It’s called at TPR valve, which is an acronym for Temperature and Pressure Release, and also sometimes referred to as a T&P valve. TPRs are designed to automatically release water in the event that pressure or temperature in the water tank exceeds safe levels.

   If a TPR valve malfunctions, water in the system may become superheated (exceed the boiling point). Once the tank ruptures and water is exposed to the atmosphere, it will expand into steam almost instantly with explosive force. This process can propel a heating tank like a rocket through multiple floors, causing personal injury and property damage.

   Water heater explosions are rare, because they require a simultaneous combination of unusual conditions and failure of redundant safety components--usually the result of negligence or malfunctioning equipment.

   The TPR valve will activate if either water temperature (measured in degrees Fahrenheit) or pressure (measured in pounds per square inch [PSI]) exceed safe levels. The valve should be connected to a discharge pipe (also called a drain line) that runs down the length of the water heater tank. This pipe is responsible for routing hot water released from the TPR to a proper discharge location.

   The TPR valve should meet these requirements:

•  Be constructed of a plumbing material rated for hot water, typically CPVC, copper, or galvanized steel. PVC and other non-approved plastics should not be used since they can easily melt.
•  Be the same diameter, or larger, than the supply pipe serving the water heater.
•  Be as short and as straight as possible so as to avoid undue stress on the valve and installed so that it drains by gravity (downward flow).    
•  Not be trapped, since standing water may become contaminated and backflow into the drinking water.
•  Discharge to within 6-inches of the floor, or to a waste receptor with an air gap, or to visible exterior location. It should not be directly connected to the drainage system to prevent backflow, which potentially can contaminate the drinking water.
•  Not have a valve, T-fitting, or threaded end, to avoid any scenario that might result in a restriction or capping of the discharge piping.

   A properly functioning TPR valve will eject a powerful jet of hot water from the discharge pipe when fully activated, not a gentle leak. If a TPR valve is just leaking slowly, it is an indication that it needs to be replaced. In the rare case that the TPR valve does activate, you should immediately shut off the water and contact a qualified plumber for assistance and repair.

Although most TPR valves never become activated, it is an essential safety component on domestic water heaters. Guidelines concerning these valves and their discharge pipes reflect real hazards that every homeowner should take seriously.

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Here’s links to a collection of more blog posts about WATER HEATERS:

• Are water heaters required to be raised off the floor? 

• Can a Temperature and Pressure Relief (TPR or T&P) valve be mounted to discharge horizontally?

• Where are gas water heaters not allowed to be installed?  

• Is the minimum size water heater inlet pipe 1/2" or 3/4" according to the building code? 

• Can you use a light switch for a water heater disconnect? 

• How can I tell if a water heater is HUD-approved for mobile/manufactured homes?

• Can you wire a 240-volt water heater with 120 volts? 

• Is it alright to have a shut-off valve on both the hot and cold water pipes at a water heater?  

• What is the minimum clearance to doors and windows for an outdoor tankless gas water heater? 

• What is required clearance for access and working space in front of an electric water heater? 

• Why is the water heater older than the house? 

• Does a water heater need a shut-off valve? 

• Why should a tankless water heater have an isolator/service valve kit installed?

• When was a gas water heater first required to be elevated 18 inches above a garage floor?  

• Can the temperature and pressure relief (TPR) valve discharge pipe for a mobile/manufactured home water heater terminate under the home? 

• What is the purpose of a thermostatic mixing valve above a water heater? 

• Does a tankless gas water heater still work with no electricity during a power outage?  

•  Why do water heaters have a sacrificial anode? 

• What is the difference between a manufactured/mobile home water heater and a regular water heater? 

• Does a tankless water heater require a pressure relief valve?

• When is a water heater drain pan required?

• Why is there water in my water heater drain pan?

• What does it mean when a water heater Temperature/Pressure Relief (TPR) discharge pipe is "trapped"? 

• Can I leave a gas water heater in place when remodeling a garage into a family room or bedroom? 

• Where do I find the water heater in a mobile home?

• Does a tankless water heater in an attic require a drain pan? 

• Does an electric water heater require a disconnect? 

• Is a catch pan and drain piping required for a replacement water heater?

• What is the difference between a single element and dual element electric water heater? 

• What is an FVIR water heater?

• What is a dielectric union?

• What's that powdery crust on the pipe connections at the water heater? 

   Visit our WATER HEATERS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.