W415-0973 / B / 08.28.13
16
IOM
IOM
8.1 DETERMINING COMBUSTION AIR
8.1.1 CASE 1: FURNACE LOCATED IN A UNCONFINED SPACE
8.1.2 CASE 2: FURNACE LOCATED IN A CONFINED SPACE
H13.1
Unconfi ned space does not necessarily mean that ventilation will not have to be introduced from the outdoors,
particularly in airtight homes. The minimum requirement for unconfi ned space is a volume of 50 cubic feet (1.42
m³) for each 1000 Btu/hr for all fuel burning appliances located within the unconfi ned area.
If the amount of combustion and ventilation air is insuffi cient to properly operate the furnace and other fuel
burning appliances within the unconfi ned area, it will be necessary to supply it from the outdoors based on the
criteria used when calculating the air supply for a confi ned space.
NOTE
If planning to use the inside air in an unconfi ned space, remember to test for proper furnace operation (as well
as other fuel burning appliances located within the unconfi ned space) with respect to adequate combustion and
ventilation air with fi replace dampers open, clothes dryer running, bathroom exhaust fans on, kitchen range hood
on, etc.
H13.2
A confi ned space, (any space smaller than the minimums discussed in Case 1), must have two air openings;
one within 12” (305mm) of the ceiling and the other within 12” (305mm) of the fl oor. The air openings must
be sized based on whether the combustion and ventilation air is being taken from indoors or outdoors, the
method outdoor air (if used) is introduced, and taking into account any other fuel burning appliances in the
confi ned space.
If suffi cient indoor combustion and ventilation air is available for the furnace and all other fuel burning
appliances, size each opening on the basis of one square inch (645mm²) of free area per 1000 Btu/hr.
(Figure 4)
NOTE
Be sure to consider all clothes dryers, bathroom fans, range hoods, etc., when making this calculation.
The minimum requirement for these openings is 100
square inches (645mm²), even for the furnace models
under 100,000 Btu/hr.
NOTE
If using grills to cover the two openings, factor in the
free area of the grill. Typically, a sidewall grill will
have a free area approximately 50% of its nominal
size. Consequently, if the required opening is 10" x
10" (254mm x 254mm), it will have to be doubled if
using a sidewall grill with 50% free area.
IMPORTANT:
IF AN EXHAUST FAN, FIREPLACE, CLOTHES DRYER OR ANY SIMILAR DEVICE IS PRESENT IN THE
INDOOR AREA FROM WHICH THE COMBUSTION AND VENTILATION AIR WILL BE DRAWN, NEGA-
TIVE PRESSURE COULD BE A PROBLEM IF NATURAL INFILTRATION FROM THE OUTDOORS DOES
NOT MATCH THE RATE AT WHICH AIR IS EXHAUSTED.
FIGURE 4 - COMBUSTION/DILUTION AIR FROM
HEATED INSIDE SOURCES (CASE 2)
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