The gas line coming into my newly redesigned kitchen and supplying a range has to be routed under the sink because there was no room behind the rear left corner to go behind the studs in the wall. The valve on the left, terminating a 1/2" black pipe, is where it enters and the nipple in the back wall is where it exits the sink to travel another approximately 10' to the range, again via 1/2" black pipe.

enter image description here

Of course, because the area under the sink is messy, busy, and complicated, as can be seen in the picture, with other pipe, cable, garbage disposal etc, running black pipe there, with a minimum of two elbows in a tight space would be inconvenient, obstructing and difficult to turn the wrench so I want to use the yellow flex connector. However, I know that flex is usually used between a black pipe terminal and an appliance and what we have here is mid-line.

So I have several questions:

  1. Is this generally permissible by the US codes (specifically mid Atlantic, relatively strict)?
  2. Considering that the two ends of black pipe it connects are 1/2", can I use 1/2" flex (which I think is smaller inside diameter than 1/2" black pipe), which I obviously prefer for spacing reasons, or should I go with 3/4", which I think corresponds more in inside diameter with 1/2" black? The final flex connector at the end that connects the range is 1/2". The picture shows a 3/4" that is 36" long, which is too long and I plan to replace it with 24" so I don't need to wrap around the PVC pipe like it shows here, which comes in both 1/2 and 3/4". Does it actually matter and can using a slightly fatter pipe between two smaller cause any adverse effects?
  3. Someone (whose advice I am not taking unverified) told me that this connector might need to be electrically grounded. I do have an adjacent junction box in a laundry bay to the left of the sink. Should I ground it using a pipe grounding clamp?
  • Just a comment as gas pipes are outside my comfort zone: Couldn't you take out the disposal and the drain pipes (except the long piece going to the wall) temporarily, put in black pipe (with 3 elbows, keeping low and near the wall most of the way) and then put disposal & drain pipes back in? Dec 20, 2020 at 18:47
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    Turn 90 degree at the stub out then add valve.
    – Kris
    Dec 20, 2020 at 18:56
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    @amphibient - It is not impossible nor impractical. If you are unwilling to except the correct way to do it then you should not ask for the correct way to do it. I would use black pipe with a union. 90 elbow, valve, run along side of cabinet, 90 elbow, union, 90 elbow into wall. If you do use the flex then you certainly should not weave it between plumbing, put it under all the plumbing.
    – Alaska Man
    Dec 20, 2020 at 19:57
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    They use a left-right nipple, as stated. Dec 20, 2020 at 20:11
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    It is a special coupling and nipple, both of which have a left-hand thread where they go together. Acts kinda like a turnbuckle in that they pull together. Specifically for joining gas pipe in the middle of an existing run. Dec 20, 2020 at 22:50

2 Answers 2


Valve at the appliance

You didn't mention whether this valve here under the sink is the only one or whether there's another shutoff at the appliance. A valve somewhere in the plumbing system is a nice optional thing, but there really must be a valve at the appliance for service or emergency use. A person unfamiliar with the house is never going to think "hmm, no valve behind the range... maybe under the sink??"

Appliance connector.. not the right fit

The flexible tube connector shown in the photo is an appliance connector. When necessary, it is customary to have some hard pipe directly attached to an appliance and then use the flex connector to make the jump from the appliance to in-wall piping. Think of a furnace, for example: hard pipe extends from the gas valve to the exterior of the furnace cabinet and flex goes from there to the building piping. It's also not unheard of to join two flex connectors to gain additional length.

It's easy to imagine a mash-up of the furnace situation and the two-connectors-for-extra-length situation and morph it into something like what you've built here. I'm no expert in the fuel gas code, but I suspect that's taking things a bit too far. An appliance connector isn't the right material to use for making this connection.

Making things neater

So, how could things be better? First, put an elbow on each pipe stub and aim them both toward that left-rear corner. This at least gets you started on the right foot, keeping the connection out of the jungle in the middle of the space.

Next, if you really must have flexible tubing, CSST (corrugated stainless steel tubing) is the right material. I know, I know, it looks like exactly the same stuff as the appliance connector. Except it is listed as building piping. You can get this in many home centers now. It comes in coils of various lengths; you trim it to fit just right and then install connectors on the ends.

Save some money

CSST may be relatively expensive for the 2 feet that you need. So, about those left-right nipple/coupling sets.. Hang on, don't leave!

You can minimize in-cabinet wrenching by strategic pre-assembly of parts. I would do it as three:

  1. 90 degree elbow with left-right nipple pre-installed. This will go on the supply pipe.
  2. Long piece of pipe, 90 degree elbow, short nipple, elbow, another long piece. This will go horizontal along the left side, upward in the rear corner, then horizontal across the rear wall.
  3. Another 90 degree elbow with left-right nipple. This will go on the stub heading out to the range.

You can see how these will come together. Install the elbow-and-nipple assemblies on the stubs first. Then lift the pipes-and-elbows assembly into place in the rear corner. Use the two left-right couplers to pull it all together.

  • Yes, of course there is a valve at the appliance. This valve is to shut off gas if work under the sink is needed
    – amphibient
    Dec 21, 2020 at 4:07
  • Thanks for the detailed response. However, the appliance connector still appears as the most optimal albeit not the ideal or textbook picture solution. Assuming this does not get inspected, what are the actual risks or consequences of using it?
    – amphibient
    Dec 21, 2020 at 5:44
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    @amphibient Because your #1 question was "Is this generally permissible by the US codes" many of us inferred that compliance to codes is important to you. This is why you've received "that's not so good; here are alternatives" responses.
    – Greg Hill
    Dec 21, 2020 at 16:29

Your p-trap looks setup to siphon. Is the pvc exiting the left side of the cabinet the drain arm or is it something dumping into your waste pipe?


Even if you could run your gas line like this and technically meet code I am sure the inspector would fail you for workmanship.

No one is going to service your under sink plumbing with a gas line run like and even if they do they'll require you to shutoff the gas. Installing or removing the garburator, adjusting the p-trap or swapping out the faucet should not require you to shut off the gas.

You have lots of options for running the gas line. Outside, in the floor assembly (assuming not slab on grade), the studs bays (don't quite understand the comment about not having space in the left rear cabinet), possibly using the kick space of the cabinets (not quite sure if there would be a code problem with that but certainly better than this).

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    I agree, the trap and piping will not work properly like that, also your garbage disposer will remain full of water at all times (unless the inevitable siphon effect due to this arrangement pulls it and the trap dry). That is not an acceptable use of a flex gas connector.If you read the flex manufacturer's instructions you will see that they are specifically and only for a connection between a stub-out valve and an appliance. Whether or not your local code directly addresses, your code certainlt requires following manufacturers' instructions. Dec 20, 2020 at 19:01
  • what do you mean "p-trap looks setup to siphon" ? does that have anything to do with the gas line?
    – amphibient
    Dec 20, 2020 at 19:06
  • do you think the P trap is not graded properly into the wall pipe? It is but you don't see it because of the angle of the picture. Notice that the floor of the cabinet is sloped up for the same reason. I shouldn't have to explain this...
    – amphibient
    Dec 20, 2020 at 19:08
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    If that is a vent it is not done correctly. A vent can't run horizontal until it is higher than the flood surface of the sink. Dec 20, 2020 at 19:18
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    The reasoning is that you get a clog downstream of the vent debris can accumulate in the horizontal section and clog it. A clogged vent allows siphoning and siphoning allows sewer gas, sewer gas is explosive and not nice smelling. It also signals to inspectors that things are not up to code and to look closer for other problems as this would never pass inspection. Dec 20, 2020 at 20:15

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