W:\ELECTRICAL\INSTRUCT\FURNACE\FURNACE GENERAL INSTRUCTIONS 2011.DOC
10 of 36
01/2014
3-5.5
Ducts handling fumes that leave a combustible deposit shall be provided with
clean-out doors and such doors should be equipped with tight fitting doors or
covers. It is important that furnaces and ducts be kept clean if they are subjected
to a build-up of flammable deposits of condensed solvent, oil vapors, dust or other
combustible debris. The build-up of condensed vapors or combustible debris is a
major cause of fires. Frequency of cleaning should be based on never allowing
build-up to exceed 1/8" thickness in any location.
3-5.6
No portions of the building shall be used as an integral part of the duct.
3-5.7
All ducts shall be made tight throughout and shall have no openings other than
those required for the operation and maintenance of the system. All interior laps in
the duct joints should be made in the direction of the flow.
3-5.8
All ducts shall be thoroughly braced where required and substantially supported by
metal hangers or brackets.
3-5.9
Ducts handling flammable vapors shall be designed to minimize the condensation
of the vapors out of the exhaust stream onto the surface of the ducts. One method
is to insulate the ducts. If flammable condensation cannot be avoided, ducts
should be pitched to drain to suitable traps or other safe locations.
3-5.10
Ducts handling combustible solids shall be designed to minimize the accumulation
of solids within the ducts.
3-5.11
Exhaust ducts that will contain combustible deposits of any type require automatic
sprinklers in accordance with NFPA 13 AStandard for Installation of Sprinkler
Systems@ (see Appendix B).
3-5.12
Exhaust ducts shall not discharge near building openings or other air intakes that
allow re-entry of effluents into the building.
3-5.13
Clearance between metal ducts and stored combustible material should be at least
2-1/2 feet. Guards should be installed to assure this clearance.
3-5.14
Multiple exhaust fans manifolded together should have manifold designed so that
operation of one or more exhaust fan does not create a hazard such as back flow
to an idle furnace or reduced exhaust flow due to increased manifold pressure.
3-5.15
Duct work should not include dampers that could be closed and restrict flow within
the duct work.
3-6 FUEL GAS SUPPLY
3-6.1
Piping from the point of delivery to the equipment should comply with NFPA 54,
National Fuel Gas Code. Local, city and state codes should be followed. Gas
source pressure must be less than 60 psig.
3-6.2
A gas filter or strainer and sediment trap with vertical leg (drip leg) at least 3 pipe
diameters long (3" minimum) of same size as supply piping, shall be installed in the
fuel gas supply piping to protect the downstream safety shutoff valves and regulator
from materials that could interfere with their operation.
