A.O. Smith GPVL-50 Service Handbook page 34

SYMPTOMS • Effects are only noticeable after hot water use followed by periods of no water use.
• Relief valve drips during any recovery cycle when no hot or cold water is used.
• Hot water pipes creak while water heater is recovering and all valves are closed.
• Tanks or other components of the water supply system fail prematurely.
• A metallic creaking noise might actually be heard in the location of the heater as the pressure is
relieved and the stretched tank returns to a natural shape.
• Faucet drips during any recovery cycle when no hot or cold water is used.
• Water surges when a faucet is fi rst opened and then pressure drops.
CAUSE The water in a water heating system expands when it is heated and increases in volume. Since water
will not compress (like air), system designers must include provisions for thermal expansion. (Water
in a closed tank at 50 psi, when heated just 10 degrees, will reach a pressure of 250 psi). Many
water supply systems have check valves or back fl ow preventers at the water meter to prevent any
possible contamination of the public water supply by the accidental back-fl ow of contaminated water
into the supply mains. These check valves are often required by code, and some cities are even
installing the check valves. They serve a useful purpose. Do not remove them! The use of pressure
reducing valves (PRV) is another cause. PRVs are designed to conserve water and prolong fi xture
life. Many PRVs also act as very effective check valves. Again, do not remove them! Water softeners
in the system may also act as back-fl ow preventers.
TEST Follow these easy steps to diagnose thermal expansion:
• Turn gas control switch to the "OFF" position, and install a water pressure gauge with dead hand
on the drain valve. Open the drain valve, so the gauge reads system pressure.
• Open a hot water tap and allow 15% to 20% of the tanks volume to run out. Shut off the drain valve
and make sure that no other fi xture in the system, hot or cold, is open. Make sure that outside
fi xtures, if they are on the same system, are turned off too. Any water leaks or use will make the
test meaningless.
• Check the water pressure gauge, and turn the pointer so it lines up with the pressure indicating
needle. Turn the gas control switch "ON", so the heater cycles on. Watch the pressure gauge.
• If the system is closed, the pressure will start to climb steadily and rapidly. A small amount of
thermal expansion control may be built into the system because of trapped air pockets or a water
hammer arrestor. In that case the pressure will increase slightly, hold steady for a short time and
then rapidly increase. The temperature and pressure relief valve (T&P) or PRV should open and
release water once the pressure reaches the maximum setting on the valve. The valve will close
once the pressure falls below the pressure setting of the valve.
THE FIX The ideal fi x involves the use of a pressure reducing valve if supply pressures are above 60 to 70 psi,
and a properly sized expansion tank. The PRV reduces supply pressures to 40 to 60 psi allowing an
economically priced and sized expansion tank to be used. The PRV also offers the benefi t of saving
water and prolonging the life of water fl ow valves. The PRV is not required if the system already has
one or if high supply pressures are desired. A supply water pressure of 80 PSI is a maximum set
by many codes. The PRV is installed between the check valve and the water heating system. The
expansion tank is installed between the PRV and the water heating system. Follow the manufacturers
instructions for installing the expansion tank. Run the thermal expansion check again. The pressure
should increase only slightly then hold steady throughout the recovery cycle. The expanded water
is fl owing back from the heater and into the pressurized storage bladder of the expansion tank. Air
pressure will force this water out of the expansion tank into the supply once usage resumes. DO NOT
DEPEND ON THE TEMPERATURE & PRESSURE VALVE (T&P VALVE) TO HANDLE THERMAL
EXPANSION! The T&P valve manufacturers designed the valve to be an emergency relief device
only. The T&P Valve could be subject to reduced effectiveness or failure.
WARNING Thermal expansion of water, if not compensated for in system design, will lead to the early failure
of components. These failures are not covered by the manufacturer's warranty, so it is extremely
important that everyone be aware of the causes, symptoms and solutions to thermal expansion in
a closed water heating system.
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THERMAL EXPANSION