally, the characteristics of an installation
are such that a natural draft vent system will not
suffice. In such cases, induced draft may be
used. The vent system is then sized with an
available "pumping" action equal to the total
theoretical draft plus the static pressure capability
of the induced draft fan. This will result in a
smaller diameter vent than for a natural draft
system. Sizing of induced draft fans should be
done using the recommendations of the fan
manufacturer and the ASHRAE Handbook.
FIG. 1.8.14: SUGGESTED APPLIANCE VENTING PROCEDURE
MULTIPLE APPLIANCE INSTALLATIONS
Bryan recommends that boilers and other gas
appliances be individually vented when possible.
See figure 1.8.14A. Individual venting provides
better draft control and fuel efficiency, and is less
likely to cause condensation in the system. When
individual venting is not possible, boilers may be
vented to a common breeching (vent connector).
See Fig. 1.8.14B for recommended design of
such a system. Note that connections of
individual boiler or
appliance vents into the common breeching
should be done with 45 elbows and not by
"bullheading" directly into the vent connector at
90 angles. "Bullhead" connections generally
cause excessive turbulence and poor draft
conditions. On vent connectors serving multiple
appliances, the diameter of the piping should be
increased at each appliance's entrance so as to
provide a relatively constant flue gas velocity
through the vent system. Using a constant
diameter breeching will often result in poor draft
at the outermost appliances.
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