Below are Pictures of relief valves one on a boiler and the other on a water heater they are both designed to relieve the pressure from the system if the pressure gets to high. this device must be on the system, and must be in good working order. A defective relief valve can cause a water heater or a boiler to explode, causing major damage to your home and can cause severe harm and death. It must be tested every year to be sure if is working properly. 
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Boiler relief valves and water relief valves are NOT interchangeable A boiler relief valve will blow of at 30 psi, A water heater Relief valve will pop at 150 psi and or 210% F If you need to replace a relief valve make sure it is the right one for your system. The following is the Warning that is on the long label attached to the relief valve.

WARNING:

FAILURE TO COMPLY WITH THESE INSTRUCTIONS REGARDING THIS VALVE CAN RESULT IN SERIOUS INJURY OR DEATH AND/OR SEVERE PROPERTY DAMAGE.

ANNUAL OPERATION OF T&P RELIEF VALVES

WARNING: Following installation the valve lever MUST be operated AT LEAST ONCE A YEAR by the water heater owner to ensure that waterways are clear. naturally occurring mineral deposits may adhere to the valve, blocking waterways, rendering it inoperative.
When the lever is operated, hot water will discharge, if the waterways are clear.
PRECAUTIONS MUST BE TAKEN TO AVOID PERSONAL INJURY FROM CONTACT WITH HOT WATER AND TO AVOID PROPERTY DAMAGE.
BEFORE operating lever, check to see that a discharge line is connected to this valve, directing the flow of hot water from the valve to a proper place of disposal.
If no water flows when the lever is operated, Replacement of the valve is required, TURN THE WATER HEATER "OFF" (SEE YOUR INSTRUCTION MANUAL) AND CALL A PLUMBER IMMEDIATELY.

REINSPECTION OF T & P VALVE RELIEF VALVES:

WARNING: Temperature and pressure relief valves should be inspected  AT LEAST ONCE EVERY THREE YEARS, And replaced if necessary, by a licensed plumbing contractor or qualified service technician, to ensure that the product has not been affected by corrosive water conditions and to ensure that the valve and discharge line have not been altered or tampered with illegally. Certain natural occurring conditions may corrode the valve or it's components over time, rendering the valve inoperative. Such conditions can only be detected if the valve and its components are physically removed and inspected. Do not attempted to conduct an inspection on your own. Contact your plumbing contractor to REINSPECTION to assure continuing safety.
 

WARNING:

FAILURE TO REINSPECT THIS VALVE AS DIRECTED COULD RESULT IN UNSAFE TEMPERATURE OR PRESSURE BUILD UP WHICH CAN RESULT IN SERIOUS INJURY OR DEATH AND/OR SEVERE PROPERTY DAMAGE.
 

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