W415-1117 / A / 03.11.14
9
IOM
3.0 FURNACE SIZING
TABLE 1 - RANGE OF TEMPERATURE RISE
Furnace Models
Temperature Rise
NBM030S2A
35 - 65°F (20 - 36°C)
NBM040S2A
35 - 65°F (20 - 36°C)
NBM40S3A
30 - 60°F (17 - 33°C)
NBM060S2A
35 - 65°F (20 - 36°C)
NBM060S3A
35 - 65°F (20 - 36°C)
NBM080S3A
35 - 65°F (20 - 36°C)
NBM080S4A
35 - 65°F (20 - 36°C)
NBM100S4A
30 - 60°F (17 - 33°C)
NBM100S5A
30 - 60°F (17 - 33°C)
NBM120S5A
35 - 65°F (20 - 36°C)
H12.15.1
H7.0.1
NOTE:
The return air system is equally as important as the
supply air system. An under-sized return air system
will prevent suf
fi
cient quantities of air from reaching
the supply air system and will consequently reduce the
service life of the furnace and its components.
The maximum hourly heat loss for each heated space shall be calculated 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 conditions, 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 retro
fi
t application, do not rely on the capacity 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. This might include additional
insulation in the attic or walls, the application 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 blower systems, operating at low RPM’s. If
replacing an existing furnace, be sure that the existing ductwork can handle the amount of air
fl
ow necessary
for a reasonable temperature rise. Most older gas furnaces operated with a system temperature rise of 70
- 100°F (39 - 56°C). These furnaces are designed to be
operated with a system temperature rise (
∆
T) of 35 - 65°F
(20 - 36°C) or 30 - 60°F (14 - 31°C) depending on what
model is installed. If the furnace selected has an identical
output capacity as the original furnace, a substantial
increase in system air
fl
ow will be necessary. See Table 1
and the air
fl
ow characteristics in Section 18 of this manual,
“Airfl ow”.
Existing ductwork should be assessed for its air handling
capabilities.
The
“Equal Friction Chart,”
as published by ASHRAE
and HRAI, is the basis for the various air duct calculators
available through heating supply companies. Following
the air velocity guide lines, according to the
“Equal Friction
Chart,”
or a slide rule air duct calculator, a typical 6”
(152mm) round duct has a capacity of approximately 100
cfm.
