Leaky unsafe relief valve (C) Daniel FriedmanRelief Valve Leaks
Causes & cures for boilers, water heaters, water pressure tanks

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Leaks at temperature / pressure relief valves:

This article describes the causes of leaks, drips, or discharges from pressure relief valves, temperature/pressure relief valves, or TP valves found on heating boilers, water heaters, or the simpler pressure relief valves found on water pressure tanks.

We list the wide variety of possible TP Valve leaks and how to find and fix each of those problems, including how to fix a leaky pressure relief valve or leaky TP valve on a boiler, water heater, or water tank - what are the possible causes of leaks at these safety devices. Safety Hazard Warnings About Dripping or Leaking Pressure Relief Valves.

How to use an expansion tank to relieve high water pressure. T&P Valves Installed on Gas Sidearm Heaters: special problems. Closed Hot Water System & Thermal Expansion Problems.

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.

Leaky or Dripping Water Heater Pressure & Temperature Relief Valves

Obsolete boiler pressure relief valveArticle Series Contents

Evidence of dangerous leaks and corrosion at water heater temperature/pressure safety valves

Causes of leaky dripping Pressure/Temperature Relief Valves

Temperature & Pressure Relief valves may open, leak, or drip for a variety of reasons including:

  1. Boiler limit control problems: On a heating boiler, the boiler temperatures are excessive, possibly due to an improperly set limit control, lack of contact between the limit switch sensor and its mounting well, or a defective control.


  2. Closed hot water systems: Periodic discharge of the temperature and pressure relief valve may be due to thermal expansion in a closed water supply system. Details are

  3. Gas sidearm heater TP valves: The T&P valve is installed on a sidearm gas heater that is causing an overheat condition at the sensor point of the valve (This thermal expansion is discussed

  4. Expansion tank defects or problems: if the expansion tank or compression tank on a hydronic heating system boiler or on other thermal expansion systems is itself defective (waterlogged, leaky, damaged internal bladder) system pressures will be excessive due to otherwise normal pressure & temperature variations during system operation, resulting in spillage at the relief valve.

    See EXPANSION TANKS for diagnosis & repair procedures

    See EXPANSION TANK WATERLOGGED, RELIEF VALVE LEAKS for examples of TP valve leaks caused by waterlogged expansion tanks.
  5. Leaks into the heating boiler or water heater: leaks into a heating of water from a higher pressure building source can cause recurrent TP valve leaking. For example, if the tankless coil in a heating boiler is leaky, higher pressure water inside the tankless coil may leak out of the coil into the heating boiler.

    Similarly, an internal leak in the heat exchanger coil of an indirect water heater can send water from the water heater's potable water into the coil and thence into the heating boiler.

    We describe these leaks into the heating boiler through the tankless coil or indirect water heater coil


    This same leak problem can cause high water pressure in a heating boiler that is used to heat water in an indirect-fired water heater. In that case, a coil containing boiler water (typically at 12-29 psi) that develops a leak may accept higher building pressure water from the building water supply to the indirect water eater tank (or cylinder) that is typically between 20 psi and 70 psi.

    Diagnose this problem by observing that when the boiler is left OFF and its own water feeder is left OFF but building water supply is left ON into the tankless coil on the boiler or ON into the indirect water heater, boiler pressure will creep up several hours. With a reader we discuss this possible TP valve leak cause in the FAQs section of this article.

    Watch out: under normal conditions, because building water supply pressures are above boiler pressures, an internal leak in the tankless coil or indirect water heater's heat exchanger coil will cause boiler pressures to rise. But there can be exceptions in the direction of water leakage, as we explain

  6. Pressure Relief Valve Defects, or Pressure/Temperature Relief Valve Defects that cause leaks
    • Dirt or debris are preventing proper closure of the relief valve valve
    • A deteriorated gasket inside the relief valve or corrosion on the valve seat can cause leaking at the valve; we find this mess occurring when someone lifts the "test lever" on a older P/T valve that has not been tested or operated for some time. A brittle piece of gasket can be spit out of the valve and it will then keep leaking.
    • The wrong T&P valve has been installed or set to too-low a working pressure. (Proper set pressure is at least 20-30 psi above the working pressure of the equipment to be protected).
  7. Thermal expansion problems: A closed water system with thermal expansion and no means of relief can cause leaks at the pressure/temperature relief valve, such as
    • Can occur on a hot water heater (hot water cylinder, calorifier, hot water tank) with some building piping arrangements, particularly where a check valve or pressure reducing valve are used.

      See THERMAL EXPANSION TPR VALVE LEAKS for an explanation of why thermal expansion leaks occur at the relief valve on hot water systems & what to do about it
    • Can occur also a hot water heating boiler (used for building heating) at which the boiler's thermal expansion tank has become waterlogged or has a ruptured internal bladder.

      See EXPANSION TANKS for diagnosis & repair procedures for expansion tanks
    • Water expansion pressure (thermal expansion) increases in any closed plumbing system, particularly where a check valve installed close to the water heater.

      The increase in plumbing system pressure to a level that opens the TP valve is called "thermal expansion pressure".

      Watts suggests installing a bypass model water pressure regulator that lets the excessive pressure head back to the street main or building water supply system - a solution that only works if the supply pressure is lower than the T&P relief valve spill pressure - which it usually is.

      See THERMAL EXPANSION of HOT WATER for an explanation of thermal expansion when water is heated
    • A failed thermal expansion tank such as an expansion vessel using an internal bladder after the bladder leaks or has burst, can cause

      a RELIEF VALVE LEAK DUE TO EXPANSION TANK failure or defect.
  8. Water hammer: The building plumbing system suffers from water hammer. Water hammer in buildings causes surging in the water piping that in turn can cause leaks at pressure/temperature or other pressure relief valves, particularly at the pressure safety valve found at water pressure tanks.

  9. Water heater temperature too high: The water heater temperatures are excessive. For example on an electric water heater a malfunctioning control can overheat the water e.g. the water heater thermostat is not working properly and is not shutting off the heat source when it should -



  10. Water pressure too high: The building water supply pressure is too high or periodically water pressure fluctuates and is too high at times. Pressures over 70 psi in a typical building tend to cause leaks at plumbing fixtures and higher pressures are likely to cause or contribute to leaks at TPR valves.



  11. Water pressure reducing valve / water feed valve problems: On a hot water hydronic heating system, if the water pressure reducer/feeder valve is not working properly it may over-feed water into the heating system causing overpressure.

  12. Combinations of factors causing TP Valve Leaks: OK so this is more than 12 causes of TPR valve leaks, but keep in mind that the temperature/pressure relief valve leak causes listed above can also occur in combination. For example high incoming water pressure alone may not cause a TPR valve to leak but if we add water hammer then the valve may be leaky.

Safety Hazard Warnings About Dripping or Leaking Pressure Relief Valves

Evidence of relief valve leakingWatch 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.

As our photo shows (above left), mineral salts left behind as hot water evaporates from the mouth of a pressure or temperature relief valve can completely clog the spring that is intended to allow the relief valve to open under excess (unsafe) pressure.

The impaction of the relief valve spring with copper and calcium salts in this photo means that the valve is almost certainly not going to open should the heater's internal pressure become unsafe.

The drip shown at the mouth of this relief valve demonstrates that a relief valve can drip and leak for a long time without anyone observing this dangerous condition.

This relief valve needs to be replaced immediately and the cause for the valve's leakage also needs to be determined.

Accidental or deliberately plugged Temperature & Pressure Relief Valves

Watch out: Never remove nor plug a pressure/temperature relief valve. This [water heater pressure/temperature relief] valve must be marked with the maximum set pressure not to exceed the marked maximum working pressure of the water heater.

Install the valve into an opening provided and marked for this purpose in the water heater, and orient it or provide tubing so that any discharge from the valve exits only within 6 inches above, or at any distance below, the structural floor, and does not contact any live electrical part. The discharge opening must not be blocked or reduced in size under any circumstance. [1] [2]

Technical note: why must the TP Valve point "down"? Take a look at the photo above. If a relief valve is dripping the deposit of minerals inside the valve will accumulate still more rapidly if the valve points to the side or upwards. The result is a clogged valve as we explain above - a dangerous situation that risks an explosion.

See BLEVE EXPLOSIONS for an explanation of why a leaky or plugged TPR valve can cause a large catastrophic explosion.

See WATER HEATER SAFETY for our complete list of water heater safety devices and water heater safety inspection advice.

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

Watch out, serious safety hazards can be caused by dripping at the TP discharge line: often the dripping is caused by thermal expansion of hot water.

For details please see THERMAL EXPANSION TPR VALVE LEAKS - an explanation of why thermal expansion leaks occur on hot water systems & what to do about it

Also 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

T&P Valves Installed on Gas Sidearm Heaters: special problems

Watts Regulator Co. offers this explanation of T&P leak problems that may be encountered on gas fired sidearm heaters. [10]

Question: What would cause an automatic T&P valve to open and close repeatedly when there is very little hot water in the storage tank?

Reply: troubles at the gas sidearm heater

With a sidearm gas heater a common problem is frequent T&P valve opening even though there is very little hot water in the hot water storage tank. That's because an overheated condition exists right at the sensor point of the T&P valve - most of the overheated water is "congested" at the top of the tank.

This problem can be caused by undersized circulation piping between the sidearm heater and the water tank. Undersized piping between the sidearm heater and the tank (supply or return) amounts to a restriction of the circulation area (or volume).

When this "too small" circulation area or volume is combined with a heater whose BTU input rate is higher than that undersized volume can carry, proper circulating does not occur through the tank nor through the sidearm heater coils themselves - the "congestion" referred to above occurs.

In turn this congestion (think of it as a hot water traffic jam) causes overheating right where the T&P valve sensor is located, thus causing the valve to spill hot water repeatedly.

In other words, hot water is accumulating at the T&P valve location rather than being distributed more evenly throughout the hot water tank.

Because there is just a small volume of "too hot" water where the valve is located, the valve opens, spills the small amount of hot water, then is cooled and closes after just a short interval.

To correct this condition install at least 3/4" piping, preferably not iron pipe, in the circulating loop.

[The system we used for years used 1 1/4" diameter copper piping - Ed.] Non-ferrous piping is preferred to reduce the chances of clogging from rust debris - a clue that warns us that even 3/4" piped gas sidearm heaters can clog from rust or scale formation and will then exhibit this problem if the piping is iron.

With an automatic gas storage heater installation, either the heater thermostat is acting abnormally (not shutting off when it should) or stacking temperature conditions are causing this TP leak. - paraphrased & adapted from information from the Watts Regulator Company. [10]


Continue reading at THERMAL EXPANSION of HOT WATER or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see RELIEF VALVE LEAK FAQs - questions & answers about leaky temperature/pressure relief valves posted originally at the end of this page

Or see these

Temperature/Pressure Relief Valve Articles

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