Drip Leg VS Sediment Trap
This is a very simple yet apparently confusing topic. Many home inspectors and quite a few plumbers and HVAC techs use the terms interchangeably and don’t really understand the difference. The general home buyer gives me a blank stare when I mention the need for and lack of a sediment trap. Sediment traps are found in the gas supply line between the gas shut off valve and the controls of gas heaters, water heaters and pool/spa or hot tub heaters. The photo below shows a water heater in use with no sediment trap.
Obviously if you have electric units this does not apply and sediment traps are not required for all gas appliances. Gas stoves or cook tops or fireplaces for instance are not required to have them.
The purpose of a sediment trap is to prevent any sediment that may be in a gas supply line from entering into the controls of a heater or water heater. They are normally built on sight by a plumber or HVAC professional when installing or replacing these units. I’ve never seen a manufactured sediment trap but they may exist.
What do they look like and how to you tell the difference? Normally they are made of ½” black or galvanized pipe and found between the gas shut of valve and where the gas enters the control unit. Let’s “build one” to help you understand.
Start with a short nipple coming out of the control unit and add a "T" and tighten until the "T" is in a vertical position. Insert another nipple of 4" to 6" on the bottom of the "T" and put a cap on that nipple. Now, bring the flexible gas line from the shut off valve and connect it to the top of the"T". You have just built a sediment trap. (There is no length requirement for the nipples. The sizes mentioned work well but they can be different lengths.) Below is a correctly constructed sediment trap.
A sediment trap will catch moisture as well as a drip leg does but a drip leg may not catch small pieces of sediment. Although no one ever does it, it is not a bad idea to occasionally remove the bottom cap from the sediment trap and tap it to knock out the trapped sediment.
So, what does a drip leg look like? It is very similar in appearance to a sediment trap so let’s “build” one of them so you can see the difference. Go back to the gas control unit on your heater or water heater and install the short nipple. This time install the "T" in a horizontal position so that the "T" has one opening facing down and one still horizontal. Install a 4" to 6" nipple with a cap from the bottom of the "T" and attached the flexible gas line to the horizontal end of the "T". The photo below shows a drip leg.
The difference is that the gas is forced to change direction at a sediment trap before it goes into the control unit. Any moisture or sediment in the gas is forced to drop out. A small lightweight piece of sediment may be carried by the gas flow over the drip leg without falling out.
Posted: Jul 8, 2013
Posted by: Ed Fryday, ACI, CMI® | TREC License: #6932
Space City Inspections, LLC