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Short version:

-I am interested in hearing your thoughts on how best to install a cold-weather sillcock/hose bib & I am running into some MIP/MNPT uncertainty.

-Situation: I can find an adapter from our water line's PVC to MNPT (SharkBite model UIP120); the sillcocks I am finding are MIP (or Shark Bite).

-Problem: I cannot find a MNPT to MIP adapter (e.g., 1/2" FNPT x FIP)

Longer version: Our home, in the south, was not designed for freezing weather. However, a hard freeze is predicted this winter (following several years of freezing winter weather) and I would like to 'upgrade' our standard hose bibs (1/2" PVC line in our crawlspace to standard hose bib) to frost free/freeze resistant sillcocks.

The crawlspace is very tight and I would like to avoid dealing with PVC primer and cement (and the possibility of not being able to do a good job making sure an appropriate join is made in the very cramped space).

It seems I have two options:

Preferred option (1):

  1. 1/2" PVC water line
  2. Shark Bite 1/2" PVC x 1/2" MNPT
  3. metal adapter that I can't seem to find and maybe doesn't exist? (1/2" FNPT x FIP)
  4. MIP sillcock

Alternate option (2):

  1. 1/2" PVC water line
  2. Shark Bite 1/2" PVC x 1/2" CPVC/PEX
  3. ~4 inches run of 1/2" CPVC/PEX
  4. Shark Bite 1/2 in. Push-to-Connect x 3/4 in. MHT x 12 in. Brass Anti-Siphon Frost Free Sillcock Valve

Question: -Am I missing something with regard to transitioning from MNPT to MIP? -I see in some places that people directly thread a MNPT fitting to FIP. (This makes me a bit nervous since the location is not readily accessible/able to be monitored) -Is there another, easier, option I have neglected?

TL;DR: all the water lines in the house are either 1" or 1/2" Charlotte Pipe PVC 1120 SCH 80. I am working with the understanding that MNPT/FNPT (male/female) National Pipe Thread is not directly compatible with Male/Female Iron Pipe given that one is tapered and the other is straight.

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    FIP = FNPT. MIP = MNPT.
    – popham
    Commented Oct 31, 2023 at 4:41
  • Don't use sharkbite fittings. I don't know who they bribed to get them into the UPC (and maybe the IPC too), but they shouldn't be in there. Installing male metal threads into PVC female threads is a recipe for disaster. Adapt the male metal threads to female metal threads before transitioning to PVC.
    – popham
    Commented Oct 31, 2023 at 4:44
  • @popham - thanks for weighing in! My understanding is that NPT is tapered and MIP is not; because of this a MNPT to FIP (or FNPT to MIP) is not the most prudent/reliable connection. Do I have this wrong?
    – Han
    Commented Oct 31, 2023 at 4:50
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    agree that FIP and FNPT are the same. Female hose thread is different. Also don't put male metal piece into female plastic as you can split the female plastic. Commented Oct 31, 2023 at 7:39
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    While you're at the big-box store, pick up a FIP and an MNPT fitting and try screwing them together. You'll find they fit quite nicely. This threw me for a while, too, until I actually tried them at the store.
    – FreeMan
    Commented Oct 31, 2023 at 12:00

1 Answer 1

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Male iron pipe (MIP) typically indicates male national standard pipe taper (MNPT), and analogously FIP typically indicates FNPT. Steel pipes have NPT threads (or straight NPSC threads, i.e. national standard pipe straight coupling threads) imposed by ASTM A53, Standard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc-Coated, Welded and Seamless, which references ASME B1.20.1, Pipe Threads, General Purpose. This pipe threads document defines NPT threads (and NPSC threads) very generically in terms of the pipe's outer diameter, but emergent products adopted "male iron pipe" or "female iron pipe" threads based on iron pipe outer diameters instead of their own pipe outer diameters. This allowed them to patch into existing steel pipe systems and interface with existing hose bibs, faucets, etc.

It's probably safe to assume MIP = MNPT and FIP = FNPT unless a fitting explicitly says that it's another thread pattern. The only other sealing thread type in that pipe threads document is NPSC threads. The most authoritative sounding source on MIP equivalence with MNPT that I found was this Pipe and Hose website. Looking at the audit trail of Wikipedia, though, I saw lots of similarities with this article. It may be an old iteration from Wikipedia that was rejected under their standards, so be cautious about believing it if you decide to check it out.

I would be angry if I hired a plumber to do some work while I wasn't around and then discovered that he had used sharkbite fittings in my water lines. It's so unprofessional that I would be tempted to withhold payment despite the fittings being code conformant. PVC is so easy to work with that you don't really have any excuse for using sharkbite fittings. You said yourself that you have no access for monitoring. Just don't do it.

Throwing away option 1 and option 2, then, you want to continue with the schedule 80 PVC. There's an issue any time you transition from metal to plastic. According to the 27th edition Machinery's Handbook, the plastic fitting transitioning to the metal fitting should be a male fitting:

When mating metal to plastics pipe threads, the threaded plastics component should be the male member, so that the plastics are in compression. If torque can be controlled during assembly, use [teflon tape] on female plastics pipe threads. If torque cannot be controlled, consider using an external hoop ring, either pressed on or molded in. Do not design flats into plastics parts for assembly purposes, because they will encourage overtightening. If some provision for improved gripping must be made, use wings or a textured surface.

You'll therefore need a metal FIP×FIP fitting to transition the frost free sillcock to PVC. It would be wise to torque this fitting and the transition PVC fitting onto the sillcock before priming and gluing the remainder of the line. This way you won't have to torque the sillcock into your relatively fragile PVC line.

If the hole in your wall isn't big enough to fit the PVC fitting through from the outside, I suggest reaming it open wide enough. By measuring through the hole, you'll only need to prime the old pipe from inside the crawlspace. After it's primed, you can install the sillcock from the outside.

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  • That was where I picked up that plumbing etiquette, @Freeman. It was the Machinist's Etiquette Manual.
    – popham
    Commented Oct 31, 2023 at 9:59
  • I have heard concerns, @Popham, about using Shark Bite connections but I haven't seen any systematic review or well cited source for them being inferior. Might you have a reference to look at or is your comment based on personal experience? Thanks!
    – Han
    Commented Oct 31, 2023 at 20:38
  • It seems worthwhile to add valves when doing work like this. @Popham (or others), if you agree, it seems that it would be reasonable to go from 1/2" PVC supply line to a PVC valve that transitions with a female PVC inlet and a PVC male MIP outlet. Then a metal FIPxFIP to the MIP sillcock. Do you agree? Does a PVC inlet x PVC male MIP outlet exist (I can't find one)?
    – Han
    Commented Oct 31, 2023 at 20:58
  • @Han, push fittings fittings aren't allowed under the gas code. Why is that? They're unreliable. The time horizon on PVC, copper, and PEX systems are 50 years plus. The time horizon on push fittings (by their own literature) is 25 years. Especially at a position where I expect a fitting to get periodically stressed, I would never use one. I love them for temporarily capping a line. That's it.
    – popham
    Commented Oct 31, 2023 at 21:33
  • @Han, such a valve's only use is for winterization, right? What purpose would your valve serve? It has a higher probability of failure with no purpose to my eye. More cost, more labor, higher risk of failure, and no purpose. Why do that?
    – popham
    Commented Oct 31, 2023 at 21:39

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