What is a Sediment Trap? Why is a Sediment Trap Required?

Sediment traps, commonly referred to as “drip legs” by tradespeople, are manually constructed fittings installed in a gas line that “trap” debris that may be floating or pushed through the gas supply piping. Natural gas pressure on residential gas supplies will typically not exceed 5 psi (static water supply pressure is typically 40-80 psi for reference), so gas isn’t being pushed through the gas lines at a high pressure, but over time, any debris present in the gas line could theoretically be pushed into the appliance controls and cause blockages or malfunctions with the equipment.

What kind of debris is floating through the gas lines? Most gas lines are made of a material called “black-iron” steel pipe that is bought in large stick lengths of a minimum of 8-12 feet and cut down to size as necessary. If you’ve never seen a plumber thread pipe, they typically have a large machine that they wheel out that clamps down the pipe and using an electric motor and lots of lubricating / cooling oil, cuts threads into the approximately 1/8th of an inch thick steel piping. This process creates little shavings of sharp metal that can easily be lodged into the pipe, especially once they get coated in oil. Although it may take a while, these pieces of metal can and likely will eventually find their way to the gas’s final destination, such as a water heater or furnace. Dirt and other minor debris may also find it’s way into the pipe just because construction is a dirty process; these things happen.

Correctly installed Sediment Trap


Where Are Sediment Traps Installed?

Current building standards require sediment traps to be installed at all gas-fired water heaters and furnaces. Here’s a snippet of the code text for reference: [2012 Mechanical Codes (408.4)]:

Where a sediment trap is not incorporated as part of the appliance, a sediment trap shall be installed downstream of the appliance shutoff valve as close to the inlet of the appliance as practical. The sediment trap shall be either a tee fitting having a capped nipple of any length installed vertically in the bottom most opening of the tee as illustrated in [the picture provided] or other device approved as an effective sediment trap. Illuminating appliances, ranges, clothes dryers, decorative vented appliances for installation in vented fireplaces, gas fireplaces and outdoor grills need not be so equipped.

Important note at the bottom: this further reinforces that you only need a sediment trap on your gas-water heater and your gas-furnace, at least here in San Antonio, Texas.

No Sediment Trap Present at Water Heater


What is the Correct Way to Install a Sediment Trap?

If you go to the hardware store and ask to buy a “sediment trap” or a “drip-leg”, you’ll get some funny looks. That’s because they don’t sell pre-made sediment traps, they’re constructed out of commonly available fittings. What you would need is a 3-sided female “tee” fitting along with a cap to plug one end (this is the downard pointing trap end) and a few “nipples” or small pieces of pipe that contain male threads on both ends. Once assembled (with thread sealant of course!), you have a sediment trap. All in all, this is certainly less than $20 in fittings.

But as a home inspector in San Antonio, Texas, I commonly find that sediment traps are installed incorrectly. Either the gas supply inlet is entering at a horizontal section of the “Tee” or the sediment trap is installed BEFORE the gas appliance connector and not after the gas appliance connector as required.

In order for a sediment trap to work, the incoming gas supply pipe has to enter vertically, from the top of the sediment trap, creating a change in direction at the bottom of the drip leg so that sediment will fall straight down into the trap and won’t fight gravity to come back up.

As for the case of the second one, you not only have the same problem as above, but the building code requires that the sediment trap be installed on the appliance side of the gas appliance connector (the yellow or steel-colored flexible gas line).

Correct installation of a sediment trap / drip-leg is important if you want the device to perform it’s job correctly: trap sediment!

sediment trap installed incorrectly

Sediment trap installed BEFORE the gas appliance connector and gas supply entering HORIZONTALLY: Both wrong

I have no Sediment Trap / The Sediment Trap is Installed Incorrectly: Do I Need To Call Someone to Fix This?

If your home inspector just inspected your New-Build home and called out the lack of a sediment trap or incorrectly installed sediment trap, then I would ask the builder to have the plumber / HVAC tech come out and fix it. It shouldn’t take more than 20 minutes.

If you’re buying a pre-owned home and the home’s water heater has no sediment trap, I wouldn’t be too concerned. At least, it’s not severe enough of an issue to call someone out to come fix the problem and I definitely wouldn’t ask the seller to correct such a minor repair in a hot market as in the San Antonio, TX area.

When the water heater / furnace next gets serviced or replaced, you may go ahead and ask the plumber / HVAC tech to install one. If they are replacing the system, they should be installing one to be code compliant.



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