Bard Bayrd Furnace 403293A - Manual (Page 61)

EXAMPLE 6: (cont'd)
Masonry Chimney. From Table 9, the Equivalent
Area for a Nominal Liner size of 8 irxihes x 12 inches is
63.6 square inches. Using Table 8, Common Vent
Capacity, read down the "Fan + Nat" column under the
Minimum internal Area of Chimney value of 63 to the row
for 30'ft height, to find a capacity value of 739,000
Btu/hr. The combined input rating of the furnace and
water heater 135,000 Btuh/hr, is less than the Table
value, so this is an acceptable installation.
Note 19 requires the common verrt area to be no greater
than seven times the flow area of the smallest appliance
outlet area. Both appliances in this installation use 4 inch
diameter outlets. From Table 9, the Equivalent Area for
an Inside Diameter of 4 inches is 12.2 square inches.
Seven times 12.2 is 85.4, which is greater than 63.6, so
this configuration is acceptable.
Note 1 specifies that the Table values are for vents or
chimneys which are not exposed to the outdoors below
the roofline. If the masonry chimney in this case were
exposed below the roofline, then the appliance
manufacturer, local gas utility, and/or authority having
jurisdiction must be consulted.
Example 7: Interpolating Between Table Values
An installer has an 80,000 Btu/hr input appliance with a 4
inch diameter draft hood outlet that needs to be vented
into a 12-ft high Type В vent. The vent connector has a 5
ft lateral length and is also Type B. Can this appliance be
vented using a 4 inch diameter vent?
Solution
Table 1 is used in the case of an all Type В vent system.
However, since there is no entry in Table 1 for height of
12 feel, interpolation must be used. Head down the 4
inch diameter "NAT Max" column to the row associated
with 10 ft. height and 5 ft. laterial to find the capacity
value of 77,000 Btu/hr. Go down further to the 15 ft.
height, 5 ft. lateral row to find the capacity value of
87.000 Btu/hr. The difference between the 15 ft. height
capacity value and the 10 ft. height capacity value is
10.000 Btu/hr. The capacity for a vent system with a 12
ft. height is equal to the capacity for a 10 ft. height plus
2/5 of the difference between the 10 ft. and 15 ft. height
values, or 77,000 + 2/5 x 10,000 = 81,000 Btu/hr.
Therefore, a 4 inch diameter vent may be used in the
installation.
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