 Thermal expansion
Heating water causes the volume of water to expand.
Since water does not compress, the pressure inside water heater builds
up.
Normally the excess pressure is not a problem because it pushes out
into the cold water line and back to city water supply (or water well
tank) where the pressure is equalized by the water supply.
Closed
system
Problems occur with water heater when the pressure cannot travel out
the cold water line.
Check
valves and incoming pressure
reducing valves will block the pressure, and create a closed system
where dangerous pressure builds up inside the water lines.
A check valve is a
one-way valve that lets water flow one direction, but not the other
direction. For example, a recirculation system and other plumbing
installations sometimes have a one-way check valve. Sometimes city
water meters come with a check valve.
The check valve stops the water from expanding back out into the cold
water line, and creates a dangerous build up of pressure especially
when tank is heating water. Evidence of a closed system
is a TP valve that releases water whenever tank is heating, or a
buldging water heater, or even a ruptured tank if it has become rusted
over time.
The
solution to a closed system is to
install a correctly-sized expansion tank anywhere on the incoming cold
water line between tank and check valve ... the result: pressure from
heated water expands into the expansion tank.
Another problem
Buy:
Expansion
tanks at Amazon
Resource:
Expansion Tank Installation pdf
Closed system / pdf |
Thermal expansion
'The water in a water heating system
expands when it is heated and has a greater volume. Since water does
not compress like air, the expansion causes high pressure inside the
water heater and plumbing system.
Thermal expansion, and the resulting high water pressure are a major
consideration when designing water heaters and plumbing systems.
For example, residential water heating tanks are rated at 300 psi (when
new), with 150 psi maximum safe operation.
The TP valve releases water pressure when pressure exceeds 150 psi ....
but 150 psi poses a serious risk to older tanks that are rusted, and
for plumbing pipes and fixtures in general. High pressure causes
plumbing failures and early demise of heater.
Maximum incoming water pressure is 80 psi, with 50 psi recommended.
If pressure is above recommended, then a pressure reducing valve
is installed, along with an expansion
tank. The pressure-reducing valve and correctly-sized
expansion tank are installed on incoming cold water line
Normal water heater operation will push the pressure back through the
incoming cold water line, and out to the water main, or push pressure
back into the water well tank.
Water expands approximately .00023% per degree F
temperature rise.
In a normal
system (not closed): ''Water expands approximately 2% in volume for a
100°F temperature rise (from 40°F to 140°F) ....40 gallon water heater,
water being heated to its thermostat setting will end up expanding to
about 40.53 gallons.'' This amount of thermal expansion, equal to about
1/2 gallon of water, can be easily absorbed back into the incoming cold
water line.
In a closed system, the water expansion dynamic is much different.
If the incoming cold water line has a check valve at water meter, a
pressure reducing valve (PRV), or sometimes a water softener can cause
the problem, the pressure from thermal expansion will get backed up.
The expanding water cannot be pushed out into the water main, and the
pressure builds up because water doesn't compress.
For example, in a closed system where water is confined to a storage
tank, a temperature rise of just 10°F (increasing from
75° to 85°F), increases pressure from 50 psi to 250 psi. This amount of
pressure will quickly exceed the TP rating, causing water to be
released in a series of short bursts, except when water is being used.'
Troubleshoot TP valve
"...water will not boil under pressure until the temperature reaches
approximately 297° F. The energy potential in the superheated water is
called latent heat energy and will flash to steam when exposed to
normal atmospheric pressure. This flash to steam (steam explosion) has
the explosive potential of over two million foot-pounds of energy."
Symptoms
of thermal expansion
• ''Expulsion of water from TP. 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 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.
• 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.''
''Periodic
discharge of the temperature and pressure relief valve may be due to
thermal expansion in a closed water supply system. The water utility
supply meter may contain a check valve, backflow preventer or water
pressure reducing valve. This will create a closed water system. During
the heating cycle of the water heater, the water expands causing
pressure inside the water heater to increase. The temperature
and
pressure relief valve may discharge hot water under these conditions
which results in a loss of energy and a build-up of lime on the relief
valve seat. To prevent this from happening, there are two
recommendations:
1. Install a 125 PSI pressure relief valve in the
cold water supply line. Make sure the discharge of this valve is
directed to an open drain and
protected from freezing.
2. Install
a diaphragm-type expansion tank that is suitable for potable water on
the cold water supply line. The expansion tank must have a minimum
capacity of 1.5 U.S. gallons for every 50 gallons of stored water.''
|
 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.''
Buy:
Water
pressure gauges
Temperature
pressure gauge
Resource
Troubleshoot and identify water heater valves |
 TP
valve / temperature-pressure
valve
Mechanical 2 in 1 protection against excessive pressure and excessive
temperature.
TP valve will release water when conditions exceed TP valve rating,
causing cold water to enter tank. This reduces both pressure and
temperature. TP valve automatically closes when emergency
conditions are over. Lift and gently lower
TP valve once each year.
Residential water heaters are pressure tested to 300 pounds per square
inch (PSI)
and have a working pressure of 150 PSI. T&P valve
will open when the pressure inside the tank exceeds
150 PSI, allowing pressure to vent safely. T&P valve will open
if water temperature at top of tank reaches 210°F depending on water
heater,
and TP valve rating. Commercial heaters can be rated for higher
temperatures and pressures than typical residential heaters. TP valve
must be rated for more BTU than tank rating so that release of
temperature and pressure can outpace ability of tank to heat water.
 Buy: TP
valves:
Replacement
TP valves at Amazon
Teflon
tape at Amazon
Resource:
How to troubleshoot and replace TP
valve
TP valve code |
Reduce
pressure with
expansion tank
Pressure
over 80 psi can damage water heater and pipes over time.
150 PSI maximum allowable working pressure on residential water heater,
but water pressure increases when heated.
Install expansion tank anywhere on
cold water line, between any check
valve or pressure reducing valve and water heater
Hot water will expand back into water line and then into expansion
tank, preventing
long-term damaging to pipes and fixtures and water heater
Closed system:
Water
confined to a storage tank or piping system, when subjected to a
temperature rise of 10°F (increasing from 75° to 85°), increases
pressure from 50 psi to 250 psi.
Normal system
without check valves etc: Water expands approximately 2%
in volume for a 100°F temperature rise (from 40°F to 140°F) ....40
gallon water heater, water being heated to its thermostat setting will
end up expanding to about 40.53 gallons.
Buy:
Expansion
tanks at Amazon
Resource:
Expansion Tank Installation pdf
Closed system / pdf |
