RESIDENTIAL GAS AND ELECTRIC WATER HEATER
SERVICE HANDBOOK
A.O. Smith Water Products Company
Training Department
©
2002
Ashland City, Tennessee
57
TECHNICAL BULLETIN
BULLETIN 45
THERMAL EXPANSION
SYMPTOMS
•
Effects are only noticeable after hot water use followed by periods of no water use.
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Relief valve drips during any recovery cycle when no hot or cold water is used.
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Hot water pipes creak while heater is recovering and all valves are closed.
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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.
•
Faucets drips during any recovery cycle when no hot or cold water is used.
Water surges when a valve is first open and then pressure drops.
CAUSE
The water in a water heating system expands when it is heated and has a greater
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 at the water meter to prevent any
possible contamination of the public water supply by the accidental back-flow 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 fixture 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-flow preventers.
TEST
Follow these easy steps to diagnose thermal expansion:
•
Turn the heater thermostat all the way down, and install a water pressure gauge with dead
hand (AOS part #4798) 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 fixture in the system, hot or cold, is open. Make sure
that outside fixtures, 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 thermostat back up to its normal position, 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.
