CPVC MIP threads compatible with FIP flexible metal faucet line? (installing undersink water filter)

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by FM1, Jun 6, 2021.

  1. FM1

    FM1 New Member

    Joined:
    Jun 6, 2021
    Location:
    WI
    I thought this question would have an obvious answer, but I can't find anything that specifically says to do it or to not do it.

    Want to install a whole house water filter under the kitchen sink cold line for extra filtering of drinking water.

    The current line is copper, to 12" of braided flexible line, to faucet line. The copper distribution line has the usual 3/8" compression. Faucet has the usual 1/2" FIP.

    The water filter housing has 3/4" female outlets. With schedule 40 PVC fittings and a short section of 3/4" schedule 40 PVC pipe, I made 3/4" male to 1/2" male threaded outlets, in the picture. That would give me 1/2" threaded male connections on the water filter housing to connect the braided flexible line(s) to.

    Is threading the stainless braided flexible FIP o-ring line onto MIP threaded PVC ok? I think so, but looking around the internet, and reading 20+ searched threads on here, etc, I can't find a picture or video of anyone actually doing it.

    They always add a metal shutoff valve, and then the threaded connection is metal to metal. But I've never seen anyone say do that to get a metal to metal threaded connection. They just say they're doing it to add a shutoff valve.

    I've seen warnings about using metal ferrules on plastic lines, and warnings about using male metal threaded fittings into female plastic threaded fittings, and don't use teflon tape or dope on o-ring line connections, etc. I've never seen a warning about this, but I've also never seen anyone do it.

    And before anyone says it, haha, yes, I'm using (schedule 40) PVC not CPVC. It's for cold water, and a temporary setup. Planning on putting the filter in-line later in the basement when redoing all the CPVC down there with PEX.

    IMG_20210603_171258188[1].jpg
     
  2. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    Your thoughts are sound and well-researched.

    Why are you replacing the CPVC with PEX?
     
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  4. FM1

    FM1 New Member

    Joined:
    Jun 6, 2021
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    WI
    It's old. The main concern is the old (and small) pressure tank, and the metal pipes that come out of the pressure tank and connect to the main in-house CPVC line.

    It's mostly a "may as well."
    Replacing the pressure tank and the couple feet of metal lines coming out of it that connect to the CPVC, may as well also replace the main CPVC line after that section since there's only 10 or so feet of it. The lines from that main in-house CPVC line to the fixtures are all copper. Those might stay, but maybe not since they're at least 30 years old and maybe much older than that. And it's well water, which supposedly wears out pipes faster.

    The CPVC and copper lines seem fine, but I don't want to mess with any of that main distribution area, (such as installing this second water filter down there), without being prepared to have something quite annoying break and have to replace it as an emergency instead of on a planned timeline. Also, the current distribution layout doesn't work well for adding another filter, and where the water softener should go, etc, so it could use a do-over anyway.

    I have confidence in CPVC and would be fine redoing it with new CPVC despite many people saying CPVC is for hacks, but PEX is the new hotness, and my brother has DIY'd it and has the tools, so definitely leaning that route.
     
  5. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    Ahh. Part of the new tank update. Makes sense.


    Common pressure tanks that are bigger than 32 gallon, such as 44 gallon, have 1-1/4 threads instead of 1 inch threads. Just a point of planning if you were going bigger than 32.
     
  6. Tuttles Revenge

    Tuttles Revenge In the Trades

    Joined:
    Oct 15, 2014
    So to answer the original question
    Yes, that would be perfectly fine. The reason is that you're not applying an incredible amount to torque to seal the fittings. Just hand tight plus a quarter turn by wrench is what most flexible supply manufactures recommend.

    Other applications of plastic threads mating with metal vary on a case by case basis as to whether they are correct or not. In this case it shouldn't present a problem
     
  7. FM1

    FM1 New Member

    Joined:
    Jun 6, 2021
    Location:
    WI
    Thanks. It seemed obviously yes, at first, but then doubt crept in, and I figured I should ask some smart people what they think.

    And the concern isn't even the o-ring leaking. It's that the braided flex line with such a short fitting and few threads makes me wonder if it will pop off entirely one day.

    There aren't many threads in there above the o-ring. It's like two turns hand-tight and it's tight, on the PVC. Connecting the braided flex line to a brass fitting, it gets seemingly an extra turn or two to be hand-tight. It feels and looks like it makes a deeper connection.

    That probably just means the metal on metal threads make a smoother connection that's easier to turn, and the metal on PVC just needs a little extra oomph. But then that's more than hand-tight + 1/4 turn. Is all this unnecessary paranoia? Most likely.

    And then compare a typical stainless flex line to the old one that was under the sink. This old one has a plastic end some people say to avoid due to potential cracks, but look how deep the fitting to the o-ring is. About twice as many threads as the metal flex line has. Screwing the deep plastic one on feels more assured than the shallow metal one. Definitely not putting that old one back on though.

    IMG_20210607_122649321[1].jpg
     
  8. Tuttles Revenge

    Tuttles Revenge In the Trades

    Joined:
    Oct 15, 2014
    If you're talking PVC male threads into any other tapered threaded fitting to create a leak free seal is much different than using a flexible supply line on the same male threads. They're not comparable.

    There are millions, if not billions of flexible supply tubes attached to faucets and toilets around the world. They are almost all identical in that they use a soft rubber cone washer at one end and typically a square cut rubber washer at the 3/8" or compression fitting end. Too much tightening will squish out the rubber gasket from the compression side.. Been there done that.
     
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