Midco Unipower A-Series - Manual (Page 7)

V Chimney and Flue Pipe The chimney should be inspected for unsafe conditions such as deteriorated masonry
and excessive soot or other blockage or potential blockage.
■ ■ ■ ■ All combustion air is furnished by the Unipower burner, and therefore the only function of the
chimney is to vent the flue products.
■ ■ ■ ■ For the conventional appliances where a slight negative pressure in
the combustion chamber is desirable, the chimney and flue pipe sizes
given in Table 3 are recommended. Where chimney height exceeds
15 feet, a draft control should be installed per Figure 10 and set to
produce a draft of approximately .04" W.C.
■ ■ ■ ■ The barometric draft control should be of the double swing type,
which opens freely outward to afford downdraft protection
regardless of the setting made to the balancing weights for updraft
control. For a chimney of the recommended diameter in Table 3, the
draft control should be sized from 80% to 100% of the flue pipe
area, or per manufacturer's recommendations. It should be located in
the same room as the heating appliance, clear of any obstruction
which could interfere with its operation.
■ ■ ■ ■ The flue pipe must be of durable construction, preferably
galvanized steel of sufficient strength, and securely supported. When
passing through a combustible wall or partition, ventilated metal
thimbles or suitable insulation and clearances must be provided to
keep surface temperatures of exposed combustible material below 194° F.
■ ■ ■ ■ Do not arbitrarily reduce the flue pipe size, since a back pressure can build up in the
combustion chamber, leading to possible
leakage of flue products into the building.
However, if an undersized flue pipe or
chimney must be used and the appliance
is carefully sealed, or if it is built for
pressurized firing, the areas can be
reduced to accommodate 14,000 BTU per
square inch or more, particularly if the
chimney is 30 feet high or more.
■ ■ ■ ■ If the flue pipe must be extra long, the
area of the pipe and chimney should be
increased, or the chimney height should
be increased, or both. Never allow the
horizontal length of the flue pipe to
exceed the height of the chimney.
■ ■ ■ ■ If the chimney is of greatly excessive
height, draft may be so great as to cause
the barometric draft control to float in the
wide open position all the time, destroying
its effectiveness. The use of an adjustable
non-floating damper in the flue pipe will
prove useful in this instance by reducing
the draft to a point where the barometric
draft control can operate effectively. It
should be placed in the flue pipe between
the barometric draft control and the
chimney. Its' area must be less than that
of the flue pipe so that total blockage of
the flue pipe is impossible. The
construction of the flue pipe damper and
its adjusting mechanism must be rugged,
so that no flutter or vibration occurs. After
final setting, the damper must be welded
into position to prevent tampering.
■ ■ ■ ■ In the case of a motorized flue pipe
damper or mechanical flue exhauster, an
electrical interlock must be provided—see
Section VI Electrical.
■ ■ ■ ■ The information given in this section is by necessity of a general nature. Special cases may
require special treatment.
Maximum
Input
MBH
Flue
Pipe
Diameter
900
1,250
1,750
2,250
3,000
3,750
4,500
6,000
12"
14"
16"
18"
20"
22"
24"
26"
Table 3: Recommended
Size of Flue Pipe and
Chimney
Figure 10: Draft Hoods
Figure 11: Barometric Dampers
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