Water Heater Supply Lines — When to Replace & Why They Fail

The pair of short flexible pipes connecting your water heater to your home’s plumbing are some of the most overlooked parts of the whole system — and they’re a major source of water damage when they fail. After 40 years of plumbing work in Burleson, I’ve cleaned up a lot of garages and water heater closets where a $20 supply line gave out and flooded the house.

Here’s what every homeowner should know about water heater supply lines — what they do, why they fail (electrolysis is the #1 culprit), what types to use, and when it’s time to replace them.

What Are Water Heater Supply Lines?

The supply lines (also called connectors, flex lines, or whips) are the two short pipes that connect your water heater to the house plumbing:

Cold water inlet — bringing fresh water into the heater
Hot water outlet — sending heated water out to your faucets

They’re usually 12–24 inches long with threaded fittings on each end. Their job is to absorb minor movement and thermal expansion so the heater doesn’t put stress on rigid pipes elsewhere in the system.

Types of Supply Lines (Choose Wisely)

There are four main types you’ll see:

1. Stainless steel braided flex lines — The current standard. Stainless braid wrapped around a polymer core. Durable, flexible, easy to install. Last 5–10 years. Cost: ~$10–$20 each.

2. Corrugated copper flex lines — Older standard. Solid copper, ribbed for flexibility. Last decades if installed properly. Common in 1980s–1990s installs. ~$15–$25 each.

3. PEX flex lines — Newer option. Flexible plastic. Long-lasting but require specific fittings. ~$10–$15 each.

4. Rigid copper or galvanized pipe — Original construction in older homes. Lasts a long time but doesn’t absorb thermal expansion well. Usually replaced with flex lines when the heater is replaced.

My recommendation for most replacements: stainless steel braided. Easy to install, durable, predictable, easy to find.

Why Water Heater Supply Lines Fail

Several causes, ranked from most to least common:

1. Electrolysis (the #1 killer) — When copper supply lines connect directly to galvanized steel fittings or vice versa, you get galvanic corrosion. Two dissimilar metals + water + minerals = small electrical current that eats away at one of the metals. This is why you see green or blue corrosion at the fittings.

The fix: use a “dielectric union” — a special fitting that separates the two metals — or use stainless braided lines that resist this kind of corrosion.

2. Age — Even good supply lines wear out eventually. The polymer core inside stainless braided lines can crack and develop pinholes after 5–10 years.

3. Pressure spikes — If your home doesn’t have a working pressure-reducing valve (PRV) or expansion tank, every time the heater fires, pressure surges stress the connections.

4. Improper installation — Over-tightened fittings strip threads or crack the line. Under-tightened ones leak slowly.

5. Kinks and twists — Flex lines that get kinked during install can develop weak spots that fail later.

Signs Your Supply Lines Need Replacement

Inspect the supply lines once a year. Look for:

Green or blue corrosion at the fittings — classic sign of electrolysis
White or chalky deposits — mineral buildup that can indicate slow seepage
Visible rust streaking down the line
Bulging or swelling anywhere along a flex line
Wet spots near the heater that can’t be traced to anything else
Drips at either connection
Age over 8–10 years for braided lines (consider preventive replacement)

If you see any of these, replace the lines BEFORE they fail. A leak from a corroded supply line in an upstairs water heater closet can dump hundreds of gallons before anyone notices.

How to Inspect Your Supply Lines (DIY)

Takes 2 minutes. No tools needed.

Look at the top of your water heater. You’ll see two pipes coming out — one cold (with a shutoff valve), one hot.
Inspect each connection point with a flashlight. Look at where the line connects to the heater AND where it connects to the house pipe.
Check the line itself for visible damage, kinks, swelling, or discoloration.
Feel the connections for any moisture — even slight dampness is a sign.

If everything looks clean and dry and the lines are less than 5 years old, you’re good. Otherwise, time to plan a replacement.

DIY Replacement of Water Heater Supply Lines

Tools:

Two adjustable wrenches
Towel and bucket
New braided stainless steel flex lines (matching length)
Teflon tape (just in case)

Steps:

Turn off the cold water supply to the heater at the shutoff valve above it.
Shut off power (electric) or gas (gas heater) to be safe.
Open a hot water faucet in the house to relieve pressure.
Place a bucket under the connection.
Use two wrenches — one to hold the heater nipple, one to unscrew the supply line. This prevents twisting and damaging the heater itself.
Disconnect the line at both ends.
Inspect the threads on the heater. If they’re corroded, you may need to call a plumber to clean or replace them.
Install the new line — most new flex lines have rubber washers built into the ends, so you usually don’t need Teflon tape. Hand-tighten first, then snug with a wrench.
Use two wrenches again when tightening, just like disassembly.
Turn the cold water back on slowly and check for leaks.
If dry after a few minutes, turn power or gas back on.

If you see green or rusty corrosion on the heater’s threaded inlets when you remove the old line, install a dielectric union along with the new flex line. That’s a simple add but it stops electrolysis from killing the new line in a few years.

When to Call a Plumber

Call us if:

The fittings on the heater are corroded and won’t come off without damaging the heater
You see active electrolysis (heavy green corrosion) — needs proper dielectric isolation
The heater itself is also showing signs of failure — might be smarter to replace the whole heater
The supply lines are rigid copper or galvanized — replacement may require soldering or threading new pipe
You see signs of leaks behind walls from the connection point
You’re not comfortable working in tight spaces with two wrenches

Cost & When to Bundle

A simple supply-line replacement on an accessible water heater is a sub-three-figure job for most homes — most of the cost is labor (driving out, shutting off, draining if needed, replacing, refilling). The lines themselves are cheap.

If we’re replacing supply lines AND installing a dielectric union, add a small amount for the extra fitting. Worth every penny on any water heater that’ll stay in service more than a few more years.

If your water heater is more than 10 years old AND the supply lines are also failing, sometimes it makes sense to replace everything together rather than do the lines now and the heater later.

A Word From Don

If you’re seeing corrosion at your water heater connections and you’re not sure how bad it is, give us a call. We can come take a look and recommend repair, replacement, or just keep an eye on it. Most supply-line jobs are same-day service on most plumbing calls. Free phone estimate before we head out.

Serving Burleson, Crowley, Joshua, Alvarado, Keene, Everman, and all of South DFW within about 20 miles of Burleson.

Call Don: 817-447-2654