Bulging Water Heater
TECHNICAL BULLETIN
BULLETIN 41
BULGING HEATER
SYMPTOM
The heater has cockeyed nipples, top of the flue is pulled out of sight or
bottom of the head bulged or reversed. These conditions may also appear
when the heater is leaking.
CAUSE
A great number of water heaters are returned as “in warranty failures”. Many
times, the failure is actually due to excessive pressure. This is evident when
the bottom head is bulged or reversed. A.O. Smith water heaters are
manufactured and tested to withstand an internal pressure in excess of 300
psi without distortion. Because of the bulged bottom, it indicates that the tank
has been subjected to a pressure in excess of 300 psi.
Water in an ordinary storage tank is under a certain static pressure,
dependent on the supply pressure in the system. As this water is heated, its
volume increases, and if there is no check valve, pressure reducing valve, or
other obstruction in the cold water line, i.e. the system is open, a portion of
the water will back up into the cold water supply line. In an open system the
water pressure in the tank will always be approximately equal to the supply
pressure.
The presence of a check valve or a water pressure reducing valve, or the
closing of a shut-off valve in the cold water line makes the system a closed
system. In such a situation, water cannot back up into the cold water line and
the pressure resulting from the normal thermal expansion is not equalized.
Should the heating continue even for a short period of time, pressure exerted
by this combined volume of water may become great enough to rupture the
storage tank. There would be no explosion, however, since the water in the
tank is below the boiling point at atmospheric pressure.
It should be noted that any blocking of the supply piping such as the jamming
of a water meter disc may change an open system into a closed system.
Also, the installation of a water conditioning unit may convert an open system
into a closed one.
Our installation instructions, and instruction plates, all include the statement
that when a water heater is installed on a closed system, a correctly sized
thermal expansion tank must be installed, or our warranty is void. The
temperature and pressure relief valve is not intended to protect against
thermal expansion.
Printed in USA 498
Part No. TC-204-41
PRESSURE
TABLE
PRESSURE BUILD UP ON 30 GALLON HEATER
CONNECTED IN A CLOSED SYSTEM
Water Temperature (°F)
Tank Pressure (psi)
74
80
75
85
76
105
78
125
80
145
82
175
85
225
88
275
90
Tank bottom will start to reverse
310
95 and eventually break a weld and leak. 400
100
520
105
625
If the tank were strong enough at 140°F, the pressure in the tank would be in
excess of 1,000 psi.
THE FIX
Since many of the symptoms noted above are often noticed when the heater
is leaking, repair to the unit is not possible. However, prior to installation of a
new water heater, it is important to install a thermal expansion tank that will
help maintain a consistent working pressure.
While temperature and pressure relief valves required to be installed on the
water heater are mechanical devices that relieve overheated water or
excessive pressure, they cannot maintain a consistent pressure. The
installation of a thermal expansion tank is the only device recommended to
maintain a constant safe operating pressure.
CAUTION
Printed in USA 498
Water heaters installed in a closed system without a properly sized and
installed thermal expansion tank will void warranty. Therefore, if the heater
displays any of the symptoms noted above, it will not be eligible for warranty
consideration.
Part No. TC-204-41
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