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Thread: Leaky Valve and Thermal Expansion Tanks

  1. #1
    DIY Junior Member leakylarry's Avatar
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    Default Leaky Valve and Thermal Expansion Tanks

    OK heres my issue, I hope someone can help me out here.

    I have a 50 gallon natural gas water tank. Its only about 1 year old.

    I do NOT have a Thermal Expansion Tank.

    The water is incoming at 44 PSI (one way, no backlflow).

    My water tank pressure release valve is set to 120 PSI. A new one was installed just a few days ago.

    The water temperature setting on the tank is at level 2 of 5 (low).

    The pressure release valve on the water tank is leaking and each morning (not always) there is approx. 100-200 ml of water in a small container underneath the pressure release tube. (1/4 to 1/2 a small bottle of waters worth).

    The other day i managed to spot the leaking from the tube. I immediatly put a water pressure gauge on both hot and cold water taps. It registered about 130 PSI.

    130 PSI seems very high to me and explains the leaking from the valve.

    Is this "normal" for modern day water heaters ?

    Do you suspect a problem with the water heater ? Maybe a thermostat problem ?

    Should i simply purchase an Expansion Tank ?

    If i purchase an expansion tank does the maximum PSI rating need to be above 130 PSI ? or is 100 PSI sufficient ? What about 60 PSI ? (I'm not sure how they actually work for max. PSI purposes).

    Should i get 2 Gallon or 4 Gallon ?

    Any help would be appreciated. Thank you.
    Last edited by leakylarry; 03-02-2011 at 10:42 AM.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    This is a classic example of why you always need an expansion tank when you have a closed system (either with a check-valve or a PRV which acts like one, at least until the internal pressure exceeds the external, IF it has an internal bypass). Add one, and the problem will go away. The tanks can last anywhere from 5-years, to 10 or more, depending on the local water and the luck of the draw. You'll know when it dies...the T&P will start leaking again. Water is not compressible, and when it expands, it has to go somewhere. Often, that may be a leaky valve, expansion of hoses, or if all of those are in good shape, the safety valve (T&P).
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Junior Member leakylarry's Avatar
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    Thank you I appreciate your time. Do you have any answers regarding the PSI rating I need for an expansion tank ? Do I need a tank that has a max. PSI rating greater than 130 PSI ? Thank you.

  4. #4
    Forum Admin, Expert Plumber Terry's Avatar
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    I would get the 2.0 or 2.2 gallon expansion tank for a 50 gallon water heater.
    You may have a check valve on the water system, allowing the water heater to pressurize the home beyond what the street pressure is.

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    The whole beauty of the expansion tank is that it prevents excessive pressure increases caused by heating the water in the first place. No home should have water pressure greater than 80#, and if you do, you need a PRV. You indicated your static pressure was lower than that, so unless it spikes on its own from the supply (not caused by expansion but from the utility), you don't need a PRV. Just adding the expansion tank, presetting the air pressure before hooking it up to the incoming static pressure value should keep it at that pressure if caused by expansion. Instead of expanding and increasing the pressure in the (barely) inelastic piping, it expands into the bladder of the expansion tank by compressing the air. Since air is quite compressible, the pressure rise is almost not measurable (since a typical WH only expands maybe a max of a pint, and usually less, depending on how much cold was introduced). The tanks generally come pre-charged to 40#...just use a bicycle pump or similar to raise it to your static incoming water pressure before installation. After you install it, and turn on the water, you'll be measuring the water pressure, not the air pressure, so you have to precharge it first.

    Any tank, properly sized for a potable water system, should work.

    Now, if the pressure DOES spike from the supply, then, you also need to add a PRV.
    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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    DIY Junior Member leakylarry's Avatar
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    Thanks alot for the info everyone. I'm sure my noobness has shown through, thats why i need to ask all these questions.

    There is already a water reduction regulator doohicky near the main so there is no spike from the outside. All things being "normal" its a static 44 PSI incoming. I have tested the PSI when it is "dormant" at 44 and when it is spiking at 130.

    My main concern is the size and PSI rating i need for the expansion tank. As mentioned earlier it spikes to 130 PSI. Does this effect the size and type of tank i get ? (i.e one rated for 60 PSI or one rated for 100 PSI or even 150). Or is this rating thats on the box only a reflection of the incoming PSI ? (i.e 44 PSI i have now). I really need to know this before i go out and purchase one.

    Thanks again .

  7. #7
    Retired Defense Industry Engineer jadnashua's Avatar
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    The tank does not need to support that high pressure, since it should never get there except if there is a failure...once installed, the pressure will stay stabilized at your PRV setting and will NOT rise as long as the PRV works and the tank is intact. This should help you size what you need http://www.watts.com/pages/support/sizing_DET.asp
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  8. #8
    DIY Junior Member leakylarry's Avatar
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    Thank you all again for helping. I've emailed Watts Canada to find out how to purchase a 2 gallon PLT-5 for my area (north of Toronto).

  9. #9
    General Engineering Contractor ballvalve's Avatar
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    The guys around here like the bag in a can method of pressure relief, but Watts offers brass valves that should out last a few of them for less money and a very quick install in just about any easy place. Your only price is a few ounces of water a day that can be run into a old campells soup can.

    http://www.watts.com/pages/_products...ls.asp?pid=803

    The problem with the ex tank is that you have no visual indicator of when they fail, they need maintenance which they DO NOT get, and its certain they will. if in doubt, use BOTH.




    At 60 pounds pressure, water boils at 307.4 degrees
    At 0 pounds pressure, water boils at 212 degrees
    At 50 pounds pressure, it releases the same energy as two pounds of dynamite.
    Last edited by Terry; 04-03-2011 at 02:43 PM.

  10. #10
    Retired Defense Industry Engineer jadnashua's Avatar
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    I've lived here about 30-years, and have had to replace the grand total of ONE tank...not a very big maintenance issue! If the goal is to maintain an even, safe water pressure and you have a closed system, the only way to keep it from rising with heating the water is an expansion tank. Most pressure relief valves are designed for fault conditions...the are not designed to get used every day. An expansion tank is...
    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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    Engineer Furd's Avatar
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    I have to agree with Jad on this one. The bladder tank will work for years whereas the relief valve, if spitting every day (which would not be uncommon) will fail in a matter of months. Install a servicing valve (ball valve) between the expansion tank and the water supply for ease of maintenance. My preferred method would include a shelf or bracket for the expansion tank and a braided SS hose between the tank and the service valve.

  12. #12
    Retired Defense Industry Engineer jadnashua's Avatar
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    While I like the idea of a valve between the tank and the line, doing that won't pass code...it sure does make servicing easier, though. The reasoning, as I understand it, is someone could close the valve, and disconnect the tank. If you did, you might take the handle off so it wouldn't happen inadvertently by someone who doesn't know better.
    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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    General Engineering Contractor ballvalve's Avatar
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    Maybe you guys should e-mail Watts and have them explain to you that these thermal expansion valves are designed just for everyday expansion and leakage, and in fact that is exactly why they last a very long time.

    Sorry, the rubber bag in the campells soup can reeks of failure to me. Springs rule!

  14. #14
    Engineer Furd's Avatar
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    I suspect that the shut-off valves with the spring-loaded reliefs cost less than the rubber bag in the soup can and in the real world initial cost is often the deciding factor rather than lifetime costs. The relief valve WILL spit every time the pressure rises to its set point and that action will erode the seat and disc until it starts dripping continuously. It also wastes water which in some areas is fairly expensive. I have read MANY tales of homeowners that have these reliefs installed and wonder why they have high water bills or why there is a valve in their home that is constantly dripping water to waste.

    Jadnashua makes the point that a service valve before the expansion tank won't pass muster with the code inspector and while I disagree with the codes in this instance it also shows that the codes recognize that replacement of an expansion tank is a fairly infrequent event.

  15. #15
    Moderator & Master Plumber hj's Avatar
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    Now, what is the DIFFERENCE between a pressure relief valve that opens to drain off the pressure, and the Watts "pressure limiting valve" which does EXACTLY the same thing? Water heater T&P valves are typically FACTORY set at 150psi, although there are rare ones at 125psi, and they are NOT adjustable, so I am not sure what kind of relief valve you have if it can be "set at 120 psi".

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