How Does a Pressure Relief Valve Work?

What is a Pressure Relief Valve?

A pressure relief valve, often referred to as a temperature and pressure relief (T&P) valve, is a safety device installed on water heaters. It’s typically located on the top or side of the tank, with a discharge pipe extending downward. This valve is designed to open automatically when the pressure or temperature inside the tank reaches unsafe levels.

Why Do You Need It?

As water heats up in the water heater tank, it expands and creates pressure. If the pressure inside the tank exceeds the water heater’s design limits, it can lead to tank failure—potentially causing explosions or significant water damage.

Similarly, excessive temperature can cause the water to reach boiling point, creating dangerous steam pressure.

The pressure relief valve acts as a safeguard against these risks by releasing excess tank pressure and lowering the water temperature in the water heater tank when needed.

How Does a Pressure Relief Valve Work?

The operation of a pressure relief valve is relatively straightforward but incredibly effective. Here’s a breakdown:

Sensing Excess Pressure or Temperature

The valve is equipped with a spring-loaded mechanism that responds to changes in pressure or temperature. When the pressure inside the water heater exceeds a preset level (usually 150 PSI for most residential models) or the temperature rises above a safe threshold (typically 210°F), the valve activates.

Releasing the Pressure or Steam

When excessive water temperature in the water heater tank builds up pressure above 150 PSI (the relief set point), that excessive pressure pushes against the pressure relief valve spring and opens the valve to release steam and water through the pressure relief pipe to the outside of your home.

This relieves the excess pressure and cold water refills the tank to lower the temperature, bringing the system back to safe operating conditions.

Automatic Reset

Once the tank pressure returns to normal levels, the spring on the pressure relief valve shuts the valve opening, sealing the tank again.

What Causes Excessive Temperature and Pressure in a Water Heater Tank?

Understanding what causes excessive temperature and pressure in a water heater tank can help you prevent dangerous situations and extend the lifespan of your water heater. A failure in the water that causes water to overheat will continue to trip the pressure relief valve until you troubleshoot and resolve the issue or schedule water heater repair and have a service technician fix the problem.

Here are some common causes of excessive temperature and pressure in the water heater tank and how to troubleshoot or prevent these problems.

Thermostat Malfunctions

The thermostat controls the temperature of the water inside the tank. It turns the electric heating elements on and off to maintain set temperature in an electric water heater and controls the burner in a gas water heater to maintain set water temperature.

If the thermostat sticks shut and causes the elements or burner to heat water constantly, the water temperature and pressure inside the tank will continue heating water beyond safe levels until the pressure relief valve trips.

When you notice overheating in an electric water heater, follow the troubleshooting tips in this DIY video:

If you’re unable to find and fix the cause of overheating in your water heater on your own, schedule water heater repair service and we’ll send a Sears Home Services technician to your home to fix the problem.

When you have a gas water heater that’s overheating, schedule service immediately to have a service technician find and fix the cause of the problem.

Excessive temperature and pressure in your water heater tank are more than just inconvenient—they’re dangerous. By understanding the common causes and taking proactive steps to address them, you can keep your water heater running safely and efficiently. Regular maintenance, proper installation, and attention to warning signs are your best defenses against potential disasters.

If you suspect an issue with your water heater, don’t delay—consult a professional plumber to ensure your system is safe and functional.

Water Expansion with No Outlet

As water heats, it expands. In a closed plumbing system, this expansion has nowhere to go, leading to a rise in pressure inside the tank.

This issue is particularly common in systems without an expansion tank, which is designed to absorb the extra pressure.

Install an expansion tank if your plumbing system lacks one or if you installed a new water heater and didn’t follow the installation guidelines to install an expansion tank above the water heater. It’s a small investment that can prevent significant problems.

If you have an expansion tank installed but you suspect that it isn’t working, have a plumber check the piping and the expansion tank to resolve the problem.

What Happens If the Pressure Relief Valve Fails?

If the valve malfunctions—either by failing to open or by leaking—serious problems can occur:

• Failure to Open: If the valve does not open during high-pressure situations, the water heater tank could rupture or even explode. This is extremely dangerous and can cause significant property damage and personal injury.
• Leaking: A constantly leaking valve could indicate a problem with the valve itself or excessive pressure inside the system. Ignoring this can lead to inefficiency, water damage, or more significant safety risks.

Maintaining Your Pressure Relief Valve

Regular maintenance ensures your pressure relief valve functions properly. Here’s what you should do.

Test the Pressure Relief Valve

Lift the lever on the valve to ensure it opens and releases water. If no water comes out or the valve doesn’t close properly, you’ll likely need to replace the valve.

Lower the lever and make sure the valve shuts. If water leaks through the valve after lowering the lever, you’ll likely need to replace the valve as well.

Replace a Bad Pressure Relief Valve

Here’s a DIY repair video that shows how to replace the pressure relief valve in a water heater:

If you’re unable to replace the pressure relief valve yourself, schedule appliance repair service and we’ll send a Sears Home Services technician to your home to replace the valve for you.

The pressure relief valve in your water heater is a vital safety component that ensures the system operates within safe limits. By releasing excess pressure or heat, it prevents dangerous conditions that could lead to severe damage. Regular maintenance and prompt replacement of a faulty valve are essential to keep your water heater running safely and efficiently.