24 • Residential Standard Gas Water Heater Service Handbook
SUPPLEMENTS
Elevated Air Temperature:
In some areas, attics can reach in excess
of 160°F. Such high temperatures can
cause atmospherically-vented water
heaters to stall and shut down.
Why? The flame arrestors, filters,
and sealed combustion chambers
that make modern water heaters
flammable vapor ignition resistant
also make them more sensitive to
high ambient temperatures. This is
because high temperatures cause
the air to expand, affecting draft and
depleting the oxygen available to the
pilot. In other words, when the pilot
flame consumes the oxygen in the
sealed combustion chamber and that
oxygen isn’t replaced by a proper draft,
the flame will go out. This, in turn, will
shut down the water heater.
This issue may be resolved by
increasing attic ventilation. Contact
a licensed contractor or a roofing
professional for guidance. See also
“Insufficient Makeup Air”
Insu
ffi
cient Makeup Air
Adequate make-up air is essential for
combustion. A water heater takes its
combustion air from the surrounding
area, and that air must be replaced
as quickly as the oxygen is consumed.
Two factors determine the availability
of makeup air: the attic’s volume
and the total combined area of the
openings that provide outdoor air.
At a minimum, the attic should have
50 cubic feet of volume per 1,000 Btu/
hr of the total input. For example, if
your water heater has an input of
30,000 Btu/hr, the attic should have
a minimum volume of 1,500 cubic
feet under normal circumstances.
This was calculated as follows:
(30,000 ÷ 1,000) 50 = 1,500.
Volume Requirement Formula:
(Inputs in Btu/hr ÷ 1,000) 50 =
minimum volume in cubic feet for
all gas appliances in the attic
Once you know the volume
requirement, you should compare it to
the actual volume of the attic. Use
this formula to calculate the volume of
the attic:
Attic Volume:
Length x Width x Height =
Attic volume in cubic feet
For example, if the attic space is 40’ x 50’
x 8’, its volume would be 16,000 cubic
feet. To provide ample combustion air,
the attic volume should be greater than
the volume requirement. If it is not,
and pilot outages have been an issue,
try increasing the attic’s ventilation.
Ventilation is always critical. Oxygen
must be replaced as quickly as it is
used. Otherwise, a pilot outage may
occur, causing the water heater to shut
down. Under normal circumstances,
a well-ventilated attic of the proper
size should provide plenty of makeup
air. However, you may need to
increase ventilation if the attic is
unusually “tight,” reaches extremely
high temperatures, or contains more
than one gas-fired appliance. Before
doing so, though, check the vents to
see if they are blocked. If they are,
unblocking them may help to improve
ventilation and lower the ambient
temperature.
If you need more makeup air, consider
an additional opening to the outdoors
as described in the National Fuel Gas
Code (ANSI Z223.1-current edition).
The opening should:
•
communicate directly with the
outdoors,
• commence within 12 inches from
the top of the attic enclosure, and
• be sized at a minimum of one
square inch for every 3,000 Btu/
hrs of total input for all gas-fired
appliances in the attic. Also, the
size of the opening must not be less
than the sum of the areas of all vent
connectors in the space.
• be protected by a louver or grille as
described in the current edition of
the National Fuel Gas Code.
Alternatively, you may be able to
increase the amount of makeup air
by increasing vent capacity. Note,
however, that your venting must
be configured to provide proper air
flow. Also, the combined area of
the vent openings should be no less
than one square inch for every 3,000
Btu/hrs of total input for all gas-fired
appliances in the attic. (Note: The
net free area will be listed in the
vent’s product specifications, making
verification easy.)
Contact a licensed contractor or a
roofing professional for guidance.
Blocked Air Screen or Flame Arrestor
Most gas water heaters today have
Flammable Vapor Ignition Resistant
(FVIR) designs. These designs include
some type of screen/flame arrestor
that reduces the likelihood that
flammable vapors in the room will
be ignited by the water heater’s pilot
flame or burner flame. Of course, the
screens, filters and arrestors used in
these designs must be kept clean so
that combustion air can flow freely
into the water heater. Dust and other
airborne contaminants can block or
reduce incoming air and cause the
water heater to shut down. Screens
and filters must be kept in place and
cleaned regularly. (See p. 18.) This
is especially important in areas that are
not cleaned regularly, such as attics.
Improper Ven ng of the Water Heater
Proper venting of the water heater
is crucial for trouble-free operation.
Verify the following:
