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Thread: PRV installation questions

  1. #1
    DIY Member bsperr's Avatar
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    Default PRV installation questions

    I've really enjoyed reading the threads and learning more about plumbing. I'm planning my first real plumbing job that will require permitting and inspection and wanted to run a couple questions by you guys. I'm going to install a PRV and expansion tank in our (old) new home. It's never had a PRV, and the water pressure is at about 90psi. I have a very accessible crawl space, and there's a nice horizontal run on the cold water supply to the hot water heater where I'm going to install the expansion tank and a nice vertical run where the line enters the house where I'll install the PRV.

    1. I wanted to buy a Watts expansion tank, and in the installation instructions I read online, it advises installing a backflow preventer on the line before the PRV. I went to Lowes, and all they carry is a Wellsaver brand expansion tank. In their instructions, it says no backflow preventer is necessary because the tank is designed for potable water (which doesn't really make sense to me). I've called the building inspector to see if a backflow preventer is necessary in our area, but haven't heard anything back yet. I also don't know if there's a backflow preventer incorporated into the meter. Would you all install the backflow preventer just to be safe? I've attached a picture of my proposed install.

    2. This is probably a stupid question, but the PRV and backflow preventer both have integral union gaskets to attach to threaded pipe, so do I use teflon tape both on the threaded adapter on my supply line and on the treaded portions on the bodies of the PRV and backflow preventer before I hook up the union (I know that you normally don't use teflon tape when you have a gasket that does the sealing, but wondered if it might be a necessary in these cases). Thanks for your help--Brad
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    In the Trades Gary Swart's Avatar
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    Interesting instructions. A PRV has a check valve built in to it that prevents the heated water from being absorbed by the water main. That's why you need an expansion tank. More over, if you do not have a PRV, you don't have a check valve so they are implying that something about the PRV makes a check valve necessary. It doesn't make sense to me at all. The only caution I would have for you is to make certain the expansion tank is well supported. I have mine hanging down between 2 ceiling joists and the pipe is clamped to the joists. I'd be interested to hear if you ever get any clarification on why Watts deems a backflow is needed.

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    DIY Member bsperr's Avatar
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    A backflow preventer is also suggested in the installation instructions for the Watts 25AUB-Z3 PRV I'm using, and in the same configuration as the Watts expansion tank instructions. I wondered if it might be a code requirement in some places, but maybe they're just trying to sell more products . I agree that it doesn't seem necessary when you have a PRV.

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    Moderator & Master Plumber hj's Avatar
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    Default prv

    The PRV acts as a check valve, so a second one would be redundant, and without either a check valve or a PRV the expansion tank would not be necessary.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Some PRV's have a bypass...if the interior pressure exceeds the supply, it opens. So, it is still possible to get water to flow out into the public supply. Not all PRV's have this (and I think it is just one more thing to fail).
    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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    DIY Member bsperr's Avatar
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    Quote Originally Posted by jadnashua View Post
    Some PRV's have a bypass...if the interior pressure exceeds the supply, it opens. So, it is still possible to get water to flow out into the public supply. Not all PRV's have this (and I think it is just one more thing to fail).
    Maybe that's why the Watts PRV recommends the extra backflow preventer, because it does have a bypass feature. According to the specs, the bypass only kicks in when the pressure in my lines rises 10psi above the supply pressure, which I can't imagine happening with a supply pressure of 90psi and my expansion tank. But I wonder if the supply pressure could drop off suddenly and create a backflow. Do you all think that the backflow preventer is still unnecessary?

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    In the Trades Gary Swart's Avatar
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    As I said in my previous answer, why would a back flow preventer or check valve be necessary at all? You certainly don't need one in a system that does not have a PRV, so what makes one necessary when you add a PRV to the system? Answer: NOTHING. Only when the supply line is split for an irrigation system, is a back flow device required, but that is on the irrigation side to prevent irrigation water from getting back into the domestic side.

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    Engineer Furd's Avatar
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    The instructions do not suggest or recommend the back flow preventer but only show one in the diagram of a suggested installation. Perhaps some water utilities require the back flow preventer but unless yours does I would not install one.

    What I WOULD do is install the PRV where it can be seen on a regular basis. Do not install it any farther from the first branch off of your water supply than is necessary. You might consider installing it so that the outside hose bibs get water before the PRV.

    I would also install gauges and shut-off valves on either side of the PRV to allow for easy servicing and indication of proper operation. A bypass to allow for continued water service in the event of PRV failure or servicing is sometimes a nice feature. I would install a tee first with the branch of the tee sized (or reduced to) 1/4 inch threaded with a ball valve and then gauge rated at 200 psi. Next would be a full-sized ball valve, the PRV, another full-sized ball valve and then another tee (with valve) for the second (regulated pressure) gauge which would be rated at 150 psi and then continuing to the existing house piping. If you wanted a bypass then the bypass would be piped from before the first full-sized ball valve to after the second full-sized ball valve with a good globe valve in between.

    Only use Teflon tape on tapered pipe threads, not on any joint that is sealed by a gasket. I recommend also using Teflon paste over the Teflon tape.

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    DIY Member bsperr's Avatar
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    Thanks Furd, that sounds like a good way to go. I thought that a backflow preventer was necessary in any residential installation, but it sounds like it's more of a specialty application or additional requirement in some areas. Thanks guys--Brad

  10. #10
    Moderator & Master Plumber hj's Avatar
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    Default bypass

    I would not install a bypass, because the customer could get to messing with the valves and open it and then leave it open.

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