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Thread: vacuum refief valve placement on bottom feed water heater

  1. #1
    One who lurks Basement_Lurker's Avatar
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    Default vacuum refief valve placement on bottom feed water heater

    I have an odd question which I hope someone will have the knowledge and experience to answer:

    I am installing a bottom feed water heater, so the cold inlet is at the bottom of the tank instead of the top. The part I am scratching my head over is the placement of the vacuum relief valve.

    For a normal top entry tank, if negative flow occurs in the system, the cold water line flowing into the tank will drain up to the point where the vacuum relief valve is installed. The vacuum relief valve will then do its job and allow air into the system to equalize pressure since it is installed at a point higher than the tank. Because of this, the water in the tank will stay in the tank, and the tank will not suffer any negative pressures.

    However, in a bottom feed tank, if negative flow occurs in the system, I feel that the cold water line flowing into the tank would drain, but that the weight of the water in the tank would force the water to flow backwards out of the bottom of the tank until the tank was empty, thereby not allowing the vacuum relief valve to function no matter where it was installed on the cold inlet line. So if I am correct, where should the vacuum relief valve be installed? Should it be on the hot outlet coming out of the top of the tank so as to allow air into the tank if there was ever a reversal of flow?

    I spent 15 mins on the phone with the technical support for the manufacturer, and they had ever encountered this question before, and by the end of the discussion I basically felt they didn't know.

    Can anyone offer any insights?

    thx
    Broken promises don't upset me. I just think, why did they believe me? -Jack Handy


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  2. #2

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    sounds like you need to install the cold inlet line higher than the top of the heater. come out of the tank and run it up to a higher point than the top of the heater and then install your watts N-36 or equivalent vacuum relief valve on the
    proper tee fitting. If I am understanding your question correctly. you can also install a swing gate check as well for further protection.
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    I can't install a check valve as that would interfere with the pressure relief valve, but thanks for the idea as I never thought of that.

    Unfortunately it's not as easy as running a branch for the relief valve higher than the tank. As I see it, if a negative flow occurred, water would flow in reverse, and because the tank is a bottom outlet, the weight of the water in the tank would force the tank to drain (aided by the suction force created by the vacuum) out of the bottom of the tank and so the cold water supply line would never empty until the tank was empty, meaning that the vacuum relief valve would never have a chance to come into play until the tank was drained. Think of it this way, if you cut the cold water line in a normal tank the water in the tank is going to stay in the tank (except for the amount that gets displaced by the hot water lines draining back into the tank), but if you cut the cold water line in a bottom entry tank...the entire tank is going to drain out.
    Broken promises don't upset me. I just think, why did they believe me? -Jack Handy


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  4. #4

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    not sure what you mean about the check valve interfering with the T&P valve.
    The only backflow that would occur would be the evacuation of the hot water distribution system. The N-36 allows the introduction of air to break the vacuum.
    the tank would not siphon dry. once the hot side drained there would be no more head pressure. thats why you install the n-36 on the outward side of the tee to allow the entry of air into the system to break the vacuum on the tank.
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    The check valve would interfere with the pressure valve (on the cold line to alleviate any thermal expansion issues that arise due to the PRV), not the T & P installed directly into the tank.

    I am not following you when you say that there would be no more head pressure in the tank once the hot lines in the house drained....the inlet is at the bottom of the tank, so all of the water in the tank will try to drain out of the bottom of the tank because of the "head pressure" caused by the weight of the water in the tank. Like I say, if you cut the cold line anywhere before the tank, even if you cut the cold line somewhere higher than the tank, the weight of the water will force the water in the tank out of the bottom inlet and back through the cold supply system. The situation you are describing, where the only displacement force on the water in the tank is that of the water in the hot lines of the house is typical of a top entry setup.
    Broken promises don't upset me. I just think, why did they believe me? -Jack Handy


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  6. #6

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    so you are not going to address thermal expansion in any way? you are relying on the thermal expansion to push against the cold inlet to relieve itself?
    I recommend an expansion tank, or watts 530-c or a governor 80. do you currently have a closed water distribution system?
    Last edited by Garrett; 02-05-2008 at 06:23 PM.
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    Thermal expansion when a PRV is installed is addressed by one of two options:

    1) use of an expansion tank, or
    2) use of a pressure relief valve on the supply line


    In an open system, any back pressure caused by thermal expansion would force cold water in the home's supply line back to the main. In a closed system, like when a PRV is installed, the back pressure will cause the pressure relief valve to open and discharge the extra volume when it become necessary.
    Broken promises don't upset me. I just think, why did they believe me? -Jack Handy


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  8. #8

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    thanks for the pointers. Now do you have an open system or a closed system?
    if you have an open system by adding a check valve to your water heater you will close that portion of the system so naturally you will need to address thermal expansion.

    But it sounds more to me like your mind has been made up the whole time and you just want to argue and oppose someone answering your question. If you already know the answer to your question why did you post?

    Now that's being Grumpy. Not even Grumpy Plumber was that grumpy.
    Terry
    Last edited by Terry; 02-06-2008 at 09:31 AM.
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    Forum Admin, Expert Plumber Terry's Avatar
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    Default Vacuum Relief Valves

    I've installed these at a high point, on the cold supply side above the water heater.
    For it to work, it should be on the supply side, not the hot.
    It's to prevent siphoning of the tank.
    The city of Bellevue requires these on electric water heater installations.




    Series N36 Water Service Vacuum Relief Valves are used in water heater/tank applications to automatically allow air to enter into the piping system to prevent vacuum conditions that could siphon the water from the system and damage water heater/tank equipment. It consists of a brass body construction with NPT male inlet connection, and a protective cap. Series N36 has a low profile design, is tested and rated to ANSI Z21.22, and is CSA certified. It is suitable for low pressure steam and water service, and is ideal for use in domestic water heaters and supply tanks, table top heaters, jacketed steam kettles, unit heaters, low pressure steam systems, and steam coil heaters. Maximum Working Pressure: 200psi (14 bar), Maximum Steam Pressure: 15psi (103.4 kPa).
    http://www.watts.com/pro/_productsFu...&pid=815&ref=2
    Last edited by Terry; 02-05-2008 at 11:48 PM.

  10. #10

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    yeah, I don't think the N-36 is rated for hot water anyway.

    Maximum Working Pressure: 200psi (14 bar), Maximum Steam Pressure: 15psi (103.4 kPa).
    Last edited by Terry; 02-05-2008 at 11:33 PM.
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    Quote Originally Posted by Garrett View Post
    thanks for the pointers. Now do you have an open system or a closed system?
    if you have an open system by adding a check valve to your water heater you will close that portion of the system so naturally you will need to address thermal expansion. But it sounds more to me like your mind has been made up the whole time and you just want to argue and oppose someone answering your question. If you already know the answer to your question why did you post?


    Garrett, thank you for your time and effort trying to answer my question. I am not trying to be argumentative with you or talk down to you. You obviously have experience that I don't, and I am not expressing myself correctly. I just felt that you weren't seeing my argument on the issue of head pressure from the water inside the tank which was an important factor for me in this whole scenario; and I want to be sure on this, which is why I posting here, because there are certainly people who are more experienced than I am out there.

    As I stated in an earlier post, the addition of the check valve was a good idea that I would not have thought of. After sleeping on it, I now feel that this would work: Install a cold shutoff, then a vacuum breaker (I feel that height would no longer be a factor so long as it was installed before the check valve), then a check valve (which would close due to the head pressure of the water of the tank and allow the vacuum relief valve to come into play), then a pressure-only relief valve, and then the connection to the tank.

    Does this make sense? The system was already closed due to the PRV; the addition of the check valve would have negated the pressure-only relief valve where I was intending to install it, but I could get around that if I add in more piping than I wanted to and place the valve somewhere where it would still remain effective.

    Thx.
    Broken promises don't upset me. I just think, why did they believe me? -Jack Handy


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  12. #12
    Moderator & Master Plumber hj's Avatar
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    Default Valve

    The purpose of a vacuum relief valve is to keep the water in the tank and prevent siphonage. With a bottom supply tank, nothing, except a check valve, is going to keep the water in the tank, once a hot water faucet is opened so air can break the vacuum in the tank. A vacuum relief valve would just open and ensure that the tank empties, which is not the intent.

  13. #13
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    Default

    Quote Originally Posted by Garrett View Post
    But it sounds more to me like your mind has been made up the whole time and you just want to argue and oppose someone answering your question. If you already know the answer to your question why did you post?

    Now that's being Grumpy. Not even Grumpy Plumber was that grumpy.
    Terry
    Try driving from house to house on estimates for half your week, explaining to customers how the job is supposed to be done, hearing them say thats too expensive and tell you how they want it done...all on your time & gas.
    Then, if it doesn't work, your the one they blame.
    I've had people call me and ask how I do the work, what type stock I use...then never hear from them again.
    I caught on to that one, no more freebie tutorials over the phone.
    I finally started charging for estimates to weed out anyone not serious.

    I toned it down here, but do understand how it is from a self employed plumbers perspective.
    Sometimes I wonder if the very customer I just gave an estimate to simply comes here to do it "his" way with a little guidance.
    "The biggest regrets we have in life are the chances we never took."

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    Default

    I wonder if the very customer I just gave an estimate to simply comes here to do it "his" way with a little guidance.

    Grumpy,
    You have to charge for quotes when you drive out. That's a given.
    No more,
    "Can you drive over and hand me a free quote and tell me how you would do it?"

    That's called a consultation, and we know how much they charge for those. Grumpy, your time is worth a lot, and you know it, and I know it.
    Terry

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    Quote Originally Posted by Terry View Post
    Grumpy,
    You have to charge for quotes when you drive out. That's a given.
    No more,
    "Can you drive over and hand me a free quote and tell me how you would do it?"

    That's called a consultation, and we know how much they charge for those. Grumpy, your time is worth a lot, and you know it, and I know it.
    Terry
    Correct!
    I was just rationalizing my old "grumpy" ways, putting the plumbers perspective into the mix.

    Relative to topic, our code requires the breaker to be 6" above the top of the tank, he'd have to offset the cold feed to accomodate.
    "The biggest regrets we have in life are the chances we never took."

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