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Thread: Replacing Gas WH Questions

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  1. #1
    DIY Junior Member sjm133's Avatar
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    Default Replacing Gas WH Questions

    Hi all,
    I have a 15 year old gas WH that's still running fine but I know the time is coming to replace it and I'm starting to formulate my game plan for it. The WH is original to the house and at the time it was built drain pans were not in the code so it doesn't have one. Now when I replace the WH I will be installing a drain pan. Plumbing the drain is the basis of my question.

    Right now my T/P release is plumbed through the wall and outside. This is what I'm thinking. My plan is to connect my drain pan line to the same line as T/P line but to place a check valve between the drain line and the T/P so that if the pressure valve opens it will not blow water, steam, or both, back through the drain line into the pan. The WH installations all reference the T/P pressure at 15 psi and the check valve is rated at 200 psi so does anyone have any thoughts, good or bad, on this potential solution? With the location of the WH in the house I do not see any other way to plumb the drain line.

    Thanks in advance.

  2. #2
    DIYer, not in the trades LLigetfa's Avatar
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    Unless there is adequate fall from the pan to the valve, there might not be enough pressure to open the valve. Can you not just enlarge the hole and push out a second pipe?

  3. #3
    DIY Junior Member sjm133's Avatar
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    The WH is on a platform about 18 inches off the floor. The T/P drain goes into the wall at 14 inches off the floor. Now I could raise the platform a little more if needed. The T/P drain goes in the wall and then bends 90 degrees and travels about 8 feet inside that wall until it exits outsilde. Since that wall is studded I can't use the same path. That's what I mean about the location and combining the drain lines as the only solution I see. Would I even need the check valve? I've never seen a T/P valve open due to failure so I don't know what kind of pressure the water exits.
    Last edited by sjm133; 08-17-2011 at 11:55 AM. Reason: punctuation...

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    Just brainstorming here...not sure I'd worry about it and just tie them together. One of the pros would know if that is to code.

    A T&P valve normally only opens if there is a fault, and often just weeps a little bit of water. Now, if you manually open it, it can come out with the full force of your supply water pressure, but then, you're doing that and can control it. If you added a WAGS valve, if water ever built up in the pan, it would shut both the water and the gas off, so there couldn't be much damage. www.wagsvalve.com
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    DIY Junior Member sjm133's Avatar
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    Hopefully a "pro" will respond. I really don't want to tear my wall apart to add an additional drain. If there isn't much pressure when/if the T/P valve opens then I don' think there will be a problem with combining them. I checked my local building plumbing codes and didn't see anything about this.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    The more common reason a T&P opens is from excess pressure. It only takes a few ounces to reduce the pressure, and then it closes. Water expands (like most everything) when it is heated...the tank and pipes are pretty stiff, and it doesn't take much increase in volume to radically increase the pressure (or reduce it). Water doesn't, for all practical purposes, compress, so when it expands, if there's no place to go, you are hydroforming your heater and pipes (i.e., expanding them). Something has to give, and the T&P is the safety valve to relieves that pressure.

    The other reason it can open is if the temperature runs away, and it opens before you would get a steam explosion (which you DON'T want to happen, as it may level the house). That would require a failure of the gas valve and it to be on constantly.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #7
    DIY Senior Member BobL43's Avatar
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    Quote Originally Posted by sjm133 View Post
    Hi all,
    I have a 15 year old gas WH that's still running fine but I know the time is coming to replace it and I'm starting to formulate my game plan for it. The WH is original to the house and at the time it was built drain pans were not in the code so it doesn't have one. Now when I replace the WH I will be installing a drain pan. Plumbing the drain is the basis of my question.

    Right now my T/P release is plumbed through the wall and outside. This is what I'm thinking. My plan is to connect my drain pan line to the same line as T/P line but to place a check valve between the drain line and the T/P so that if the pressure valve opens it will not blow water, steam, or both, back through the drain line into the pan. The WH installations all reference the T/P pressure at 15 psi and the check valve is rated at 200 psi so does anyone have any thoughts, good or bad, on this potential solution? With the location of the WH in the house I do not see any other way to plumb the drain line.

    Thanks in advance.
    Just out of curiousity, and not exactly related to your actual question, but are you sure the temp pressure valve for the water heater is spec'd at 15 PSI? I would thing that is WAY too low for a water heater, but as I say, I am not a pro plumber!
    I am definitely not a pro plumber, but I am a pro crastinator

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    DIY Junior Member sjm133's Avatar
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    I knew I shouldn't have typed that!! I can't remember where I read that, but I believe it was regarding the drain pipe attached to the valve, not the valve itself. Sorry for the confusion. The point I was trying to get at was that I wasn't sure of the force of whatever would go into that drain line in the event of the valve coming into play and whether it would be of enough force to blow back into the drain pan if the two were connected together. From what I've read I don't think it would be.

    As a side note to my original question, I've recently seen other installations that simply place the T/P drain into the drain pan itself and then just plumb the pan outside. I guess that would be my other option??

  9. #9
    Retired Defense Industry Engineer jadnashua's Avatar
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    Depends on local codes...the release point on the T&P valve is a lot higher than 15psi - (without looking, I guess I should remember), it's about 150psi. But, generally, discharge into the pan is likely okay.

    WRT the max pressure of the water coming out, it depends on what tripped it, temp or pressure. If it was temp, then it could be anything up to the incoming line pressure (assuming you have an expansion tank). If it is pressure, since the pressure drops almost immediately as you release a little water, it would most likely just weep out as it expanded (this would assume you either didn't need or had an ineffective expansion tank, if one was needed based on the plumbing configuration). IOW, it's unlikely it would come out with more force than if you manually opened it (like a hose bib).
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  10. #10
    Moderator & Master Plumber hj's Avatar
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    The drain line from a T&P valve will range between city pressure and 150 psi, depending on WHY it opened. There are three things you can depend on as far as check valves are concerned;
    1. if the do NOT operate frequently they will be stuck closed, and,
    2. if they DO operate frequently the pivot will wear and the flap will become misaligned, and,
    3. you cannot depend on them sealing 100%.

    A "pressure line" and a "gravity line" should NEVER be interconnected, and as an aside, you are not even allowed to connect two relief valve discharge lines together.
    Licensed residential and commercial plumber

  11. #11
    Nuclear Engineer nukeman's Avatar
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    Most of what you want to do wouldn't fly here (and probably the same for your location). Our code is IPC based, and I bet AL is the same:

    504.6 Requirements for discharge piping. The discharge piping serving a pressure relief valve, temperature relief valve, or a combination thereof shall:

    1. Not be directly connected to the drainage system.

    2. Discharge though an air gap located in the same room as the water heater.

    3. Not be smaller than the diameter of the valve served and shall discharge full size to the air gap.

    4. Serve a single relief valve device and shall not connect to piping serving any other relief device or equipment.

    5. Discharge to the floor, to an indirect waste receptor or to the outdoors. Where discharging to the outdoors in an area subject to freezing, discharge piping shall first be piped to an indirect waste receptor through an air gap located in a conditioned area.

    6. Discharge in a manner that does not cause personal injury or structural damage.

    7. Discharge to a termination point that is readily observable by the building occupants.

    8. Not be trapped.

    9. Be installed as to flow by gravity.

    10. Not terminate more than 6 inches (152 mm) above the floor or waste receptor.

    11. Not have a threaded connection at the end of such piping.

    12. Not have valves or tee fittings.

    13. Be constructed of those materials listed in Section 605.4 or materials tested, rated and approved for such use in accordance with ASME A112.4.1.

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