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My dishwasher's hot water supply line is braided stainless steel going to a separate cut-off valve. I assumed it would be a compression fitting, but it looks like it's been tapped with PTFE tape, and I've read in numerous places that you're not supposed to tape compression fittings.

Hot water cut-off with what looks like PTFE tape

It's been like this since I moved in 8 years ago. I rented the property for three years before buying it from my landlord, and (a) I know that he liked to save money by doing work himself, and (b) I can fully believe he incorrectly taped a compression fitting because he didn't know better.

If I disconnect the supply line (e.g., to replace the dishwasher) how do I reconnect it properly?

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    that looks suspiciously like a quick connect type
    – Traveler
    May 14, 2023 at 21:35

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Typically in the US, those water supply line shutoffs (like yours) have a male threaded "compression" outlet. New ones come with the threaded male outlet, a soft metal compressible "ferrule" and a nut that fits over the ferrule. In days gone by, the line running to faucet etc. would actually be a hard or semi hard metal tube attached via a real compression connection using the ferrule and compression nut onto this threaded outlet.

In that case, the watertight seal is a metal-to-metal outlet/ferrule/metal tube unthreaded seal (the threads are used only to drive the outer compression nut). Since the threads are not making the seal, no tape/dope is necessary on the threads, but wouldn't hurt as long as it does not get into the ferrule sealing area.

Nowadays, almost always the line running to the faucet etc is a flexible braided hose with attached nut on the end matching the shutoff outlet's "compression" drive threading, but not actually making a true compression connection. On first installation, one first discards the ferrule and loose nut that came with the male end. Instead of a true compression connection, the hose-attached nut makes a standard tubing end seal using a rubber gasket inside the base of the nut. There is no ferrule and no metal-to-metal seal; as the seal is with the rubber washer inside, the threading is also not part of the seal. Thus again, teflon tape or pipe dope is not necessary but doesn't hurt unless it gets between the tubing end and the rubber gasket.

Such rubber-gasketed seals are vulnerable to over-tightening, which can dislodge the rubber gasket and cause a leak. Proper tightening of such a connection is to first hand tighten nut with fingers so the male outlet is seated against the bottom gasket, then use a wrench to tighten an additional 1/2 turn or so.

You will not need teflon tape or pipe dope when you re-connect. However, since the inner plastic/rubber tubing in the braided supply hose degrades over time, it's strongly recommended to purchase a new replacement braided hose for your work.

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