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Thread: Gas shutoff of HWT if leak drains tank

  1. #1

    Default Gas shutoff of HWT if leak drains tank

    Does anyone know or heard of this situation? A gas supplied hot water heater develops a leak in the tank and drains down because the main water supply is off. The heater is heating at the time or a call to heat is in effect. Isn't there a over temp or a override safety system for this? If not this sure sounds dangerous to me.

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    DIY Senior Member Murphy625's Avatar
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    Interesting scenario.

    I am not aware of any type of safety to prevent a burner from running under that condition.. That could get ugly fast.

    That said, since the gas control valve and temp sensor assembly are near the bottom of the heater, I would think that once the flame has steamed off any remaining water, it would simply cycle on and off as the metal turned red hot, radiated heat to the thermocouple and shut the unit off until it cooled a bit.. then repeat the cycle.

    Waste of gas, but I'm not to sure it would burn a building down..

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    Moderator & Master Plumber hj's Avatar
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    All you would have is a tank full of hot air. It could NOT develop any pressure because the tank had a hole in it. But as a practical matter, unless the water was off and a hot water faucet was open or leaking, the water would not drain out of the tank anyway.
    Licensed residential and commercial plumber

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    Retired Defense Industry Engineer jadnashua's Avatar
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    At least some (maybe all) gas WH have an overheat sensor that will shut them off. http://www.whirlpoolwaterheaters.com...water-heaters/ and on this Bradford White WH, they say it has an "energy cut out device to prevent overheating." (from page 26 http://www.bradfordwhite.com/sites/d...-44219-00K.pdf you can check out others.

    Any of these things can fail, but in general, should the thing be left on without the water supply AND there was a failure in the system, it should protect itself from overheating.
    Jim DeBruycker
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    DIY Senior Member dj2's Avatar
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    Could it be this: as long as water surrounds the prob/sensor extending from the gas valve, the valve will open and the burner will come on. But if the level of the water in the tank goes below the prob (due to the leak), the gas valve won't open.

  6. #6

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    I'm not sure how the probe could sense that there is water around it or not. I would think if all that was in the tank was air, the air temp would rise to a point that the probe/sensor would eventually heat up enough to shut off the burner but it would also cycle on/off till someone shuts it off and repairs the leak or replaces the HWT.

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    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by hj View Post
    But as a practical matter, unless the water was off and a hot water faucet was open or leaking, the water would not drain out of the tank anyway.
    This thread is in response to a hypothetical situation I raised in another thread. Two scenarios where I can see the tank getting emptied is 1. the tank has a leak at the bottom, or 2. there is a recirc loop connected to the drain port.

    I'm guessing the ECO fuse could blow but in the case of #1, the tank is toast anyway so no concern of further damage to it. In case #2, a plumbing leak that drains the tank could incur the additional cost of replacing parts on the HWT. The OP is designing a leak detection and shutoff system for a well pump. I simply pointed out that just turning off the pump may not be enough.
    Last edited by LLigetfa; 10-29-2013 at 10:02 AM.

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    DIY Senior Member Murphy625's Avatar
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    Quote Originally Posted by LLigetfa View Post
    The OP is designing a leak detection and shutoff system for a well pump. I simply pointed out that just turning off the pump may not be enough.
    I didn't see that in this thread.. But, if that's the case then how about.... (give me a second to think)

    1. A simple float switch like what every boiler system has that cuts power to the controller.

    Don't go after cutting the gas.. its too expensive and would require a bi-stable motorized valve.

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    Master Plumber Caduceus's Avatar
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    If your tank is leaking it is still under pressure from the cold water supply. Since it is under feed pressure, as water leaks out water will also be filling the tank...unless you shut the cold supply valve off. So there should not be a void in the tanks filled with steam, but your tank will continue fire more frequently and longer.

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    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by Caduceus View Post
    If your tank is leaking it is still under pressure from the cold water supply...
    Perhaps you missed this little detail in the OP?
    Quote Originally Posted by Lucky Don View Post
    Does anyone know or heard of this situation? A gas supplied hot water heater develops a leak in the tank and drains down because the main water supply is off...
    Generally, for the tank to drain, there needs to be air let into it. That means a faucet or other valve (or vacuum breaker) opened.

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    Master Plumber Caduceus's Avatar
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    Quote Originally Posted by LLigetfa View Post
    Perhaps you missed this little detail in the OP?


    Generally, for the tank to drain, there needs to be air let into it. That means a faucet or other valve (or vacuum breaker) opened.
    Yes I did miss that detail when I read the post.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    If you have a gas valve that is compatible with it, a WAGS valve, when it detects a leak (it requires a pan to collect the water so it gets high enough to trigger the valve - about 1/2" I think), it has an internal switch that can disable the gas valve. It's primary function is to turn off the cold water inlet, but if that's already off, IF the switch can be made to work with your WH, it would disable the valve. I think the intent is to put it in series with the thermocouple, and when open, the valve would never sense the flame, and thus not turn on the main burner. Might sit there and keep trying to light the pilot though, but that shouldn't be a big issue.
    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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    quote; "energy cut out device to prevent overheating.

    The ECO will open ANY TIME the temperature, air or water, exceeds its setting, but that would normally ONLY occur if the thermostat had failed so that the burner did not shut off a the set temperature.
    Licensed residential and commercial plumber

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    DIYer, not in the trades LLigetfa's Avatar
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    I've had the ECO trip twice on my current gas water heater. If it was not covered under support contract I would have been out of pocket for several hundreds of dollars.

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