TABLE 3B - AIR FLOW FOR TEMPERATURE RISE
Furnace
Model
High Fire (HF) CFM Required for a
▲
T of F
35
45
55
65
60
1501
1167
955
808
80
2001
1557
1274
1078
100
2502
1946
1592
1347
120
3002
2335
1910
1616
Furnace
Model
Low Fire (LF) CFM Required for a
▲
T of F
25
35
45
55
60
901
700
573
485
80
1201
934
764
647
100
1501
1167
955
808
120
1801
1401
1146
970
Existing ductwork should be assessed for its air handling capabili-
ties. For residential applications, the recommended air velocity of
a supply air trunk duct is 700 feet per minute (fpm), and should
not exceed 900 fpm. Th
e recommended air velocity of a supply air
branch run is 600 fpm, and should not exceed 900 fpm. Th ese values
are slightly lower for fl exible ducting. Th
e recommended air veloc-
ity of a return air trunk duct is 600 fpm, and should not exceed 700
fpm. Th
e recommended and maximum air velocity of a return air
branch 600 fpm.
Th
e “Equal Friction Chart,” as published by ASHRAE and HRAI, is
the basis for the various air duct calculators available through heat-
ing supply companies. Following the air velocity guide lines, accord-
ing to the “Equal Friction Chart,” or a slide rule air duct calculator, a
typical 6” round duct has a capacity of approximately 100 cfm.
NOTICE
Th
e return air system is equally as important as the supply
air system. An under-sized return air system will prevent
suffi cient quantities of air from reaching the supply air
system and will consequently reduce the service life of the
furnace and its components.
4 - FURNACE SIZING
Furnace Sizing
Th
e maximum hourly heat loss for each heated space shall be calcu-
lated in accordance with the procedures described in the manuals of
the Heating, Refrigeration and Air Conditioning Institute of Canada
(HRAI), or by any other method which is suitable for local condi-
tions, provided the results obtained are in substantial agreement
with, and not less than those obtained using the procedure described
in their manuals.
In the United States, “Manual J - Load Calculation,” published by
the Air Conditioning Contractors of America, describes a suitable
procedure for calculating the maximum hourly heat loss.
If the installation is a retrofi t application, do not rely on the capac-
ity of the existing heating equipment as a method to size the new
furnace. Many of the heat transfer multiples listed in earlier versions
of load calculation manuals were much higher than those listed in
more recent editions. It is possible that energy saving measures have
been completed since the installation of the existing furnace. Th is
might include additional insulation in the attic or walls, the applica-
tion of sprayed foam insulation, the addition of storm windows and
doors, weather-stripping, caulking, etc.
Many of the older furnaces were equipped with large belt drive blow-
er systems, operating at low RPM’s. If replacing an existing furnace,
be sure that the existing ductwork can handle the amount of airfl ow
necessary for a reasonable temperature rise. Most older gas furnaces
operated with a system temperature rise of 70° - 100°F. Th
is furnace
is designed to be operated with a system temperature rise (ΔT) of
High Fire 35° - 65°F, Low Fire 25° - 55°F. If the furnace selected has
an identical output capacity as the original furnace, a substantial
increase in system air fl ow will be required. See
Tables 3A and B
and the airfl ow characteristics in Section 23 of this manual,
“Airfl ow.”
TABLE 3A - RANGE OF TEMPERATURE RISE
Furnace Model
Temperature Rise
60, 80, 100, 120
High Fire (HF) 35 - 65°F
Low Fire (LF) 25 - 55°F
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