Gas shutoff of HWT if leak drains tank

Discussion in 'Water Heater Forum, Tanks' started by Lucky Don, Oct 28, 2013.

  1. Lucky Don

    Lucky Don New Member

    Messages:
    22
    Location:
    Nashua NH
    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.
  2. Murphy625

    Murphy625 Member

    Messages:
    149
    Location:
    Michigan
    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..
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,270
    Location:
    Cave Creek, Arizona
    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.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,817
    Location:
    New England
    At least some (maybe all) gas WH have an overheat sensor that will shut them off. http://www.whirlpoolwaterheaters.com/learn-more/gas-water-heaters/6th-senseā„¢/gas-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/default/files/product_literature/238-44219-00K.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.
  5. dj2

    dj2 Member

    Messages:
    399
    Location:
    California
    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. Lucky Don

    Lucky Don New Member

    Messages:
    22
    Location:
    Nashua NH
    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.
  7. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    3,833
    Location:
    NW Ontario, Canada
    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: Oct 29, 2013
  8. Murphy625

    Murphy625 Member

    Messages:
    149
    Location:
    Michigan
    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.
  9. Caduceus

    Caduceus Master Plumber

    Messages:
    99
    Location:
    Pittsburgh, PA
    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.
  10. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    3,833
    Location:
    NW Ontario, Canada
    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.
  11. Caduceus

    Caduceus Master Plumber

    Messages:
    99
    Location:
    Pittsburgh, PA
    Yes I did miss that detail when I read the post.
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,817
    Location:
    New England
    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.
  13. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,270
    Location:
    Cave Creek, Arizona
    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.
  14. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    3,833
    Location:
    NW Ontario, Canada
    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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