USA Expert Plumbing

Water heater connections explained: flex lines, nipples and unions

By USA Expert Plumbing Editorial Team. Updated . 1 min read.

Short answer

A water heater connects to the house through hot and cold water lines, usually with flexible connectors, and often dielectric nipples or unions that prevent corrosion where copper meets the steel tank. Gas heaters also have a gas connector, a shutoff valve and a sediment trap. Leaks at these connections are common and usually easy to fix.

In this guide
  1. Water connections
  2. Gas connections
  3. Common connection problems
  4. Signs a connection needs attention

The top of a water heater can look like a tangle of pipes and fittings. Each piece has a job, and knowing them helps you spot leaks and understand a plumber's quote.

Diagram of gas and electric tank water heater parts
Where the common parts sit in gas and electric tank water heaters.

Water connections

PartWhat it does
Cold inlet and hot outletBring cold water in and send hot water out
Flexible water connectorsMake installation easier and absorb movement
Dielectric nipples or unionsSeparate copper from the steel tank to limit galvanic corrosion
Heat trap nipplesReduce heat loss up the pipes when water is not flowing
Cold water shutoff valveIsolates the heater for service

Gas connections

  • Manual gas shutoff valve near the heater.
  • Sediment trap (drip leg), a short capped pipe that catches debris before it reaches the gas valve. Codes generally require one.
  • Gas connector, which must be an approved appliance connector sized for the heater. Old connectors should not be reused.

If you smell gas, do not touch switches or light anything. Leave the house and call your gas utility from outside.

Common connection problems

  • Corroded fittings with green or white crust, often where dissimilar metals meet
  • Drips from worn flex connector washers
  • Kinked connectors that reduce flow
  • A cold water valve that leaks at the stem or will not close

Small drips at fittings are usually simple repairs. A leak from the tank itself is not; see our water heater leaking repair cost guide.

Signs a connection needs attention

What you seeWhat it usually means
Green or white crust on fittingsSlow leak and corrosion
Rust streaks down the tank jacketLeak at a top fitting or corroded nipple
Drips from the flex line nutWorn washer or loose nut
Hissing or gas smell at the gas connectorPossible gas leak: leave and call

Connections are also a common upgrade point during a replacement: plumbers often add new shutoffs, dielectric fittings, an expansion tank and a drain pan. See our water heater replacement cost guide.

Common questions

Why are there dielectric fittings on a water heater?

They separate copper pipes from the steel tank to reduce galvanic corrosion at the connection.

Can I reuse the gas connector when replacing a water heater?

It is best practice, and often required, to install a new approved connector.

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