How Do I Size Furnace Gas Pipes?

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Calculating the size gas pipe needed for your furnace may seem complicated, but if you follow a few simple guidelines you will have your answer quickly and without much difficulty. It is important to properly calculate the size of the gas pipe going to your furnace in order for it to operate safely and efficiently. You should also check with your local municipality to learn about building codes related to gas piping in your area.

Things You'll Need

  • Tape measure
  • Nut driver
  • Pen
  • Notepad
  • Find the product information plate on your furnace. The plate is typically located in the burner section of the furnace (where the gas pipe enters). You may need a nut driver to remove the cover panel.

  • Look for the BTU ratings listed on the plate and write down the input BTUs for your furnace. You will need this information later.

  • Measure from the gas valve in the burner section of your furnace to the gas meter located outside your house and write this down. This will be the length of your pipe and will affect the size gas pipe needed. Be sure to add 10 feet of pipe to your measurement for every 90-degree turn and 5 feet for every 45-degree turn when making your final calculations. This is to compensate for the additional pressure drop created by the change in direction. In other words, a 90-degree turn offers the same resistance as 10 feet of pipe.

  • Determine what size pipe you need. For furnace input BTUs of 50,000 to 60,000, use a 1/2-inch diameter pipe if the pipe length is less than 40 feet. If the pipe length is over 40 feet but not more than 200 feet, then use a 3/4-inch diameter pipe. For furnace input BTUs of 61,000 to 80,000, use a 1/2-inch diameter pipe if the pipe length is less than 20 feet. If the pipe length is over 20 feet but not more than 90 feet, then use a 3/4-inch diameter pipe.

Tips & Warnings

  • The average house (1,600 square feet) will typically have a furnace that is 80,000 BTUs or less.
  • Check with your local building officials and gas company before doing any work on your gas piping system. Most areas of the U.S. require you to obtain a permit before making alterations to your system, and many require you to hire a licensed mechanical contractor.
  • Never do work that you are not qualified for. Gas piping that is incorrectly installed can cause fires or explosions.

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References

  • Photo Credit plumbing fittings image by Greg Pickens from Fotolia.com
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