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Thread: Hot water tank expansion tank vs T&P valve.

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    DIY Senior Member Mikebarone's Avatar
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    Default Hot water tank expansion tank vs T&P valve.

    I’m a (want to be) handyman and I have been reading the post on, “Imploding water heater”. I’m still a little confused why the T&P valve didn’t open, before the damage accrued to the water heater. Did the valve malfunction? If so, would it be a good idea to install a “T” coming out of the tank, and install two valves, for a back-up valve?

    Mike

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    Master Plumber Redwood's Avatar
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    The valve has a temperature probe that extends out the back setion of the valve. You would need multiple openings in the tank at the top or, on the side near the top...


    T&P Valve looking at two sides.
    Last edited by Terry; 04-24-2009 at 10:25 AM.

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    DIY Senior Member Mikebarone's Avatar
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    Quote Originally Posted by Redwood View Post
    The valve has a temperature probe that extends out the back setion of the valve. You would need multiple openings in the tank at the top or, on the side near the top...


    T&P Valve looking at two sides.
    True...The "T" idea wouldn't work, because it wouldn't be in the tank itself...good point!
    Was it a bad T&P valve, (along with probably a bad expansion tank) that caused the damage to the water heater on the other post?
    Last edited by Terry; 04-24-2009 at 10:25 AM.

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    In the Trades Gary Swart's Avatar
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    A T/P and an expansion tank are two separate devices and although both are related to the water heater, they serve different purposes. The T/P, has Redwood has pictured, protects the tank for exploding in the event that the temperature or pressure exceeds safe limits. It is required on every water heater. A thermal expansion tank is only necessary if the water heater is part of a closed system. A closed system is created when there is a check valve in the incoming water supply that prevents the expansion of heated water from being absorbed by the water main. This is commonly caused by a pressure regulator valve. Not all home have or need a pressure regulator valve as their water supply is already at a safe, usable pressure. When a system is closed and there is no expansion tank, when the water heater heats the water, that water expands and the expansion being blocked by the check valve in the PRV causes the pressure in the water heater to rise to the limit of the T/P valve which then trip to relieve that pressure. Even if you are willing to lose the water, you can not rely on the T/P alone to do the job because they sometimes do not close after the pressure is relieved.

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    Master Plumber Redwood's Avatar
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    And sometimes when the T&P has failed it won't open.
    Test your T&P by lifting the handle and don't worry about it!
    Some may say that testing makes it possible that the valve leaks.
    Oh well!
    If it leaks because debris lodges in it and it can't be stopped by opening it fully with the handle and snapping it shut then it needed replacement anyway.

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    DIY Senior Member Mikebarone's Avatar
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    Quote Originally Posted by Gary Swart View Post
    A T/P and an expansion tank are two separate devices and although both are related to the water heater, they serve different purposes. The T/P, has Redwood has pictured, protects the tank for exploding in the event that the temperature or pressure exceeds safe limits. It is required on every water heater. A thermal expansion tank is only necessary if the water heater is part of a closed system. A closed system is created when there is a check valve in the incoming water supply that prevents the expansion of heated water from being absorbed by the water main. This is commonly caused by a pressure regulator valve. Not all home have or need a pressure regulator valve as their water supply is already at a safe, usable pressure. When a system is closed and there is no expansion tank, when the water heater heats the water, that water expands and the expansion being blocked by the check valve in the PRV causes the pressure in the water heater to rise to the limit of the T/P valve which then trip to relieve that pressure. Even if you are willing to lose the water, you can not rely on the T/P alone to do the job because they sometimes do not close after the pressure is relieved.
    Thanks Gary / Redwood. I should have mentioned that I have installed many PRV’s and expansion tanks for my clients, (so poor Gary didn’t have to type his fingers to the bone…LOL). I install a expansion tank on all the systems that I install a PRV, just because it is the right thing to do, but I always wonder why the tank is needed when the PRV valve should blow off if excess pressure is built up.
    Still wondering why the damage accrued on the hot water tank on the other post. It had to be a malfunctioning T&P valve? Call me dense...it's OK.
    Last edited by Mikebarone; 09-29-2008 at 08:35 AM.

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    TROJAN WORLDWIDE SALES RP MACPLUMB 777's Avatar
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    Default Water heater t&p valve

    Per the i.p.c. And the u.p.c. The t&p valve is not supposed to act
    for thermal expansion, ! !
    Because of the good possibility that over time the valve will lime up and freeze closed making it inoperative when needed to
    prevent explosion, ! ! !

    Because of check valve installed on this water heater, with a
    water hammer that lasts appox. A nano second the t&p doe's
    not trip fast enough to prevent damage to water heater

    ALSO WHY VOIDS MFG'S WARRANTY ON SAID WATER HEATER EVEN IF JUST INSTALLED

    macplumb 777
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    DIY Senior Member Mikebarone's Avatar
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    Quote Originally Posted by MACPLUMB 777 View Post
    Per the i.p.c. And the u.p.c. The t&p valve is not supposed to act
    for thermal expansion, ! !
    Because of the good possibility that over time the valve will lime up and freeze closed making it inoperative when needed to
    prevent explosion, ! ! !

    Because of check valve installed on this water heater, with a
    water hammer that lasts appox. A nano second the t&p doe's
    not trip fast enough to prevent damage to water heater

    ALSO WHY VOIDS MFG'S WARRANTY ON SAID WATER HEATER EVEN IF JUST INSTALLED

    macplumb 777
    Thanks all for the info! I think it’s starting to turn on that little 20 watt light bulb in my head….LOL. Never considered the water hammer issue, that even if you don’t hear the pipes knocking, the hammer effect is still there.

    Thanks again all,

    Mike

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    DIY Senior Member Mikebarone's Avatar
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    Quote Originally Posted by Redwood View Post
    And sometimes when the T&P has failed it won't open.
    Test your T&P by lifting the handle and don't worry about it!
    Some may say that testing makes it possible that the valve leaks.
    Oh well!
    If it leaks because debris lodges in it and it can't be stopped by opening it fully with the handle and snapping it shut then it needed replacement anyway.
    When replacing an element in hot water tanks, I use to pull the handle on the T&P valve to assist the draining in the tank, but most of them wouldn’t’ shut after that so I stopped doing that. You are absolutely correct, that they should be tested like that, and replace if they don’t close again. I’ll be carrying a few with me from now on, and replacing them if they don’t shut. THANKS FOR THAT THOUGHT !

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    Retired Defense Industry Engineer jadnashua's Avatar
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    A T&P valve also won't open when a vacuum condition exists (which can also damage a tank). So, in some places, a vacuum breaker is also required (it is where I live along with a tempering valve). I dont' think I'd ever consider reusing a T&P valve when installing a new tank, either, as one thread indicated.
    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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    Moderator & Master Plumber hj's Avatar
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    Default T&p

    Many agencies RECOMMEND that the plumbers trip the T&P valve lever in EVERY building they work in, to check for proper operation. IF it leaks afterwards it WAS defective and should be replaced. But many will not even open or will not drain because the discharge pipe is plugged. Those are the ones that give a false sense of security and which will have the demolished homes or bulged tanks when something goes wrong.

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    Plumber jimbo's Avatar
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    Quote Originally Posted by jadnashua View Post
    A T&P valve also won't open when a vacuum condition exists (which can also damage a tank). So, in some places, a vacuum breaker is also required (it is where I live along with a tempering valve). I dont' think I'd ever consider reusing a T&P valve when installing a new tank, either, as one thread indicated.

    We drifted away, but I think one of the original "diagnose" on this thread was that it was a vacuum IMPLOSION, not an expansion. I concurred with that because of the angle of the nipples. We won't know unless someone cuts the casing open and posts a picture. But it is plausible, because there is a least a probablility that if the tank was over pressured, the TP would have prevented damage.

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    DIY Senior Member Mikebarone's Avatar
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    Default Thanks Jimbo for the reply

    Quote Originally Posted by jimbo View Post
    We drifted away, but I think one of the original "diagnose" on this thread was that it was a vacuum IMPLOSION, not an expansion. I concurred with that because of the angle of the nipples. We won't know unless someone cuts the casing open and posts a picture. But it is plausible, because there is a least a probablility that if the tank was over pressured, the TP would have prevented damage.
    If the hot water tank, (on the other thread, “Imploding water heater?”) imploded, wouldn’t the nipples on the top of tank be pushed outwards, (away from the vent)?
    If it did implode, I can see why the T&P valve and the expansion tank wouldn’t have saved it. But if it did expand with excessive pressure, that means that two devices failed. I have learned on the forum, that even the expansion tank will only relieve so much excessive pressure, so that means that there is only one device really saving the tank from excessive pressure….the T&P valve….kind of scary!


    Mike

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    Retired Defense Industry Engineer jadnashua's Avatar
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    If the top was sucked down, the nipples would slant in.
    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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