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MFC Series Boiler Vent and Combustion Air Guide
6. Exhaust Vent and Combustion Air Systems
The MFC Series supports several venting and combustion air options, and although the
application parameters vary, there are basic similarities among all systems. Tables 4 through 8 in
Section 19 at the end of this Guide address the pressure drop of most applicable vent and duct
fittings and sizes. The losses in the vent exit and air duct entrance are also included.
It should be noted that flow and vent or duct diameter have the most significant effects on overall
system pressure drop. When using fittings or terminations not listed in Tables 4, 5 and 6, consult
the device manufacturer for actual pressure drop values. If a rectangular duct is to be used,
consult Table 8 in Section 19 for a round diameter duct size that has the identical pressure drop
per length of rectangular duct.
The pressure drop values in Section 19 are in equivalent feet for varying exhaust vent sizes based
on each MFC Series model. Consult your local AERCO representative if you require additional
venting information.
7. Gross Natural Draft
Flue gases have a lower density (and are lighter) than air and will rise, creating "gross natural
draft." Gross natural draft is created when flue gases exit the vent at an elevation above the MFC
boiler. The amount of draft depends upon the height of the stack and the difference between the
flue gas temperature and the surrounding air temperatures (densities). Gross natural draft values
for stacks at various heights above the MFC unit are presented in Table 6 in Section 19, Part 1
and Part 2. These draft values are based on an installation site at sea level.
Adding the gross natural draft (negative) to the vent and air system pressure drop (positive)
determines if the total system will be positive pressure or negative pressure ("net natural draft").
As with most combustion equipment, negative pressure (net natural draft) systems should be
treated differently from positive pressure systems when the discharge vents are manifolded. Note
that sidewall vent terminations, as well as some vertical terminations, are positive pressure
systems.
Contact your AERCO sales representative or AERCO International for design assistance
and approval when designing manifolded exhaust vent systems.
CAUTION!
Do NOT install a non-sealed draft control damper.
8. Acceptable Pressure Ranges
For individually vented units, the exhaust system must be designed so that pressure measured
at every point is in the range from -0.04" W.C. to +0.81" W.C. For common vented units, the
exhaust system must be designed so that pressure measured at every point is in the range from
0" W.C. to +0.25" W.C. Pressures below 0" W.C. (negative) may cause flame instability. Pressures
above +0.25" W.C. for common vented units, or +0.81 W.C. for individually vented units (more
positive), will prevent flue gases from exiting.
9. Exhaust Fans
If the MFC series boiler exhaust system incorporates an exhaust fan, the system designer must
size the vent pipe diameters, select the fan and determine the location of the fan sensor to
maintain a 0" to +0.25" W.C. pressure range at the outlet of each boiler. Also, the designer must
ensure that the exhaust fan material is acceptable for use with Category IV appliances.
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3/29/2023
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