What is a gas drip leg, and is it required?

Yes, on most appliances. Phoenix enforces the 2024 International Fuel Gas Code, and the city's amendments leave the piping chapter's sediment trap rule as written. Section 408.4 says that where a sediment trap is not built into the appliance, one "shall be installed downstream of the appliance shutoff valve as close to the inlet of the appliance as practical."

The same section lists the appliances that do not need one:

The exemption is a code floor, not a ban. A plumber can still add a trap ahead of an exempt appliance, and the appliance's own manual has the final word on its hookup.

The manufacturer's instructions matter as much as the code. The code requires water heaters and furnaces to be installed to those instructions, and A.O. Smith's residential gas water heater manual says there must be "a sediment trap ahead of gas control valve/thermostat to help prevent dirt and foreign materials from entering the gas control valve/thermostat."

It protects the gas valve. New piping carries cutting chips, thread compound, and scale, and older steel pipe sheds rust over time. A.O. Smith's manual is blunt about the stakes: "Contaminants in the gas lines may cause improper operation of the gas control valve/thermostat that may result in fire or explosion."

The trap works on a simple idea. Gas turns the corner at the tee and keeps going. Heavier debris does not make the turn and falls into the capped nipple, where it sits until someone cleans it out. That's the whole job.

The code sets no minimum length. Section 408.4 describes "a tee fitting having a capped nipple of any length installed vertically in the bottommost opening of the tee." Manufacturer instructions can set a floor, though. A.O. Smith's installation figures for its residential gas water heaters mark the capped nipple at 3 inches minimum, for both a flexible connector hookup and a black pipe hookup.

Here is what a correct trap looks like, based on the code figure and the manufacturer diagrams:

• Tee, not an elbow. Gas enters one side of the tee and leaves through the other side toward the appliance. The nipple hangs from the bottom opening.
• Vertical nipple. The code says the nipple is "installed vertically in the bottommost opening of the tee." A nipple turned sideways cannot hold debris.
• Capped end. A pipe cap on the bottom, so the trap can be opened and cleaned.
• After the shutoff valve. The trap goes downstream of the appliance's shutoff valve, as close to the appliance inlet as practical.
• Before the connector or union. On a water heater, A.O. Smith's figures show the order: shutoff valve, sediment trap, then the union or flexible connector to the gas control valve.

So, can a drip leg be horizontal? No. The whole point is gravity, and the code wording requires the nipple to point down.

### Drip leg or drip: what's the difference?

People use "drip leg" for two different things, and the code keeps them apart.

• Sediment trap (408.4): the capped tee at each appliance that catches solid debris. This is what almost every Phoenix house has, and what people usually mean by a drip leg.
• Drip (408.2): a trap for liquid condensate, required only "where wet gas exists." When one is needed, the code also calls for "a drip at the outlet of the meter," with ready access for emptying.

In a house, the everyday question is almost always about sediment traps. If a plumber or inspector mentions a drip at the meter, ask which one they mean.

On a pool heater, the drip leg sits outside the cabinet, next to the heater's shutoff valve. Pentair's MasterTemp manual tells installers to "Install a manual shut-off valve and a sediment trap and union located outside the heater panels," and warns in capital letters not to put the gas line union inside the heater cabinet. It describes the trap the same way the code does: "a tee fitting with a capped nipple in the bottom outlet which can be removed for cleaning."

A few pool heater details go with it:

• Order of parts. Shutoff valve first, then the sediment trap, then the union, all outside the heater panels.
• Reduce near the heater. The supply line is usually bigger than the heater's gas valve inlet, and Pentair says to make that reduction as close to the heater as possible.
• Protect it during testing. Pentair says the heater must be disconnected from the gas piping during any pressure test above half a psi.

The same logic applies to a furnace or a tankless water heater: the trap goes after the appliance shutoff valve and as close to the gas inlet as the manual allows. When in doubt, the manual's diagram settles it.

A missing sediment trap on an appliance that needs one is a code and installation issue. It does not mean gas is leaking. It means debris has nothing to stop it before the gas valve.

Here is how to handle it:

1. 1Look at the gas line where it drops to the appliance. After the shutoff valve you should see a tee with a short capped pipe pointing down. If the line runs straight into a flex connector, there is no trap.
2. 2Leave the gas piping alone. Adding a trap means breaking into gas piping, and in Phoenix any alteration beyond installing an appliance connector needs a permit. The work also ends with a leak check.
3. 3Fix it at the next service or replacement. A licensed plumber can add the trap when the appliance is serviced or swapped. A replacement is a natural time to do it, since the manufacturer instructions call for one.

If you smell gas near the appliance at any point, stop, leave the building, and call 911 and Southwest Gas at 877-860-6020 from outside.

HQ Plumbing & Air installs gas water heaters and gas lines across the Phoenix area to code, sediment traps included. We're licensed and insured, and you get the price up front before work starts.

A tee, a nipple pointing down, and a cap. Small part, real job.