Residential Standard Gas Water Heater Service Handbook • 25
SUPPLEMENTS
• The venting system must be installed
according to the water heater’s
installation instructions and local
codes. In the absence of local codes,
follow the National Fuel Gas Code
(ANSI Z223.1-current edition).
• The draft hood must be installed
properly and must not be modified
in any way.
• The water heater must not be
common vented with any power
vented appliance.
• Horizontal runs of vent should be
sloped upward from the water heater
by at least ¼-inch per foot. Ensure
that there are no dips or sags in the
vent run.
• The vent run should include as
few bends as possible. Each bend
increases resistance to the flow of
flue gasses. Refer to the applicable
installation code and the installation
instructions for the water heater.
• Venting must be free of obstructions
and corrosion.
• There must be no separation of joints.
Each joint must be fastened by sheet
metal screws or other approved
means. Also, vent must be supported
properly to prevent sags and joint
separation. Use of support straps or
other approved means is acceptable.
Decompression
Decompression can cause or contribute
to pilot outage. Decompression occurs
when the air near the water heater is
at slightly less pressure than the air
outside the building. Inside the home,
a classic decompression condition can
be caused by a large kitchen exhaust
fan. The negative pressure can cause
air to flow backwards through the
water heater vent, stall the air in the
flue, and cause the pilot to go out. The
same is true for a water heater in an
attic, but for different reasons.
For example, an attic may be cooled by
an attic fan, designed to pull hot air out
of the attic and move it to the outdoors.
This is a good solution for cooling an
attic, but it could cause problems for
an atmospheric gas water heater. If the
attic doesn’t have adequate free-air
openings, the attic fan could cause air
to flow backwards through the water
heater vent, causing the pilot to go out.
This, in turn, will force the water heater
to shut down. Increasing the number
or size of free air openings in the attic
may help to prevent this problem. See
“Insufficient Makeup Air” on p. 24.
Wind
Wind is only more critical for attic
installations when it occurs in
combination with one of the above
factors. The pressure needed to stop a
flue draft is less than 1/500th of an inch
of water column. If the draft through
the flue stops, the water heater will
shut down.
Water Damage
A water heater must be installed in
such a manner that if the tank or
connections leak, the flow will not
damage the living spaces below. A
suitable metal drain pan should be
installed under the water heater. Metal
drain pans are available at your local
hardware store. The pan must have a
minimum length and width of at least
2” (5.1 cm) greater that the water
heater dimensions and must be piped
to an adequate drain. Drain pan depth
must provide clearance for servicing
the igniter and burner. However, even
with the use of a drain pan piped to a
drain, water damage can still occur.
This risk can be reduced further by
installing a leak detector or water shut-
off device. You can buy such devices
from plumbing supply wholesalers or
retailers. Examples are listed below:
• Sensors mounted in the metal drain
pan that trigger an alarm or turn
off the incoming water to the water
heater when leakage is detected.
• Sensors mounted in the metal drain
pan that turn off the water supply
to the entire home when water is
detected in the drain pan.
• Water supply shut-off devices that
activate based on the water pressure
differential between the cold water
and hot water pipes connected to
the water heater.
• Devices that will turn off the gas
supply to a water heater while at
the same time shutting off its water
supply.
Elevated Water Temperature
DANGER!
• Water temperature over 120°F can
cause severe burns instantly or death
from scalds.
• Children, disabled and elderly are at
highest risk of being scalded.
• Thermostatic mixing valves at each
point of use reduce the risk of
scalding.
If the ambient temperature in the attic
is extremely high, it may gradually affect
the temperature of the water inside the
water heater. For example, assume that
a water heater is set to 120°F, but the
attic maintains a temperature of 130°F
or more during the summer months.
Over time, the water temperature in
the water heater may increase beyond
the desired 120°F. As a result, the
water temperature at the outlet may
be higher than expected.
A remedy would be to decrease the
ambient temperature in the attic. This
may be accomplished by improving
ventilation. However, in doing so,
be sure that you do not create a
decompression situation (e.g., using
an attic fan without adequate free
