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Example:
(294,000 / 1000) x 50 = 14,700
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 suffi cient air.
Use Op on 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 ven la on.
A3: Check that combus on
ven la on is adequate
Because modern homes are o en
well-sealed to prevent dra s, a
large room may not provide enough
combus on air without ven la on.
Confi rm that your installa on has
enough combus on air.
Op on B: Installa on with
outside ven la on
Ven la on with outside air is
recommended, and, for most
installa ons, is needed. There may be
exis ng ven la on that is adequate,
or you may need to add more
ven la on.
Supplying outside air typically requires
two openings. One opening must be
within 12 inches from the fl oor 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 6. Two openings
must be used when ven la ng with
air from another room.
B1: Determine type of
ven la on
There are several types of ven la on
that can be used:
1. Direct to outdoors
2. Ver cal ducts
3. Horizontal ducts
4. Single opening (not recommend-
ed); must be at least 100 square
inches. Not appropriate for
confi ned spaces smaller than 50
cubic feet per 1,000 BTU/HR as
calculated in sec on A or when
ge ng air from another room.
5. From a larger room inside the
house (not recommended —
refer to sec on A to determine
if the combined volume of the
room may be adequate)
B2: Determine minimum free
area required for each vent
opening
The size of the vent openings depends
on the total BTU/HR ra ng of all
appliances in the space (use your
calcula on from "Before Beginning")
and the type of vent used. Table 6
provides the minimum free area for
each vent opening depending on the
type of ven la on.
B3: Calculate minimum size of
vent openings and ducts
The vent cross-sec onal 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
grille 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-sec onal area:
Cross-sec onal area = minimum free
area required ÷ percent free area of
covering (in decimals – e.g., 60% = .6)
Residen al Gas Tankless Water Heater Use and Care Guide • 19
INSTALLATION
For example, an installa on 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
your louver or grille, 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
fl ow:
• Each vent opening must be no small-
er than 100 square inches.
• Ducts must have the same cross-sec-
onal 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.
• Movable louvers must be locked
open or interconnected with the
equipment so that they open auto-
ma cally during opera on.
• Keep louvers and grilles clean and
free of debris or other obstruc ons.
B4: Check that air source is
clean and free of chemicals
Air for combus on and ven la on
must be clean and free of corrosive
or fl ammable chemicals. A failure due
to corrosive chemicals in the air is not
covered by the warranty. Combus on
air must be free of acid-forming
chemicals such as sulfur, fl uorine, and
chlorine. Be sure that air at the vent
inlets is free of such chemicals.
B5: Check that combus on
ven la on is adequate
To confi rm that your installa on has
enough combus on air.
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