Water Heater Temperature & Pressure Relief Valves

Water heaters have the ability to become a pressure vessel should their internal pressure or temperature exceed safe limits, so water heaters have a device mounted either at the top or on the side (within 6” of the top) that helps prevent an unsafe condition. This device is a temperature and pressure relief valve, more commonly known in the home inspection and plumbing industries as a “T&P Valve”. T&P valves are typically designed to open at a pressure of 150 psi and at a temperature of 210° F. By opening, amounts of water (small or large) will discharge out of this valve. Water exceeding this pressure or temperature (if not discharged through the T&P valve) may allow the water heater to shoot through your home like a missile hundreds of feet up into the air. Several videos on the internet (including a demonstration done on the Discovery Channel show MythBusters) exist that demonstrate this extreme situation.

In normal operation of the water heater, the valve should remain closed and discharge no water, however, in some cases, the valve will open for safety reasons. The T&P valve may be more prone to open if no thermal expansion tank is installed. When water is heated, it also expands. An expansion of almost a gallon of water may occur in a 50 gallon water heater. If there is no place for this expansion to occur, such as within a thermal expansion tank, this pressure builds within the water heater tank and something must give.

An uninstalled top-mounted T&P valve

The T&P valve’s thermostat arm extends into the top 6” of the hot water tank and is exposed to the water at the top of the unit. Plumbing standards require a T&P valve on all water heaters; these same standards also require a proper discharge pipe connected to the T&P valve to aim this potentially dangerous hot water to a safe location. Two common flaws found during home inspections are either: (1.) No installed discharge pipe, or (2.) the T&P discharge pipe is not hot water rated (i.e., PVC). The lack of a proper discharge pipe can injure (such as 2^nd or 3^rd degree burns due to scalding) or kill someone standing near the water heater should the T&P valve open. Sometimes, the T&P valve is sticking out more than a few inches from the water heater. Some manufacturers do make T&P valves with a longer-than-normal sensor to allow for this situation; the T&P valve’s tag or model number may indicate if the T&P valve with the longer sensor is installed. If the inspector can’t verify this, it should be written up for review by a licensed plumber to ensure the proper T&P valve is installed.

The T&P discharge pipe must be rated for hot water; PVC pipe is not hot water rated and must not be used as a T&P discharge pipe. PVC pipe is only cold water rated and can melt or burst if used for pressurized hot water. Almost any other type of plumbing supply pipe may be used for the T&P valve’s discharge pipe, such as: copper, PEX, CPVC, and polybutylene (PB). PVC pipe is white and will have “PVC” stamped on the pipe’s exterior for easy identification. Some white piping is actually approved for T&P valve discharge use and will specifically state this on the pipe normally in red print.

The discharge pipe must be the same size as the T&P valve. A smaller pipe may not be used as the pressure will increase if the valve opens and the water is now constricted through a smaller pipe as it is discharged. The discharge pipe should also have few bends and must drain downwards to a readily visible location. In a typical basement installation, the discharge pipe should terminate about 6” above the basement floor. If there is living space nearby, such as in a mobile home installation, the discharge pipe will normally drain through the floor into the crawl space below the home. Also, the bottom of the discharge pipe may not be threaded; this is to prevent possible capping of the discharge pipe. Doing so, would prevent proper T&P valve operation and could present a hazard should an over-temperature or over-pressure situation exist.

T&P valve with a proper copper discharge pipe installed.

The T&P discharge pipe must be the same size (not smaller) as the T&P valve. Think of a garden hose with your thumb over the end; constricting the flow will greatly increase the water pressure.

The photo below shows a T&P valve with no discharge pipe installed. As you can see, someone standing near the water heater could get scalded if the valve were to open. The valve could either aim very hot water directly at someone or could aim this very hot water toward a wall where it could splash back and scald someone standing nearby.

Since our local water supplies, either public or private well, are typically very hard, T&P valves should be tested at least yearly to ensure that minerals haven’t stuck to the valve’s interior or blocked it preventing proper operation. T&P valves have a little metal arm or tab on them that will open the valve. Discharging only a small amount of water (maybe a cup) is needed for this test. If no water comes out when this test is performed, then the T&P valve should be promptly replaced by a licensed and qualified plumber.

In normal day-to-day operation of the water heater, no water will come out of the T&P valve and its drain pipe. Should a few drops of water be noted on the floor below the discharge pipe, this likely indicates that the valve is doing its job, however monitoring this area is suggested. Should large amounts of water be noted, however, this likely indicates an issue warranting prompt professional repair. A new T&P valve or a pressure regulator may be needed or a thermal expansion tank may be needed depending on the cause. A licensed and qualified plumber should be consulted to determine the cause and perform the needed repair.

As part of a home inspection, the home inspector will not test (open) the temperature and pressure (T&P) relief valve, although the inspector should confirm that one is installed on the water heater and note whether the installed discharge pipe is the proper material and size and terminates to an appropriate location.

IRC section P2804.6.1 covers Water Heater T&P Valve Discharge Pipes.

Matthew Steger is a Certified Level 1 Infrared Thermographer, an ASHI Certified Inspector (ACI), and an electrical engineer. He can be reached at matthew@thehomeinspectorsnotebook.com. No article, or portion thereof, may be reproduced or copied without prior written consent of Matthew Steger.