22 • Residential Standard Gas Water Heater Service Handbook
SUPPLEMENTS
room to get a better estimate of the air
available.
Room Volume – Object Volume = Air
Volume
A2: Calculate required air volume
A water heater installed in an unconfined
attic or garage requires that the space
be at least 50 cubic feet per 1,000 BTU/
HR of the total input for all gas burning
appliances in the same area.
[Total BTU/HR/1000] x 50 = Cubic feet
of air required.
Example:
(135,000 / 1000) x 50 = 6,750
If the air volume of the room is less
than the required air volume, you must
provide two permanent outside air
openings that draw in sufficient air. Use
Option B.
If the air volume of the room is greater
than the required air volume, it may
be possible to install the water heater
without outside ventilation.
A3: Check that combustion
ventilation is adequate
Because modern homes are often
well-sealed to prevent drafts, even a
large room may not provide enough
combustion air without ventilation.
To confirm that your installation has
enough combustion air, conduct the
vent draft test on page 11 when
installation is finished.
Option B: Install with outside
ventilation
Ventilation with outside air is
recommended, and, for most
installations, is needed. There may be
existing ventilation that is adequate, or
you may need to add more ventilation.
Supplying outside air to typically
requires two openings. One opening
must be within 12 inches from the floor
and the second opening must be within
12 inches from the ceiling. Although a
single opening is not preferred, you may
use a single opening to outside air if the
minimum free area is sized according
to Table 5. Two openings must be used
when ventilating with air from another
room.
The outside air can be taken from a crawl
space or attic open to the outdoors and
adequately ventilated. You may use
vertical or horizontal ducts.
B2: Determine type of
ventilation
There are several types of ventilation
that can be used :
1. Direct to outdoors
2.
Vertical
ducts
3.
Horizontal
ducts
4. Single opening (not recommended;
must be at least 100 square inches.
Not appropriate for confined
spaces smaller than 50 cubic feet
per 1,000 BTU/HR as calculated in
section A or when getting air from
another room.)
5. From a larger room inside the
house (not recommended – refer
to section A above to determine
if the combined volume of
the rooms may be adequate).
B3: Determine minimum free
area required for each vent
opening
The size of the vent openings depends
on the total BTU/HR rating of all
appliances in the space (use your
calculation from “Before beginning”)
and the type of vent used. Table 5
provides the minimum free area for
each vent opening depending on the
type of ventilation.
B4: Calculate minimum size of
vent openings and ducts
The vent cross-sectional area needed
to provide the free area depends on the
covering on the vent openings. Typical
vents use louvers or grilles to protect
the opening. The louver or grill itself
blocks some of the free area, so the
opening may need to be larger to meet
the minimum free area requirements.
Use the following formula to calculate
the required cross-sectional area:
Cross-sectional area = minimum free
area required ÷ percent free area of
covering (in decimals – e.g., 60% = .6)
For example, an installation area that
requires openings with 100 square
inches of free area would need 134
square inch openings if using metal
louvers rated at 75% free area (100 sq.
in. ÷ .75 = 134 sq. in.).
If you do not know the % free area for a
louver or grill, use the following values:
•
For wood louvers or grilles: 20%
•
For metal louvers or grilles: 60%
Follow these rules to ensure that vents
and ducts provide adequate air flow:
• Each vent opening must be no
smaller than 100 square inches .
• Ducts must have the same cross-
sectional area as free area of the
opening.
• Rectangular ducts must have a
minimum dimension of no less
than three inches .
• All screens must have mesh ¼” or
larger.
• Moveable louvers must be locked
open or interconnected with the
equipment so that they open
automatically during operation.
• Keep louvers and grills clean and
free of debris or other obstructions.
