Step-by-step fix
Step 1: Make sure the hot supply line is really the problem
- Open the hot side of the faucet and note the symptom: visible dripping under the sink, a bulged or kinked line, corrosion at the nuts, or weak hot flow from a damaged line.
- Look under the sink and trace the hot line from the hot shutoff valve up to the faucet connection. The hot side is usually on the left at the faucet.
- Dry the line and fittings with a towel, then run the hot water briefly and watch for fresh water forming on the line or at either end.
- Check that the leak is not coming from the shutoff valve body, the faucet itself, or the drain above the line.
If it works: You have confirmed the hot supply line or its end connections are the source of the problem.
If it doesn’t: If the valve body is leaking, the faucet is leaking from above, or the drain is dripping onto the line, fix that problem instead of replacing the supply line first.
Stop if:- The shutoff valve will not close fully.
- The valve, wall pipe, or faucet shank is badly corroded or feels loose in the sink.
- You see cabinet damage, mold, or signs of a long-term hidden leak that may need broader repair.
Step 2: Shut off the hot water and set up the work area
- Turn the hot shutoff valve clockwise until it stops. Do not force it hard.
- Open the hot side of the faucet to relieve pressure and let the remaining water drain out.
- Place a bucket or bowl under the hot supply line and lay down a towel in the cabinet.
- Use a flashlight so you can see both ends of the line before you start loosening anything.
If it works: The hot line is depressurized and the area is ready for a clean removal.
If it doesn’t: If water keeps running strongly from the hot side, the shutoff valve is not holding and should be repaired or replaced before this job continues.
Stop if:- The shutoff valve leaks around the stem or body when you turn it.
- You cannot access the fittings safely without forcing tools into a tight space.
Step 3: Remove the old hot supply line
- Hold the shutoff valve steady if needed, then loosen the supply line nut at the valve end with an adjustable wrench.
- Loosen the upper nut where the line connects to the faucet shank or faucet inlet.
- Let the remaining water drain into the bucket, then remove the old line completely.
- Compare the old line to the new one for overall length, nut size, and connection style.
If it works: The old hot supply line is out and you have a matching replacement ready.
If it doesn’t: If the new line does not match both ends and the length, pause and get the correct replacement before installing anything.
Stop if:- A fitting starts turning with the nut and you cannot hold it securely.
- The faucet shank spins in the sink deck or the shutoff valve twists at the wall.
- The threads on the valve or faucet connection are damaged.
Step 4: Install the new supply line without twisting it
- Start the top connection by hand first so the threads engage cleanly. Then start the bottom connection by hand.
- Make sure the line follows a gentle curve with no sharp bend, kink, or stretch.
- Tighten each nut snugly with a wrench. Do not overtighten; the goal is a firm seal, not crushed threads or distorted fittings.
- If one fitting wants to move while tightening, hold the stationary part with pliers or a second wrench so the valve or faucet does not twist.
If it works: The new hot supply line is installed straight, supported, and not cross-threaded or kinked.
If it doesn’t: If a nut will not thread on smoothly by hand, back it off and realign it before tightening again.
Stop if:- The line is too short to reach naturally or so long that it has to loop awkwardly into nearby plumbing.
- A connection will not tighten because the threads are stripped or mismatched.
Step 5: Turn the water back on and check for leaks
- Close the faucet handle.
- Slowly open the hot shutoff valve and watch both supply line connections as pressure returns.
- Wipe both fittings dry, then run the hot water for a minute while checking again with a dry finger or towel.
- If you see a small drip at a nut, gently tighten that connection a little more and recheck.
If it works: The line holds pressure and both ends stay dry while the hot water runs.
If it doesn’t: If a connection still drips after a small retightening, shut the valve back off, relieve pressure, and inspect for a crooked connection or the wrong line.
Stop if:- Water sprays, the leak gets worse quickly, or the shutoff valve fails while reopening.
- The leak is coming from the valve body or faucet rather than the new supply line connection.
Step 6: Confirm the repair holds in normal use
- Run the faucet on hot and mixed temperatures several times.
- Check under the sink immediately after use, then again after 10 to 15 minutes.
- Make sure the line is not rubbing sharply against the cabinet, drain, or another supply line.
- Leave the cabinet dry so any new drip will be easy to spot later.
If it works: The bathroom sink hot supply line stays dry during use and after the water has been off for a few minutes.
If it doesn’t: If moisture returns later, dry everything again and pinpoint whether it is from the upper connection, lower connection, valve, faucet, or drain so you can correct the right part.
Stop if:- You find recurring leaks from surrounding plumbing or signs that more than the supply line needs repair.
Replacement Parts
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FAQ
Do I need thread tape on a bathroom sink supply line?
Usually no on the supply line nuts themselves. Most faucet supply line connections seal at a washer or built-in gasket, not on the threads. Use the replacement line as designed and do not add tape unless the fitting specifically calls for it.
How do I know which line is the hot line?
At the faucet, hot is usually on the left. Under the sink, trace the line from the hot shutoff valve to the faucet connection. If needed, run the faucet briefly before shutting it off and feel which supply line warms up.
What if the shutoff valve will not close all the way?
Do not remove the line until you can stop the water reliably. A shutoff valve that will not hold should be repaired or replaced first, or the water may need to be shut off upstream.
Can I reuse the old supply line if it only leaked at one end?
It is better to replace it. Supply lines are inexpensive compared with water damage, and an older line that has already leaked may be worn, kinked, or corroded elsewhere.
How tight should the new supply line nuts be?
Hand-start them first, then tighten them snugly with a wrench. They should be tight enough to seal without crushing the fitting or damaging threads. If it drips, tighten a little more rather than forcing it hard all at once.