HTP CG300N65 - Manual (Page 9)

9
LP-436 REV. 3.21.14
additional fresh air may need to be provided. For instructions
on obtaining additional air supply, see the requirements for
confined spaces.
Requirements for Confined Spaces
A confined space is an area where the volume is less than 50
cubic feet for each 1,000 BTU/H of the total input rating for all
gas appliances installed in that space. Water heaters installed
in confined spaces require additional combustion and
ventilation air. This can be provided in two ways:
1. All Air from Inside the Building
The confined space shall be provided with two permanent
openings communicating directly with one or more rooms of
sufficient volume, so that the combined volume of all spaces
meets the criteria for an unconfined space. The total input
rating of all gas appliances installed in the combined space
shall be considered in making this determination.
Each opening shall have a minimum free area of one
square inch per 1,000 BTU/H of the total input rating of all gas
appliances in the confined space, but not less than 100 square
inches. One opening shall commence within 12” (30.5 cm) of the
top and one within 12” (30.5 cm) of the bottom of the enclosure.
See Figure 2.
2. All Air from Outdoors
The confined space shall be provided with two permanent
openings, one commencing within 12” (30.5 cm) of the top and
one commencing within 12” (30.5 cm) of the bottom of the
enclosure. The openings shall communicate directly, or by
ducts, with the outdoors or spaces (crawl or attic) that freely
communicate with the outdoors.
a. When communicating directly with the outdoors, each
opening shall have a minimum free area of one square inch per
4,000 BTU/H of the total input rating of all gas appliances in the
enclosure. See Figure 3.
b. When communicating with the outdoors through vertical
ducts, each opening shall have a minimum free area of
one square inch per 4,000 BTU/H of the total input rating of all gas appliances
in the enclosure. See Figure 4.
c. When communicating with the outdoors through horizontal ducts, each
opening shall have a minimum free area of 1 square inch per 2,000 BTU/H of
the total input rating of all gas appliances in the enclosure. See Figure 5.
When ducts are used, they shall be of the same cross-sectional area as the
free area of the openings to which they connect. The minimum short side
dimension of rectangular air ducts shall not be less than 3” (7.62 cm).
Louvers and Grilles
In calculating free area for ventilation and combustion air supply openings,
consideration must be given to the blocking effect of louvers, grilles, or
screens protecting the openings. Screens must not be smaller than ¼” (6.4
mm) mesh. If the free area through a particular design of louver or grille is
known, it should be used in calculating the size of opening required to provide
the free area specified. If the design and free area is not known, it may be
assumed the wood louvers and grilles will allow 20-25% free area and metal
louvers and grilles will allow 60-75% free area. Louvers and grilles must be
installed in the open position or interconnected with the water heater so that
they are opened automatically during water heater operation.
Figure 4 – Communicating with the Outdoors through Vertical Ducts
Figure 5 – Communicating with the Outdoors through Vertical Ducts