LP-517 Rev. 000 Rel. 019 Date 7.21.20
27
Figure 14 - Unbalanced Venting - Roof Exhaust and Sidewall Intake
Figure 18.
Combustion air requirements are based on the latest edition of
the National Fuel Gas Code, NFPA 54 / ANSI Z223.1, CGA Standard
CAN/CSA B149.1 in Canada. Check all local code requirements for
combustion air.
All dimensions based on net free area in square inches. Metal
louvers or screens reduce the free area of a combustion air opening
a minimum of approximately 25%. Check with louver manufacturers
for exact net free area of louvers.
Where two openings are provided, one must be within 12” of the
ceiling, and one must be within 12”of the floor of the mechanical
room. Each opening must have a net free area as specified in Table
10. Single openings shall commence within 12” of the ceiling. The
minimum dimension of air openings should not be less than 3”.
Under no circumstances should the mechanical room ever be
under negative pressure. Particular care should be taken where
exhaust fans, attic fans, clothes dryers, compressors, air handling
units, etc., may take away air from the unit. Failure to follow these
instructions could result in property damage or personal injury.
Combustion air supply must be completely free of any flammable
vapors that may ignite, or chemical fumes which may be corrosive
to the boiler. See Table 1 for a list of combustion air contaminants.
These chemicals, when burned, form acids which quickly attack
the stainless steel heat exchanger, headers, flue connectors, and
the vent system. The result is improper combustion and premature
boiler failure. Such failure IS NOT covered under warranty.
EXHAUST FANS:
Any fan or appliance which exhausts air from the
mechanical room may deplete the combustion air supply and/or
cause a downdraft in the venting system. Spillage of flue products
from the venting system into an occupied living space can cause a
very hazardous condition that must be corrected immediately.
Minimum Recommended Combustion Air Supply to Mechanical
Room
Model
Figure 15
Figure 16
*Outside Air from 2 Open-
ings Directly from Outdoors
1
*Outside Air from 2 Ducts
Delivered from Outdoors
1
Top Opening,
in
2
Bottom
Opening, in
2
Top
Opening, in
2
Bottom
Opening,
in
2
750
188
188
375
375
1000
250
250
500
500
Model
Figure 17
Figure 18
Inside Air from 2 Ducts Delivered from
Interior Space
2
*Outside
Air from 1
Opening
Directly
from
Outdoors,
in
2 1
Same Story
Different
Stories
Top Opening,
in
2
Bottom
Opening, in
2
Top
Opening, in
2
750
750
750
1500
250
1000
1000
1000
2000
335
Table 11 - Indoor Combustion Air Sizing
The above requirements are for the boiler only; additional gas fired
appliances in the mechanical room will require an increase in the net
free area and/or volume to supply adequate combustion air for all
appliances.
No combustion air openings are needed when the boiler is installed
in a space with a volume NO LESS than 50 cubic feet per 1,000 BTU/hr
of all installed gas fired appliances. Buildings MUST NOT be of *”Tight
Construction”.
1
Outside air openings shall communicate with the outdoors.
2
Combined interior space must be 50 cubic feet per 1,000 BTU/hr
input. Buildings MUST NOT be of *”Tight Construction”.