Watts AERCO MFC Series User guide

Heating and Hot Water Solutions
USA: T: (845) 580- -
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© 2020 AERCO
Pre-Installation Application Guide
MFC Series Boiler
Vent and Combustion Air Guide
Applies to Models 3000 through 10000
Exhaust venting, combustion air and piping
requirements for MFC Series boilers
Other documents for this product include:
OMM-0104 GF-148 MFC-Series User Manual
OMM-0108 GF-148-IN MFC-Series Installation Manual
TAG-0077 GF-148-E MFC-Series Electrical Power Guide
TAG-0078 GF-148-G MFC-Series Gas and #2 Oil Supply Guide
TAG-0081 GF-148-B MFC-Series Boiler Application Guide
Disclaimer
The information contained in this manual is subject to change without notice
from AERCO International, Inc. AERCO makes no warranty of any kind with
respect to this material, including, but not limited to, implied warranties of
merchantability and fitness for a particular application. AERCO International is
not liable for errors appearing in this manual, not for incidental or consequential
damages occurring in connection with the furnishing, performance, or use of
these materials
TAG-0080_OG • GF-148-V • 3/29/2023

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Table of Contents
1. General .............................................................................................................................3
2. Approved Vent Materials...................................................................................................3
3. Code Required Vent Terminations....................................................................................4
4. Combustion Air Supply......................................................................................................5
4.1 Combustion Air from WITHIN the Building..................................................................6
4.2 Combustion Air from OUTSIDE the Building..............................................................7
4.3 Two-Permanent-Openings Method (USA Only).........................................................7
4.4 One Permanent Opening Method ..............................................................................9
4.5 Opening a Louver Through the MFC Series Boiler....................................................9
5. Direct Vent/Ducted Combustion Air ................................................................................10
5.1 Optional Air Intake Adapters ....................................................................................10
6. Exhaust Vent and Combustion Air Systems ...................................................................12
7. Gross Natural Draft .........................................................................................................12
8. Acceptable Pressure Ranges..........................................................................................12
9. Exhaust Fans ..................................................................................................................12
10. Corrections for Altitude....................................................................................................13
11. Manifolded Systems........................................................................................................13
12. Elbow Quantity and Separation.......................................................................................13
13. Exhaust Muffler And Air Inlet Attenuator Guidelines.......................................................13
14. Vent and Combustion Air System Design Requirements................................................14
15. Condensate Removal......................................................................................................14
16. Individually Vented Systems ...........................................................................................15
16.1 MFC Series Pressure Drop Calculation Example..................................................15
17. Manifolded Ducted Combustion Air.................................................................................17
18. Common Vent Breeching (Manifolded)...........................................................................19
19. Pressure Drop and Draft Data Tables.............................................................................21

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1. General
The AERCO MFC gas-fired or fuel oil-fired (backup) boiler is a high efficiency, forced draft,
hydronic-heating unit with unique venting capabilities. All MFC venting options (which include
horizontal and vertical discharges, direct vent, and manifolded vent breeching), typically exceed
the capabilities of competing combustion equipment. These and other features enable MFC
boilers to provide extremely high thermal efficiencies and optimum temperature control under
widely varying conditions. It is therefore critical that the flue gas vent and combustion air system
be designed to maintain these objectives.
MFC’s high efficiency is achieved through air/fuel modulation and the release of energy from
the moisture condensing in the combustion products. Because condensation can occur in the
exhaust vent system, means must be provided to remove the moisture accumulation. Each MFC
model is fitted with a condensate removal trap, as indicated in Figure 1, which illustrates the
vent connection and condensate removal connection for the MFC 3000, MFC 4000, MFC 5000,
MFC 6000, MFC 8000, and MFC 10000 models.
The design guidelines in this bulletin provide broad latitude while meeting the objectives of
safety, longevity and optimum performance.
2. Approved Vent Materials
The MFC boiler is a Category II, III, and IV appliance, which requires special attention to exhaust
venting and combustion air details. The exhaust vent MUST be UL listed for use with Category II
and IV appliances. The following materials are allowed:
The MFC 3000, 4000, 5000, 6000, 8000, and 10000 boilers can use only AL29-4C venting
materials. Polypropylene, PVC, and CPVC are NOT allowed.
TABLE 1: Flue System Requirements –Minimum Thickness for AL29-4C
DIAMETER
WALL
THICKNESS
DIAM.
WALL
THICKNESS
DIAMETER
WALL
THICKNESS
3"
0.015"
9"
0.020"
20"
0.024"
4"
0.015"
10"
0.020"
22"
0.024"
5"
0.015"
12"
0.020"
24"
0.024"
6"
0.015"
14"
0.020"
26"
0.034"
7"
0.015"
16"
0.020"
28"
0.034"
8"
0.015"
18"
0.024"
30"
0.034"
Proper clearances to combustibles must be maintained per UL and the vent manufacturer
requirements. The UL, National Fuel Gas Code (ANSI Z223.1/ NFPA54)1, NFPA 31 Standard for
the Installation of Oil-Burning Equipment, and CSA B149.1-10 guidelines are often the basis for state
and local codes. AERCO's recommendations follow the guidelines of these agencies, unless
more stringent codes govern the installation site. The venting and combustion air systems must
meet all applicable code requirements. AERCO will not be responsible for failure of the system
design to meet code requirements.
All Canada installations must comply with CSA B149.1 installation code.
MFC Series boilers have the capability to condense on #2 fuel oil if the fuel oil has a low sulfur
content (<15 ppm) and return temperatures are no less than 140 degrees Fahrenheit. For
applications which shall take advantage of this capability, please be sure to utilize the appropriate
venting specified above that has been approved for oil fired equipment and condensing on fuel
oil.

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3. Code Required Vent Terminations
The guidelines provided in this bulletin should be followed to comply with AERCO, UL, NFPA 54
(National Fuel Gas Code, ANSI Z223.1), NFPA 31, and in Canada: CSA B149.1-10
recommendations and regulations.
Vent terminations should be at least 4 feet below, 1 foot above or 4 feet removed horizontally
from any window, door or gravity air inlet of a building. Such terminations should extend beyond
the outside face of the wall by at least 6 inches.
Figure 1: Location and Piping of the Two Condensate Drain Ports
The bottom of the vent termination should be at least 12 inches above both finished grade and
any maximum snow accumulation level to avoid blocking the vent or air intake. The vent
termination should be least 3 feet above any forced-air building inlet within 10 feet. Design must
prevent flue gases from recirculating through the boiler air intake.
Vents should not terminate over public walkways or areas where condensate or vapor could
create a nuisance or be detrimental to the operation of regulators, meters or related equipment.
Discharges should not be located in high wind, wind-blocked areas or corners, or be located
directly behind vegetation. Discharges in these locations may cause the flue pressures to fluctuate
and result in flame instability. As a general rule, designs should minimize wind effects.
Wall and roof penetrations should follow all applicable codes and the vent manufacturer's
instructions. Vents should never be installed at less than required clearances to combustible
materials, as enumerated in UL, NFPA, CSA B149.1-10 or local codes "Double-wall" or “Thimble”
assemblies are required when vents penetrate combustible walls or roofs.
Vertical discharges should extend at least 3 feet above the roof through properly flashed
penetrations, and at least 2 feet above any object within a 10-foot horizontal distance.
Vertical and horizontal discharges should be designed to prevent rain from entering the vent.
Large-mesh screens can be applied to protect against the entry of foreign objects but the 'free
area' should be at least equivalent to the flue cross-sectional area when unrestricted.
CONDENSATE TRAP
VENT OUTLET
CONNECTION
MFC BOILER
TO DRAIN
UNDER SIDE OF EXHAUST CHAMBER
SHOWING THE TWO CONDENSATE PORTS
CAUTION! PORTS MUST BE PIPED
TOGETHER BEFORE THE TRAP.
DRAIN
PIPING
CONDENSATE
DRAIN PORTS

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If the vent system is to be connected to an existing stack, the stack must be UL listed for Category
II and IV appliances (capable of 350F, positive pressure and condensing flue gas operation).
Masonry stacks must be lined, and the vent penetration must terminate flush with, and be sealed
to, this liner. Vents may enter the stack through the bottom or side. All side connections must
enter at a 45-degree connection in the direction of flow and must enter at different elevations, with
the smallest vent connection at the highest elevation. MFC vents must not be connected to other
manufacturer’s equipment.
The exhaust vent must be pitched upward toward the termination by a minimum of ¼ inch per
foot of length. Condensate must flow back to the MFC unit freely, without accumulating in the
vent.
4. Combustion Air Supply
The MFC boilers require the following combustion air volumes when operated at full capacity.
TABLE 2: Required Combustion Air
Volumes (at Full Capacity)
UNIT
VOLUME
MFC 3000
700 SCFM
MFC 4000
935 SCFM
MFC 5000
1170 SCFM
MFC 6000
1400 SCFM
MFC 8000
1870 SCFM
MFC 10000
2335 SCFM
These flows MUST be accommodated. Air supply is a direct requirement of NFPA, CSA B149.1-
10 (Canada) and local codes that should be consulted for correct design implementation.
In equipment rooms containing other air-consuming equipment ― including air compressors and
other combustion equipment ― the combustion air supply system must be designed to
accommodate all such equipment when all are operating simultaneously at maximum capacity.
Combustion air intakes must be located in areas that will not induce excessive (>0.10" water
column (W.C.)) intake air pressure fluctuations. Designs should take into account equipment
blowers and exhausts when using room air for combustion.
Intakes should be located to prevent infiltration of chlorides, halogens or any other chemicals that
would be detrimental to the operation of combustion equipment. Common sources of these
compounds are swimming pools, degreasing compounds, salts, plastic processing and
refrigerants. When the environment contains these types of chemicals, the air MUST be supplied
from the outdoors using direct-vent/ducted-combustion ductwork.
Air intakes must not be located in the proximity of garages, industrial and medical hood venting,
loading docks or refrigerant vent lines. Boilers should not be installed in the proximity of activities
that generate dust if that dust can enter the boiler intake. Boilers should be located to prevent
moisture and precipitation from entering combustion air inlets.
When a boiler is used, temporarily, to provide heat during ongoing building construction or
renovation, accumulated drywall dust, sawdust and similar particles can:
•Accumulate in the unit’s combustion air intake and block combustion air flow.
•Accumulate over the burner surface and restrict flow of air/fuel mixture.

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In these situations, AERCO recommends that a disposable air intake filter be installed,
temporarily, above the boiler combustion air inlet. Air filters may be required year-round in
instances in which dust or debris can enter the combustion air tube. Consult the boiler Operations
and Maintenance Manual for details.
Combustion air temperatures as low as -30°F can be used without affecting the integrity of the
equipment; however, the combustion settings may require adjustment to compensate for site
conditions.
4.1 Combustion Air from WITHIN the Building
Where combustion air will originate from within the building, air must be provided to the equipment
room from two permanent openings to an interior room (or rooms). Openings connecting indoor
spaces shall be sized and located in accordance with the following:
•Each opening shall have a minimum free area of 1 inch2per 1,000 BTU/hr (2,200
mm2/kW) of total input rating of all appliances in the space, but not less than 100 inch2
(0.06 m2).
•One opening shall commence within 12 inches (300 mm) of the top of the enclosure, and
one opening shall commence within 12 inches (300 mm) of the bottom. (See Figure 2).
•The minimum dimension of air openings shall be not less than 3 inches (80 mm).
Figure 2: All Combustion Air from Adjacent Indoor Spaces through Indoor
Combustion Air Openings
INDOOR
COMBUSTION
AIR OPENINGS

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4.2 Combustion Air from OUTSIDE the Building
Outdoor combustion air shall be provided through opening(s) to the outdoors in accordance with
the methods described below. The minimum dimension of air openings shall not be less than
3 inches (80 mm). The required size of the openings for combustion air shall be based upon the
net free area of each opening. When the free area through a louver, grille, or screen is known, it
shall be used to calculate the opening size required to provide the free area specified. For
additional details, consult NFPA 54, or in Canada, CSA B149.1-10, paragraphs 8.4.1 and 8.4.3.
4.3 Two-Permanent-Openings Method (USA Only)
Two permanent openings shall be provided; one commencing within 12 inches (300 mm) of the
top of the enclosure and one commencing within 12 inches (300 mm) of the bottom. The openings
shall communicate directly ― or by ducts ― with the outdoors, or spaces that freely communicate
with the outdoors, as show on the following pages:
•When communicating directly with the outdoors, or when communicating to the outdoors
through vertical ducts, each opening shall have a minimum free area of 1 inch2per
4,000 BTU/hr (550 mm2/kW) of total input rating of all appliances in the space (see Figures 3
and 4).
Figure 3: All Combustion Air From Outdoors - Inlet Air From Ventilated Crawl
Space and Outlet Air to Ventilated Attic

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Figure 4: All Combustion Air from Outdoors - Through Ventilated Attic
•When communicating with the outdoors through horizontal ducts, each opening shall have a
minimum free area of 1 inch2per 2,000 BTU/hr. (1100 mm2/kW) of total input rating of all
appliances in the space (see Figure 5).
Figure 5: All Combustion Air from Outdoors through Horizontal Ducts
OUTDOOR OPENINGS
TO HORIZONAL AIR
DUCTS

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4.4 One Permanent Opening Method
One permanent opening shall be provided, commencing within 12 inches (300 mm) of the top of
the enclosure. The appliance shall have clearances of at least 1 inches (25 mm) from the sides
and back of the appliance, and a clearance of 6 inches (150 mm) from the front. The opening
shall communicate with the outdoors directly or through a vertical or horizontal duct to the
outdoors or spaces that freely communicate with the outdoors (as shown in Figure 6) and shall
have a minimum free area as follows:
•1 inch2per 3,000 BTU/hr (700 mm2/kW) of the total input rating of all appliances located
in the space.
Figure 6: All Combustion Air from Outdoors through Single Combustion Air
Opening
4.5 Opening a Louver Through the MFC Series Boiler
A louver can be opened using the auxiliary relay contacts, part of the MFC burner controls. These
contacts are provided by a single pole double throw (SPDT) relay that is energized when there is
a demand for heat and is de-energized after that demand is satisfied. The relay contacts are rated
for 120 VAC at 5 amps, resistive.
NOTE:
Do NOT power the louver directly using the Auxiliary Relay. An external relay (supplied by
others) must be employed for this purpose. The boiler power cannot support external
accessories.

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If the louver features a proof-of-open switch, it can be connected to the boiler’s Enable/Disable
terminals. The Enable/Disable terminals must be closed for the unit to fire. If the proof-of-open
switch does not prove within the programmed time frame, the boiler will shut down.
For wiring connections and further details regarding the auxiliary relay and limit circuit, refer to
the MFC Series boiler Operations and Maintenance manual.
If an AERCO supplied sequencing panel is being used to manage a multiple boiler installation,
the louver can be opened using the System Start relays. Refer to the MFC Series Operations and
Maintenance Manual, GF-148, for wiring connections and further details.
5. Direct Vent/Ducted Combustion Air
The MFC is approved for direct vent installation; i.e., it can draw all combustion air from the
outdoors through a metal or PVC duct connected between the MFC unit(s) and the outdoors. This
configuration is useful for situations in which room air is insufficient or otherwise unsuitable for
combustion. The minimum ducted combustion-air duct sizes for the MFC boilers are as follows:
•MFC 3000 = 10-inch diameter
•MFC 4000 = 12-inch diameter
•MFC5000 = 12-inch diameter
•MFC 6000 = 14-inch diameter
•MFC 8000 = 16-inch diameter
•MFC10000 = 16-inch diameter
In many installations, the combustion air duct can be manifolded for multiple unit applications.
The length and restriction of the ducted combustion duct directly impact the size, length and
restriction of the discharge venting. The direct vent air intake should be located at least 3 feet
below any vent termination within 10 feet.
A screen with mesh size not smaller than 1” x 1” must be installed at the inlet of the ducted
combustion air duct.
5.1 Optional Air Intake Adapters
The following Air Inlet Adapters are available from AERCO:
Air Inlet Adapters
MFC Model
Riello Burner
Part Number
3000 Dual Fuel
RLS 120
201044004
NOTE:
Rectangular to circular transition piece shall be field-fabricated.
Adapters for other Riello burners shall be field-fabricated.

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Figure 7: Air Inlet Adapter for MFC 3000 (P/N 201044004)

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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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10. Corrections for Altitude
Table 7 in Section 19 lists correction factors for installation altitudes above sea level. These
factors must be applied to both the natural draft and pressure drops of vent and air ducts. The
pressure drop through vents and combustion air ducts will increase at higher elevations, while the
natural draft will decrease.
11. Manifolded Systems
In many instances it may be practical to connect multiple units using a manifolded vent or exhaust
configuration. However, when multiple units are connected by a manifolded air intake or exhaust
vent, the operation of a given unit can be affected by the others, if the venting or combustion air
system is not designed properly. Properly designed common vent and air supply systems can be
installed that will prevent "operational interaction" between units.
Do not use static regain method on common ductwork, but rather, use one duct size for
the common run (See Figure 11 in Section 18).
Contact your AERCO sales representative or AERCO International for design assistance
and review when designing manifolded exhaust and manifolded combustion air systems.
12. Elbow Quantity and Separation
The quantity and angle of elbows and the distances between them can influence the system’s
exhaust and combustion air pressures, as well as its acoustical behavior. Designers should
consider minimizing the quantity of elbows in the design and the use of angles less than 90°,
whenever possible. Five or fewer elbows are recommended for individual venting/connections;
five or fewer are recommended for common sections. The minimum distance required between
two elbows is five feet.
13. Exhaust Muffler And Air Inlet Attenuator Guidelines
MFC Series boilers require an exhaust muffler when installed in a noise-sensitive application and
when the exhaust vent ducting is relatively short in length. The following criteria should be used
to determine when to include a field-installed muffler in a MFC series installation:
•The exhaust is sidewall vented and the vent is terminated in close proximity to
residences, offices, hotel/hospital rooms, classrooms etc.
OR
•The total vertical and horizontal section of exhaust vent is less than 25 linear feet in
length from the last unit, and the vent terminates in close proximity to residences, offices,
hotel/hospital rooms, classrooms etc.
For manifolded exhaust systems, the total vertical section length includes both horizontal and
common vertical; individual boiler vertical connectors are included in the determination as well.
EXAMPLE: For an installation that has a 20-foot common vertical, 5-foot common horizontal
after the last boiler, and each boiler has a 10-foot vertical connector, the total section linear
length considered is 35 feet. Because this length is greater than 25 linear feet, a muffler is not

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required.
For manifolded ducted combustion, the total section length includes both horizontal and
common vertical; individual boiler vertical connectors are included in the determination.
EXAMPLE: For a manifolded ducted combustion that has a 10-foot common vertical, 5-foot
common horizontal after the last boiler and each boiler has an 8-foot vertical connector, the
total section linear length considered is 23 feet. Because this is less than 25 linear feet, an
attenuator is required.
Contact your local AERCO sales representative for more information on the AERCO
exhaust muffler and air inlet attenuator.
14. Vent and Combustion Air System Design Requirements
The minimum exhaust vent and combustion air duct sizes for MFC Series boiler models are as
follows:
TABLE 3: Minimum Exhaust Vent and Combustion Air Duct Sizes
MODEL
MINIMUM DIAMETER
Combustion Air Duct Diam.
Exhaust Vent Diam.
MFC 3000
10 inch dia.
10 inch dia.
MFC 4000
12 inch dia.
12 inch dia.
MFC 5000
12 inch dia.
12 inch dia.
MFC 6000
14 inch dia.
14 inch dia.
MFC 8000
16 inch dia.
16 inch dia.
MFC 10000
16 inch dia.
16 inch dia.
A 1/4-inch NPT combustion test hole is provided on each unit’s exhaust manifold connection. A
24-inch length of straight vent is recommended downstream of the exhaust manifold.
The vent system should always be pitched up ¼ inch per foot of run towards the vent termination
to enable condensate to drain back to the unit for disposal. Lowspots in the vent must be avoided.
Periodic inspection should be performed to assure correct drainage.
MFC Series vents should not be interconnected to those of other manufacturers' equipment.
Horizontal vent and ductwork should be supported to prevent sagging, in accordance with local
code and the vent manufacturer’s requirements. Vertical vent and ductwork should be supported
to prevent excessive stress on the horizontal runs. The exhaust manifold and inlet air adapter
should never be used as weight-supporting elements. The supports should be so arranged and
the overall layout designed to assure that stresses on the vent and combustion air connections
are minimized.
The vents and combustion air ducts may be insulated in accordance with the vent manufacturer's
instructions and local codes.
15. Condensate Removal
The exhaust vent system must be pitched back toward the MFC Series unit by a minimum of
¼-inch per foot of duct length to enable condensate to drain back to the unit for disposal. Low
spots in the vent must be avoided to prevent the condensate from collecting.

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The condensate trap assembly is located directly below the exhaust manifold. Plastic hose should
be connected to the trap assembly and run to drain. Care should be taken to avoid hose kinks
and to avoid raising the hose above the trap assembly. Condensate should flow freely to drain.
The condensate-to-drain run must not be hard-piped so the trap can be removed periodically for
maintenance purposes.
If the condensate must be lifted above the trap assembly to a drain, it should be drained into a
sump. From there, a pump can lift the condensate away.
Each unit will produce the following approximate condensate quantities in the full condensing
mode:
•MFC 3000 = 20 gallons per hour
•MFC 4000 = 27 gallons per hour
•MFC 5000 = 34 gallons per hour
•MFC 6000 = 40 gallons per hour
•MFC 8000 = 54 gallons per hour
•MFC 10000 = 68 gallons per hour
Condensate drain systems must be sized for full condensing mode.
In multiple boiler applications, it is common to manifold these drains together in a plastic pipe
manifold to a floor drain. Condensate manifolds must be large enough to handle the anticipated
flow and must be properly secured and protected. Manifolds are generally located behind the
boilers so that short runs of plastic tubing into the manifold can be used for the condensate drain.
A base drain must be installed at the bottom of vertical common flue piping.
The pH level of the condensate, produced by MFC Series boilers, ranges between 3.0 and 3.2.
The installation should be designed in accordance with local codes that specify acceptable pH
limits. If required, any type of commercially available neutralizer may be used.
16. Individually Vented Systems
Systems with individual vents may be used with any of the combustion air systems described
previously and illustrated in Section 4. The maximum combined pressure drop of the vent and
combustion air system must not exceed 140 equivalent feet of length.
To calculate the pressure drop:
1) Calculate the exhaust vent pressure drop.
2) Calculate the combustion duct pressure drop.
3) Divide the vent pressure drop by the altitude correction factor (CF) listed in Table 7 to
correct for installations above sea level.
4) Determine the natural draft, if any, from Table 6 and multiply it by the altitude CF.
5) Add the altitude corrected vent pressure drop (positive) and the draft (negative) to get the
total vent pressure drop.
6) Add the total vent pressure drop to the altitude corrected combustion air duct pressure
drop.
The total system pressure drop must not exceed 140 equivalent feet.
16.1 MFC Series Pressure Drop Calculation Example
Calculate the maximum pressure drop for a single boiler installation at 500 feet above sea level
having a winter design temperature of 20°F. The duct system of an MFC 3000 consists of:
1) An 10-inch diameter exhaust vent with two (2) 90° elbows, one (1) 45° elbows, 75 feet of

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horizontal run, 20 feet of vertical run
2) A rain cap termination
3) A 10-inch diameter ducted combustion air duct with two 90° elbows and 50 feet of run
MFC Series Pressure Drop Calculation
10-INCH DIAMETER EXHAUST VENT PRESSURE
Two 90°elbows: 2 x 2.68 = 5.36 ft.
One 45°elbow: 1 x 2.05 = 2.05 ft.
95 feet total run (75 horizontal + 20 vertical):
95 x 0.4 = 38.00 ft.
Rain cap exit loss: 1 x 8.81 = 8.81 ft.
Vent drop subtotal: = 54.22 ft.
Altitude correction: 54.22 = 55.21 ft.
0.982 (CF)
Natural draft for 20 feet @ 20°F outside temperature:
= 12.6 ft.
Altitude correction: - 12.6 x 0.982 CF= - 12.37 ft.
TOTAL VENT DROP: = 42.84 ft.
10-INCH DIAMETER COMBUSTION AIR DUCT PRESSURE
Two 90elbows: 2 x 1.88 = 3.76 ft.
50 feet total run: 50 x 0.3 = 15.00 ft.
Entrance loss: 1 x 4.46 = 4.46 ft.
Combustion air drop subtotal: = 23.22 ft.
Altitude correction: = 32.22 = 23.65 ft.
0.982 CF
COMBUSTION AIR DROP TOTAL: = 23.65 ft.
SYSTEM TOTAL PRESSURE DROP
Vent drop + combustion air duct pressure drop:
= 42.84 + 23.65
FINAL PRESSURE DROP TOTAL: = 66.49 ft.
CONCLUSION:
Pressure drop is less than 140 equivalent feet = SYSTEM OK.

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17. Manifolded Ducted Combustion Air
For systems using manifolded ducted combustion ductwork, use the longest length of common
duct and the individual branch to the furthest boiler to calculate the pressure drop.
Figure 8a: Individual Vents –Preferred Installations

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Figure 8b: Individual Vents –Acceptable Installations
NOTES:
1. For high wind, wind blocked sites, a tee may be installed at the fresh air inlet. The leg of
the tee connects to the combustion air intake.
2. On the flue vent side, a tee or exit cone (velocity cone) may be utilized in place of a rain
cap for high wind or wind blocked sites.
3. The branches of the tee can be in the horizontal or vertical direction, as determined by the
system designer and site conditions.

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18. Common Vent Breeching (Manifolded)
Connections to common vent breeching or duct work must be accomplished with a 45° elbow in
the direction of flow in the main breeching. “Tees” should not be used to accomplish these
connections - see Figure 9a. The recommended minimum individual boiler vent length before
joining the common vent breech is 10 feet.
Figure 9a: Recommended Connections to Common Vent Breeching
Interconnection of groups of units must never be accomplished via a “tee”. As shown in Figure 9b,
change the direction with one of the mains and then connect the second three diameters (common
section diameter) from this turn via a 45° connection.
Figure 9b: Required Interconnection of Groups of Units
REQUIRED
NOT
APPROVED

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Figure 10 illustrates the preferable “transition vent section” when making the 45° connection into
a main. The main can also remain at one diameter, as long as it is sized for the total number of
units vented and the 45° branch connection is retained. Use of the recommended “transition”
assembly will reduce the overall system pressure drop.
Figure 10: Required Transition Vent Sections
The vent system should always be pitched up ¼-inch per foot of run towards the vent termination
(see Figure 11). This will enable condensate to drain back to the unit for disposal. Low spots in
the vent must be avoided. Inspect periodically to ensure correct drainage.
As shown in Figure 11, the unit at the end of the vent main must be connected via an elbow. An
end cap must not be used as it may cause vibration and flue pressure fluctuations.
As discussed previously, the static regain method should not be used for common ductwork, but
rather, the one duct size should be used for the common run.
MFC vents should never be interconnected to those connected to other manufacturers’
equipment.
Figure 11: Connection of Unit at End of Vent Main