Combustion Air From Outside The Building; Two-Permanent-Openings Method (Usa Only) - Watts AERCO MFC Series Pre-Installation Application Manual

MFC Series Boiler Vent and Combustion Air Guide

4.2 Combustion Air from OUTSIDE the Building

Outdoor combustion air shall be provided through opening(s) to the outdoors in accordance with
the methods described below. The minimum dimension of air openings shall not be less than
3 inches (80 mm). The required size of the openings for combustion air shall be based upon the
net free area of each opening. When the free area through a louver, grille, or screen is known, it
shall be used to calculate the opening size required to provide the free area specified. For
additional details, consult NFPA 54, or in Canada, CSA B149.1-10, paragraphs 8.4.1 and 8.4.3.

4.3 Two-Permanent-Openings Method (USA Only)

Two permanent openings shall be provided; one commencing within 12 inches (300 mm) of the
top of the enclosure and one commencing within 12 inches (300 mm) of the bottom. The openings
shall communicate directly ― or by ducts ― with the outdoors, or spaces that freely communicate
with the outdoors, as show on the following pages:
•
When communicating directly with the outdoors, or when communicating to the outdoors
through vertical ducts, each opening shall have a minimum free area of 1 inch
4,000 BTU/hr (550 mm
and 4).
Figure 3: All Combustion Air From Outdoors - Inlet Air From Ventilated Crawl
TAG-0080_OG • GF-148-V •
2
/kW) of total input rating of all appliances in the space (see Figures 3
Space and Outlet Air to Ventilated Attic
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