035-15241-003 Rev. A (201)
8
Unitary Products Group
FURNACE SIZING AND DUCT SYSTEM
DESIGN
The duct system must be installed in conformance with
ASHRAE/NFPA 90, Standard for Installation of Warm Air
Heating and Air Systems and other applicable local codes.
Failure to adhere to proper duct system design standards can
reduce airflow, resulting in reduced system performance and
possible furnace damage.
Consideration should be given to the heating capacity
required and also to the air quantity (CFM) required if A/C is
to be installed along with the furnace or at some future time.
These factors can be determined by calculating the heat loss
and heat gain of the home or structure.
If these calculations are not performed and the furnace is
oversized, the following may result:
1.
Short cycling of the furnace.
2.
Wide temperature fluctuations from the thermostat set-
ting.
3.
Reduced overall operating efficiency of the furnace.
The supply and return duct system must be of adequate size
and designed such that the furnace will operate within the
designed air temperature rise range and not exceed the max-
imum designed static pressure. These values are listed in the
table below.
Additional information, values and data necessary for heat
loss, heat gain and duct system design may be found in the
ASHRAE HANDBOOK OF FUNDAMENTALS or in other
nationally recognized publications recognized by municipal,
state, provincial and federal code authorities.
If possible, it is recommended that the supply air duct
attached to the furnace be provided with a removable access
panel. The opening should be accessible when the furnace is
installed in service and should be large enough that smoke or
reflected light may be observed inside the casing to indicate
the presence of leaks in the heat exchanger. The cover panel
for this opening should be attached in such a manner as to
prevent leaks.
A/C USAGE DUCT SYSTEMS
1.
When a single (common) duct system is used, one of the
following methods shall be used:
a.
A plenum type cooling coil must be installed on the
air discharge side, or
b.
A blower-coil type cooling coil must be installed in
parallel with and isolated from the furnace, or
c.
A self-contained A/C unit must be in parallel with
and isolated from the furnace.
When the furnace is installed in an attic or other
insulated space, make sure that all insulation is at
least 12" away from furnace combustion air open-
ings. Failure to do this could cause asphyxiation
or fire.
FIGURE 6: AIR OPENINGS
OPENING FOR
VENTILATION AIR
OPENING FOR
COMBUSTION AIR
EXT. STATIC IN. W.C.
INPUT BTUH
MINIMUM
MAXIMUM
50,000
.10
.50
75,000
.12
.50
100,000
.15
.50
125,000
.20
.50
150,000
.20
.50
Dampers must be installed when a coil-blower or
self-contained unit is employed to prevent condi-
tioned cool air from coming in contact with the
heat exchanger to avoid moisture condensation
and rust-out. This can allow products of combus-
tion to be circulated into the living area by the fur-
nace blower resulting in possible asphyxiation. If
dampers are of a manually operated type, a
means must be provided to prevent either the fur-
nace or A/C unit from operating unless dampers
are in full heat or cool position.