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Relief Valve: Definition & Professional Guide

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A relief valve is a safety device that automatically opens to discharge fluid when pressure or temperature inside a closed system exceeds a predetermined setpoint, then recloses when conditions return to safe levels. In residential plumbing, the most critical relief valve is the temperature and pressure (T&P) relief valve installed on every storage water heater, which prevents catastrophic tank rupture by releasing water when temperature exceeds 210 degrees Fahrenheit or pressure exceeds 150 PSI.

Spring-Loaded Disc, T&P Sensing & Thermal Expansion Overpressure

Relief valves function as the last line of defense against overpressure conditions that could cause equipment failure, property damage, or injury. When a closed system — such as a water heater or boiler — generates pressure beyond its design limits, the relief valve spring compresses and the valve disc lifts off its seat, opening a discharge path to atmosphere. Water or steam escapes through the valve and discharge pipe, reducing system pressure. When pressure drops below the setpoint, the spring pushes the disc back onto the seat, resealing the valve.

The T&P relief valve on a residential water heater is arguably the single most important safety device in the home plumbing system. A water heater with a failed thermostat can continue heating water past the boiling point. In a closed system (one with a check valve or backflow preventer on the supply line and no expansion tank), thermal expansion generates pressure that can exceed the tank’s burst strength. Without a functioning T&P valve, the superheated water can flash to steam upon tank failure, creating an explosion with enough force to launch the water heater through multiple floors of a building.

The T&P valve must discharge through a pipe that terminates within 6 inches of the floor or to an approved exterior location. The discharge pipe must be the same diameter as the valve outlet (typically three-quarters of an inch), cannot be reduced in size, and cannot be connected to the drain system. These requirements ensure that the discharge can flow freely without restriction.

T&P, Pressure-Only, Thermal Expansion & Boiler Safety Valve Types

Temperature and pressure (T&P) relief valves combine both temperature and pressure sensing. A temperature probe extends into the tank water, and a pressure-sensing element monitors system pressure. Either condition exceeding its setpoint opens the valve.

Pressure-only relief valves respond only to system pressure and are used on systems where temperature is not a concern, such as cold water supply lines and pressure vessels.

Thermal expansion relief valves are small-capacity valves installed on closed water supply systems to relieve the modest pressure increases caused by thermal expansion during heating cycles.

Boiler safety valves are heavy-duty relief valves rated for steam applications, with higher capacity and faster response than standard water heater T&P valves.

T&P Testing, Discharge Pipe Inspection & Water Heater Safety Services

Bonded Plumbworks’ plumbers inspect and replace T&P relief valves during water heater services — both routine maintenance visits and water heater installations. T&P valves have a limited service life and should be tested annually by lifting the lever to verify they open and reseat properly. The valve should be replaced every 5 years or whenever it fails to operate correctly during testing.

During plumbing inspections, Bonded Plumbworks checks for missing discharge pipes, improperly terminated discharge lines, and T&P valves that have been removed, plugged, or replaced with standard valves — all dangerous conditions that require immediate correction.

ASME/ANSI Z21.22, ASSE 1017 & State Code Discharge Requirements

ASME/ANSI Z21.22 governs relief valves for hot water supply systems. The State building codes require T&P relief valves on all storage water heaters and specifies discharge pipe requirements. ASSE 1017 covers temperature conditioning valves. The valve must be rated to match the water heater’s BTU input and storage capacity.

Watts 100XL, Cash Acme & Apollo ASME-Rated Relief Valves

Watts manufactures the 100XL series T&P relief valves, the most widely installed brand in residential water heaters. Rheem and A.O. Smith include Watts valves as OEM equipment. Cash Acme produces the F-82 Nozzle series. Apollo offers ASME-rated relief valves for commercial applications.