How much water comes out of the pressure relief valve

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by qwertyjjj, Oct 7, 2020.

  1. qwertyjjj

    qwertyjjj Member

    Joined:
    Jul 11, 2014
    In testing my pressure relief valve, the water goes down the down pipe, hits the pan and just sprays everywhere. The pan barely contains any of it.
    In a pressure relief situation, does it open full like this or does it just leak a bit?
    Is there anything I can do like lengthen the pipe, increase size?
    3/4 PEX entering the heater, the downpipe on the tp is 3/4, the pan pipe floor drain is 1.5
     
    Last edited: Oct 7, 2020
  2. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    Except rarely, the only time water comes out at a high rate is when you test. At that time, one of the things you are testing for is whether the T&P valve closes back on its own. Be ready to close the supply valve when you test.

    I recently changed out my T&P valve, because it would not open during a test.

    What you can do will vary with locality. The rules tend to vary according to the likelihood of outdoor freezing in the state. Here is some past discussion: https://terrylove.com/forums/index.php?threads/water-heater-relief-valve.26152/
     
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  4. Tuttles Revenge

    Tuttles Revenge In the Trades

    Joined:
    Oct 15, 2014
    Best practice for a water heater temperature and pressure relief valve drain is for it to be piped to the exterior of the house or into an appropriately sized indirect drain inside.

    The purpose of a T&P valve is to relieve the excessive pressure if a water heater for some reason begins to overheat the water to prevent that water from boiling and becoming steam. Mythbusters did a show on the effects of a water heater explosion. The valve can in rare cases open up fully and dump a lot of water well beyond what will fit in that pan. In most cases the valve spring becomes weakened by repetetive thermal expansion acting against it and it will dribble slowly until noticed. A simple battery operated alarm with its sensor in the pan will alert you to that early failure before.
     
  5. Jeff H Young

    Jeff H Young In the Trades

    Joined:
    Feb 27, 2020
    Location:
    92346
    never ran a tand p to a drain pan. piped to code
     
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    How you run the discharge pipe depends on where you live. If you live where it can get below freezing, if the T&P valve is weeping for some reason off and on, going outside, it could freeze up and plug the line depending on how long it is which could be disastrous. An indirect termination to a drain with an air gap may be the safest.

    The WAGS valve will shut the inlet water off if water accumulates in the pan to about 1/2" deep, so if that were the case, it would limit how much water came out. Depending on the type of WH, it may also be able to disable the burner or heating element to keep it from heating any more water.

    If the T&P valve is working, it will stay open until the thing that caused it is no longer there which could be over pressure, or over temperature. Over temp, incoming water should cool things off. Over pressure, unless it's steam, won't need much. Steam should not occur as the valve should open before the water is hot enough unless you happen to live way up in altitude that drops the boiling point. The T open point is nominally 205-degrees. The P is 150-psi. To have it boil before the T part opened up, you'd have to be above around 4000', but since the water is under pressure in the tank, it's considerably higher until you open a valve and lower the pressure some, the water could flash to steam as it comes out sort of like a rocket.
     
    Last edited: Oct 8, 2020
  7. Sylvan

    Sylvan Still learning

    Joined:
    Sep 23, 2004
    Occupation:
    plumbing - fire suppression - boiler inspector
    Location:
    New York
    First, they respond to excessive pressure by opening at the pressure set point of the valve, typically 150 psi, to prevent further pressure increase. When a relief valve is dripping, it is typically due to thermal expansion. When water is heated, it expands. This causes excess pressure in the system. When the T&P relief valve senses this excess pressure, it opens relieving the thermal expansion* and returning the pressure back to normal conditions.


    Second, they respond to excessive temperature. When the temperature of the
    water in the water heater reaches 210°F,
    the T&P relief valve’s internal thermostatic
    element expands, lifting the valve’s disc
    off its seat to discharge the overheated
    water. This allows cooler water to enter
    the tank and moderate temperatures.
    When the temperature returns to a safe
    level (under 210°F), the thermostat contracts, allowing the loading spring to
    reseat the valve. At this point the automatic temperature relieving element is ready
    to protect the system again. If a T&P relief
    valve is relieving a high volume of water, it
    is due to excessive water temperature,
    an unsafe water heater condition.

    Discharging outside there is a risk of scalding someone and also if the valve is seeping no one will know unless they physically see it which seldom happens
     
  8. Sylvan

    Sylvan Still learning

    Joined:
    Sep 23, 2004
    Occupation:
    plumbing - fire suppression - boiler inspector
    Location:
    New York
    A Relief valve on a boiler for example may seep as the pressure reaches the set point before fully discharging

    A Safety valve steam /vapor/ air pressure they " POP" open full flow
     
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