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Thread: Dripping thermal expansion relief value

  1. #31
    Forum Admin, Expert Plumber Terry's Avatar
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    Ballvalve loved those pressure releasing valves that needed drains.
    He didn't like the expansion tanks. I would rather install an expansion tank, and not worry about where to drain the "pop off" water.

    But if the vavle is releasing too often, I would look at house pressure first. It may be too high.

  2. #32
    DIY Senior Member wjcandee's Avatar
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    Quote Originally Posted by Terry View Post
    The T&P Valve?
    Not unless you want that water heater to launch itself through the roof and land on your neighbors home.
    Like this (and other photos and videos out there show that this is no joke):

  3. #33
    DIY Junior Member psyq's Avatar
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    Got the reading. Holding steady at 70psi

  4. #34
    In the Trades Wrenched's Avatar
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    To meet code in Vancouver, auxillary thermal expansion relief valves have to dump at 80psi or less. The tag on the valve will tell you what the factory set point is.

    When a residential tank is heating from cold a valve can dump as much as a cup of water, but usually it's less than that.

    A steady drip or stream when the tank is not heating can be a sign of valve failure -if- the system pressure is below relief point.

    Typically plumbers here will set house pressure at 60psi - a system pressure above that means either someone cranked the PRV, or the PRV isn't regulating properly.

  5. #35
    Sprinkler Guy Wet_Boots's Avatar
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    Most descriptions of the PRV's I've had mentioned a bypass feature that was designed to allow excess downstream pressure to bleed back through the device. Since expansion tanks are de rigueur in a system with a PRV, does that mean that the bypass feature of PRV isn't really being counted on to accomplish anything?

  6. #36
    Retired Defense Industry Engineer jadnashua's Avatar
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    The bypass, if it exists, can only open if the house pressure exceeds the incoming pressure. The goal is to keep the house pressure fairly constant, which is typically the reason a PRV is installed in the first place. That entails an expansion tank unless you want to deal with dumping water periodically along with the periodic rise in pressure.

    One poster here recently said his incoming water pressure was peaking at something like 190. At that pressure, before the PRV would ever open the bypass, the WH's T&P would have relieved the pressure or something else would have leaked or busted.

    Certainly, a pressure relief valve can keep the pressure from maxing out, but if anything, I think the preference is to keep it steadier, and not have to deal with periodic dumping of water. With a PRV and no expansion tank, it is not uncommon at all to have a weeping T&P valve.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #37
    DIY Junior Member psyq's Avatar
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    The thermal expansion relief valve is made by Apollo and has a rating of 80psi. Since my pressure is WAY below that I should probably look at either replacing that valve, or buying an expansion tank. Thanks everyone, huge help!

  8. #38
    Retired Defense Industry Engineer jadnashua's Avatar
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    A closed system can easily get high enough to cause the T&P valve to open (at 150psi) unless there's some other path to relieve the pressure from thermal expansion. Fluids like water do not compress to any significant amount like a gas does...expansion correlates directly with raising the pressure in a closed, primarily rigid system. The expansion tank, containing air, provides an easy place for that water to be accommodated. To keep from dripping through that valve, install an expansion tank. You can keep that valve or take it out if you want...if you did keep it, it would act as a secondary backup. Some people like the redundancy, but it is not required. Without it or an expansion tank, your T&P valve would be weeping each time the WH ran after you shut the hot water tap off. How much would depend on the aquastat setting and how much and at what temperature the incoming cold water was. Say you were to raise 1 gallon of water from 12C to 60C, you'd have about 1.32oz or almost 40ml of expansion http://www.engineeringtoolbox.com/vo...ion-d_315.html Dealing with that is typically a nuisance, especially when an expansion tank takes care of it nicely with no leaks.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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