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W415-0973 / B / 08.28.13

IOM

3.0 FURNACE SIZING

H7.0.2

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 retroﬁ 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 airﬂ ow necessary

for a reasonable temperature rise. Most older gas furnaces operated with a system temperature rise of 70 -

100°F (39 - 56°C). This furnace is designed to be operated with a system temperature rise (∆T) of 35 - 65°F

(20 - 36°C). 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 airﬂ 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.

NOTE:

The 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.

H12.15.5

TABLE 1 - RANGE OF TEMPERATURE RISE

Furnace Models Temperature Rise

ALL 35 - 65°F (20 - 36°C)

Napoleon 9600 Series User Manual

Questions and Answers: