A sediment trap is a short piece of gas pipe installed near an appliance that helps to collect debris in the gas line. The idea is that any debris in the gas line will collect in the sediment trap before that debris can foul up the appliance’s gas valve and cause trouble.
You’ll often hear the term “drip leg” instead of sediment trap. In fact that’s what I usually call it, but it’s not really the same thing.
A sediment trap is just a T fitting in the gas pipe with a capped pipe nipple at the bottom of the T. It’s required at a furnace, boiler, and water heater, but not at a clothes dryer, a kitchen range, or a fireplace.
The sediment trap needs to be installed downstream of the appliance’s shutoff valve and as close to the appliance as practical.
And it needs to be installed so that the gas flow changes direction, so that the debris will fall into the trap. If you install it so that the gas flows horizontally (like in the picture below) then the debris might just jump over the trap and get into the gas valve.
This sediment trap is not installed correctly.
There’s no excuse for the lack of a sediment trap. A competent plumber knows it’s required by all relevant codes and by the appliance manufacturer’s instructions. And you don’t want an incompetent plumber installing your gas pipes. So if it’s missing that’s one strike against the plumbing system as we work on telling the story of the house.
But how bad is this problem, really? Well, I’ve heard that this might have been an issue many years ago, but today the natural gas delivered to your house generally is clean and dry. And I’ve never personally heard of a problem that occurred because of a missing sediment trap, although I have heard stories of clogged gas valves.
So have a sediment trap installed the next time the appliance is replaced. In the meantime it’ll just be one of the many things in your house that’s not perfect.