Leaky unsafe relief valve (C) Daniel FriedmanRelief Valve Leaks from Thermal Expansion
Hot water expansion TPR valve leak cause & cure

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Why the relief valve leaks: thermal expansion of hot water

This article explains hot water thermal expansion that can cause leaks, drips, or discharges from pressure relief valves, temperature/pressure relief valves, or TP valves found on heating boilers, water heaters.

We explain how to find and fix leaky relief valves on hot water heating systems.

Watch out: a dripping or frequently spilling T&P valve is dangerous because those very leaks can eventually cause the valve to clog and then to fail to open when it should.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Closed Hot Water Systems & Thermal Expansion Problems - Why is the Water Heater Relief Valve Leaking?

Water heater pressure temperature safety valve (C) Daniel Friedman Watts and other manufacturers of TP valves warn of serious safety hazards caused by dripping at the TP discharge line and point out that often the dripping is caused by thermal expansion of hot water.

When water is heated in a closed plumbing system, it expands, causing an increase in pressure. This increase in system pressure over that regulated by the pressure reducing valve is called thermal expansion pressure. The T&P valve is a safety device and is not intended for continuous operation to relieve thermal expansion.

Continuous dripping can cause the build up of mineral deposits on the valve and may render it useless. - Watts (2011)

See HOT WATER PRESSURE EXPANSION RATE - for an explanation of just how much pressure increase to expect when heating water. In explaining why the relief valve on a water heater may be dripping, American Water Heater Co. [1] explains:

Periodic discharge of the temperature and pressure relief valve may be due to thermal expansion in a closed water supply system. The water utility supply meter may contain a check valve, backflow preventer or water pressure reducing valve. This will create a closed water system.

During the heating cycle of the water heater, the water expands causing pressure inside the water heater to increase. This may cause the temperature and pressure relief valve to discharge small quantities of hot water. To prevent this from happening, there are two recommendations:

Note: While a 125 psi pressure relief valve on cold water supply piping is fine to protect piping from rupturing, if building incoming water pressures are too high, rather than relying just on a relief valve (wasting water) we prefer to install a pressure reducing/control valve at the building entering water supply piping. Limit incoming water pressure to 60 psi at a typical one or two story residential home.

How to detect that a TPR valve is dripping due to thermal expansion

Draining pressure from the water system to determine the cause of a leaky TPR valve (C) Daniel Friedman There are several reasons that a temperature/pressure relief valve or TPR valve might be dripping or leaking.
See RELIEF VALVE LEAK for a complete discussion of leaky TP valves.

Watts (2011) suggests two methods for testing for thermal expansion of hot water as the cause of a TP valve drip. The text below is adapted from that source.

  1. Open a water faucet and see if the dripping stops: If the TPR valve is leaking due to thermal expansion of hot water, simply opening a water faucet or fixture anywhere in the plumbing system will immediately stop the drip.

    That's because opening the hot water faucet reduces the system pressure back to normal operating levels - we are no longer heating water in a "closed" piping and tank system.

    Why would opening either a hot or cold water faucet work? Because when no water is running, the entire plumbing system is "closed" - unless there is an expansion tank already installed on the hot water heating system to accommodate thermal expansion, heating water in the water cylinder (water heater for Yanks) causes an increase in system pressure.
  2. Monitor actual water piping system pressure using an temporarily or permanently-installed pressure gauge:

    Install a pressure gauge on a convenient hose bib such as at the washing machine hook-up or if weather permits, outdoors. Install & use the pressure gauge to monitor system water pressure over at least a full 24-hour period. You don't have to sleep by it. If you select a pressure gauge with a re-settable "telltale" needle, the gauge's re-settable indicator will indicate the highest pressure that has been reached by the water system since the water pressure test gauge was installed.

    See MEASURE WATER SYSTEM PRESSURE & PRESSURE MAXIMUM for tips on how to make or buy an inexpensive recording-type water pressure test gauge for this purpose.

Solutions for Dripping TPR Valves due to thermal expansion of hot water are given
at HOT WATER EXPANSION TANKS where we describe both hot water expansion tanks and hot water expansion relief valves designed to handle thermal water pressure increases and thus to stop dripping TP valves.

Watch out: get advice from a licensed plumber and your local water supplier. Never remove or plug a leaky pressure/temperature relief valve on a water heater - doing so risks dangerous or even fatal

See RELIEF VALVE LEAK DUE TO EXPANSION TANK where in a series of steps a homeowner figures out that a leaky TPR valve is due to the failure of the internal bladder of an expansion tank attached to the boiler.

Also see the FAQs below where we discuss closed plumbing systems caused by check valves on the cold water line.

If the building water pressure gauge reading is ever found at 80 psi or higher, you will want to install a water pressure regulator at the point where water supply enters the building.

If your building already has a water pressure gauge installed, it may be defective or it may be set too high. The articles listed just below discuss how to adjust a water pressure regulator:

Cures for Water Heater Relief Valve Leaks due to Thermal Expansion: Expansion tanks to relieve high water pressure

An alternative to installing or changing a water pressure regulator when building water pressure is occasionally 80 psi or higher is the installation of an expansion tank to temporarily absorb that pressure increase.

Proper use of an expansion tank can help avoid unnecessary opening of the pressure/temperature relief valve on a hot water heating tank or a hot water heating boiler.

Reader Comment: why is a water heater expansion tank ever necessary?

2017/01/08 GaryC said:

You go through alot of effort to explain how heated water expands and the resulting increase in pressure. All that is well and good for theory. In a domestic (North American) water supply system, to the best of my knowlege there is nothing to prevent the expanding water/pressure from the hot water heater from just backing up into the supply plumbing and hence the water pressure in the hot water heater would NEVER exceed the supply pressure. Or is there a required one way valve somewhere in the system I am not aware of say in the hot water heater?

This comment was posted originally at RELIEF VALVE, WATER HEATER


Good question, Gary. Thanks.

Domestic water systems in most cities are required to have a backflow preventer or check valve on the system. So when no water is being run the system is in effect closed.

My original interest and intent was to show that the actual expansion in volume from heating water is actually very small. Nevertheless, on some systems, the water heater can cause enough of a pressure increase that some installations include a small water heater expansion tank.

Even where a backflow preventer is not installed, the pressure/reducer valve at the municipal supply acts as a check valve.

And on a water system on a private well, the system pressure encounters a check valve at the water pump or a foot valve in the well.

Where expanding hot water pressure is causing trouble the common solution is the installation of a small expansion tank, usually on the hot water supply line above the water heater, though it could go almost anywhere so long as it's not isolated from the heater by a shutoff valve.

Amtrol's Therm-X-Trol expansion tank is sold specifically for this application. Here is what Amtrol says about that product:

Thermal expansion occurs as domestic water is heated. Left unchecked, the resulting pressure increase can cause relief valve discharge and other potentially hazardous conditions. Therm-X-Trol® expansion tanks absorb this expanded water, keeping water pressure at a safe level.
Water heater manufacturers and plumbing codes require the installation of an expansion control device if a backflow preventor, pressure reducing valve or check valve is installed in the domestic supply line.

Watch out: a leaky relief valve on a water heater, heating boiler, or any other closed vessel is dangerous. Leaks ultimately can lead to relief valve clogging, failure, and worse, BLEVE EXPLOSIONS. Beginning at RELIEF VALVE LEAKS we describe all of the causes of and cures for leaky relief valves or TP valves.

Standards & Referencs on Hot Water Expansion Requirement for Expansion Tanks or Control Valves on Water Heater Systems


Continue reading at HOT WATER EXPANSION TANKS for a solution to thermal expansion leaks at relief valves on water heaters, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see EXPANSION TANKS for diagnosis & repair procedures

Or see HOT WATER PRESSURE EXPANSION RATE - how much does water expand when you heat it?

Or see RELIEF VALVES - TP VALVES - topic home where we include additional relief valve information including for hydronic heating and steam heating boilers used for central heating.

Or see THERMAL EXPANSION of HOT WATER for an explanation of the amount by which water expands when heated

Or see this

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