A dirt leg is a short piece of pipe, normally five to six inches long, that is designed to collect debris that is flowing along with the gas in a gas line. Its purpose is to prevent this debris from reaching and collecting in the sensitive parts of the gas control valve where they can prevent the valve from operating properly. The dirt leg, also called a sediment trap, is a simple device made from a piece of pipe and a couple of fittings. You will normally see these on gas water heaters and gas furnaces. Some people incorrectly call dirt legs “drip legs”, but this is not correct since they are not designed to collect moisture.
In the photo on the right, the gas flows in from the top and makes a 90-degree turn to reach the gas appliance. Any solids flowing with the gas should continue downwards and accumulate in the dirt leg. In the meantime, the gas will make a turn and flow to the appliance. Since solid debris will be denser than the gas, the debris will have a much more difficult time making the 90-degree turn, resulting in it continuing downward and to the dirt leg. This allows only clean, debris-free gas to continue to the appliance.
Requirements for Dirt Legs
The requirements for a dirt leg or sediment trap are as follows:
- The gas leg should be installed as close to the equipment as possible.
- It must be installed before the pressure regulator.
- The gas must flow in from the top.
- The gas leg must be below and in line with the pipe where the gas is coming from. The gas should be at least 3-inches long.
- The connection to the appliance must be at a 90-degree angle.
- There should be a cap at the bottom of the dirt leg that can be removed to empty any dirt or debris that has accumulated.
The diagram below from the NFPA illustrates some of these requirements.
What if you don’t have a dirt leg on your gas appliance?
I probably see gas appliances without these dirt legs more often than I see gas appliances with dirt legs, so apparently, there are a lot of plumbers who don’t install them, and a lot of code enforcement officials who do not require or enforce them. I would not be too concerned if I had a gas appliance without a dirt leg. I have had several gas appliances over the years in my own homes that did not have a dirt leg. It would not be a bad idea, however, to ask your plumber about it the next time you have to replace your gas water heater or furnace.
How NOT to install a sediment trap
The photo to the right shows a sediment trap that I saw at a recent home inspection. This configuration will not do any good as the gas and debris can flow right over the top of the dirt leg, making this dirt leg useless.
Dirt legs or sediment traps are useful, but not critical devices to help keep small particles from causing problems with our gas appliances. If you have them, great!! If not, well, its not the end of the world, and you shouldn’t be too concerned. If you don’t have dirt legs on your gas appliances, you are actually very likely in the majority and not the minority, so don’t lose too much sleep over it.
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© 2020 Mike Morgan
This article was written by Mike Morgan, the owner of Morgan Inspection Services. Morgan Inspection Services has been providing home, septic and well inspection services throughout the central Texas area since 2002. He can be reached at 325-998-4663 or at email@example.com. No article, or portion thereof, may be reproduced or copied without prior written consent of Mike Morgan.