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Thread: inline hot water heater pressure relief drain

  1. #1

    Default inline hot water heater pressure relief drain

    Hi. I've been lurking out on your site for several weeks now and have had many questions answered from viewing old post. I am currently adding an addition to my house and I am interested in installing a 2.5 gal inline hot water heater so I don't have to wait forever for hot water. My question is how can a properly hook up a emergency use only drain for the pressure relief valve? I am fortunate that the location of the new hw heater is in the same 2x8 wall where the drain stack is located and I still have access to all the drain pipes the plumber installed for a sink. There is a 1 1/2" waste line that runs directly below the location of the new HW heater and a vent line that runs 46" above the new HW heater. I am afraid that if I just add a T and run a flex line from the relief value to a open pipe it will let out a smell where the two join or water could come out if I ever have a back-up. Should I be worried this much about the relief valve ever needing to operate and cause me issues? Should I call my plumber back for a beer and get his professional opinion?

    Any code I should be worried about with installing a Bosch 2.5 gal electric heater? I planning on just extending the 2x8 wall where I'll create a shelf with a door to set the unit on.



  2. #2
    Plumber jimbo's Avatar
    Join Date
    Aug 2004
    San Diego


    Most places require the T/P relief valve from ANY water heater to be piped at a minumum to a floor drain if there is one, and in most cases it just has to pipe to the outside of the building. Be careful with that run, because it must not go UP at any point/ You can NOT pipe it into a vent as you described.

  3. #3

    Default T&P relief valve piping

    And do not reduce the pipe size from what the T&P relief valve is, and of course no valves or similar in the piping.

  4. #4


    Thanks for the input. I don't have a floor drain, however there is a drain mounted in the wall for a washing machine very close, could I just drain it to that?
    Or, if a run the drain outside it will be coming out of the soffit on the second story, does it have to go to the ground or could I just leave it at the soffit area?

  5. #5
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default drain

    You should not and would not want to pipe it to a dedicated drain and trap. You cannot just hang it out of the eave. It has to be piped with continual downward slope and then terminate about a foot off the ground. As a practical matter, I have never seen one of those heaters make the valve pop open, so any existing drain you pipe it to should work.

  6. #6
    Commercial Plumber markts30's Avatar
    Join Date
    Sep 2006
    Phoenix, AZ


    Just make sure you pipe it with an air gap when you do it - do NOT connect the T&P discharge pipe directly to the drain...

  7. #7


    Looks like I will be able to run a 3/4" plastic pipe down the outside wall and paint it red. It will be next to the chimney so it won't show too much once painted.


  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    Consider that the pipe would only get used if the tank malfunctioned. That can happen if the pressure gets too high, or the temperature exceeds the safety setting. This means that whatever you use must be rated for the high temp of the T&P valve plus (although the paint might prevent it) it needs UV protection so it doesn't degrade. Some can get brittle when they get cold, so that is a concern, too.
    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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