Solving Problems Caused By High Water Pressure Due To Thermal Expansion
All liquids and most other substances, expand when there temperature is increased. Mercury and alcohol thermometers are excellent examples of thermal expansion.  As the temperature of the liquid in these thermometers is increased, it expands.  The expanding liquid rises in the thermometer's calibrated tube to indicate the temperature.  If the liquid is heated above a certain temperature, the thermal expansion will create sufficient pressure to literally blow the thermometer apart.  The thermometer blew apart because the liquid was enclosed in a sealed system (the glass tube): there was no room for further expansion.
A home's water system becomes a sealed system when the main supply line to the home has either a Backflow Preventer, water meter with a one way valve or a pressure reducing valve, which has no bypass.
Why do water companies install backflow preventers? The Environmental Protection Agency, in some areas, requires local utilities to install them at the water meter to prevent water from flowing back into the main supply from the home's water system.
If a home has a backflow preventer, water heated by the water heater will be allowed to expand only within the home's water system.  Since none of the expansion will be able to back into the main supply line, there will be a pressure increases throughout the system.  This increased pressure can cause dripping faucets, failure of appliance water inlet valves. banging pipes and periodic water flow from the water heater's temperature pressure relief valve.
In cases of sudden pressure surges, a gas water heater's flue could callapse and in an electric water heater, the bottom of the water heater can reverse.
GAS
(NOTICE)
Piping drawn together
and T&P valve dripping
ELECTRIC

(NOTICE)
T&P valve dripping
and bottom reversed

A Customer Complaint Of Water Weeping Or Discharging Periodically From The Temperature Pressure Relief Valve Should Be Handled In The Following Manner:

1. Check the water heater's temperature pressure relief valve for proper operation
a. If the water heater is gas, set the gas control to the pilot position.  If the water heater is electric, shut off power to the heater.
b. Open a hot water faucet and allow the flow of 4-5 gallons of water. then, shut off the faucet.
c. Manually open the water heater temperature pressure relief and determine if the valve will completely reset without water leakage,  If the valve dose not seal properly, a new valve should be installed.  If the valve closes properly, check water pressure.
d. Turn gas control to on, or turn on power to electric water heater.
2. Check the incoming water pressure.
a. Water pressure should be 80 PSI or less.
Important:  Since water pressure can fluctuate greatly, depending on water usage, water temperature, etc., a special water pressure test gauge, made by WATTS should be used.  Following the instructions packed with the gauge, install the gauge on a cold water sillcock and leave installed for a least 24 hours.  The red indicator on the gauge will indicate the highest pressure measured while installed.
3. If water pressure exceeded 80 PSI during the time period pressure was checked, thoroughly explain the problem and advise the customer of the following possible solutions.
* Install a pressure only relief valve anywhere within the closed system.  The setting on this pressure only valve should be approximately 10 PSI below the water heater's combination T&P valve.  The discharge pipe from the pressure only valve should be piped to a suitable drain or outside the home.  Explain that flow through this valve will usually not exceed a few quarts per day.
* If a functioning pressure reducing valve is not already in the supply line suggest that, if permitted by codes, one with a suitable bypass be installed.
* Explain that the best and surest way to solve the problem is the installation of an expansion tank suitable for potable water.