Choosing the right size gas line is not simply a matter of choosing a tube large enough to sustain a flame. It’s a question of how gas flows: Every fitting you add and each additional foot of pipe create resistance, which can starve your furnace. Many homeowners focus solely on the diameter of the pipe… But fail to consider length of run or the number of fittings. If this results in sputtering burners or cold floors under increased demand, it may be due to this oversight.
As you’ll see from the chart, both distance and width impact capacity. With propane, you have more energy packed into every cubic foot. That means propane requires smaller diameter piping to produce the same BTUs as a natural gas system. That means propane require smaller diameter piping to produce the same BTUs as a natural gas system.
How to Choose the Right Gas Line Size
But here’s where the rub: it comes down to pressure. For residential applications, low pressure (typically about eleven inches of water column) is standard. At lower pressures, friction become a bigger factor. As you lengthen your supply line, there is less momentum behind the gas. And if you check out the reference guide, the longer the run, the lower the capacity.
A half-inch line can sustain short bursts, but stretch that pipe an additional fifty feet and you’ve lost a significant chunk of its delivery potential. You’re pushing gas against friction. Turbulence caused by fittings detract from efficiency. Straight-line distance between house and the tank is what people measure and then stop there. They fail to consider the additional resistance caused by elbows, valves, and tees.
Every 90 degree elbow equates to several feet of additional pipe, consuming pressure. Size plumbing accurately by accounting for any elbows. To convert fittings to equivalent length, use the chart as a guide. Failing to account for virtual feet will cause your calculated pipe size to be insufficient. When only the pilot is on everything works fine; however, once you start running multiple appliance such as the pool heater and dryer, pressure goes down.
That equation alters with materials. For example, above ground runs can be run in Black Iron Schedule 40 which is readily available and quite robust. Underground or indoor runs can also be done in copper (Type L), which is cleaner to install, but has a lower capacity due to its thickness rating and smooth wall.
CSST offers flexibility, though you have to respect its tighter constraints on flow. Flexible corrugated stainless steel tubing will bend around corners and into tight spots well, but again has tighter restrictions on flow. Rigid iron can forgive some bad planning where flexible tubing cannot. So you should of choose material based off application location and then size it out accordingly for the amount of work that needs to be done.
Pressure systems are another animal entirely. A little pressure in a big pipe moves big energy. Small pipes in high pressure does too. That’s why big pool heaters has small lines and so do commercial generators. The pressure does all the work. But that means you need special regulators at each appliance to back the pressure down to something safe enough to ignite. It is powerful stuff, but it must be respected. Don’t take your low pressure regulator from one line and throw it on a high pressure line expecting it to do its thing.
First: Add together the BTU demand of all appliances that may be used simultaneously. Don’t guess. Look for nameplates for accurate info.
Second: Measure the longest route from the source to the farthest device. Add the equivalent feet for each valve and bend in the route.
Third: Locate where your total load and length fall on the chart. Choose the smallest pipe equal to or larger than your need. (Note: It’s safe to oversize; it’s frustrating to undersize.)
Gas plumbing is all about balance. How much do you need? You need enough to provide a stable, efficient flame while all the appliances in your home are running at once. Not hard science, just physics. It is important to remember to get this part correct initially to avoid issues down the road. Give the furnace sufficient fuel, follow the laws of physics, and don’t block the way.
