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Accessibility clearances, which are typically greater, may exceed fire protection
clearances. Therefore, consider providing at least 24 inches of clearance from the front
(and the rear, for lowboy furnaces) of the furnace to obstructions and surfaces for
adequate service and maintenance access.
The minimum clearances from furnace casing surfaces to combustible materials are:
• 3 inches from casing sides and rear
• 8 inches from front casing of furnace to the closet door, with exception of:
o the lowboy rear flue model only requires 3 inches from front casing of the
furnace to the closet door
o the horizontal / counterflow furnace requires 22 inches from front casing of
the furnace to the closet door
• 9 inches from flue pipe/vent connector
• 3 inches from casing top
• 3 inches from any side of supply air plenum
• 3 inches above supply air ducts, within 6 feet of furnace
• For the horizontal / counterflow furnace only, when the optional combustible
surface-mounting base is used with the furnace in the counterflow configuration,
adequate clearances from the supply end surface of the furnace casing to
combustible surfaces are provided by the design of the mounting base.
Though these are approved clearances for these furnaces, space must be provided at
the front (18 inches minimum, 24 inches recommended) and at the rear of the furnace
for access, service, and replacement of the oil burner, air filter(s), circulating air blower,
and fan motor.
Air for Combustion and Ventilation
c CAUTION: Relief openings in the front or top of the furnace casing should not
be obstructed or blocked. These openings supply combustion and ventilation air
to the furnace.
Chloride, fluoride, iodide, and bromide bearing compounds when present, even in low
concentrations, in air supplied for combustion to the furnace, can result in accelerated
and severe corrosion of the heat exchanger and/or the venting system. This condition
can occur with less efficient furnaces, but is more likely to happen on furnaces with
efficiencies of 80% or greater.