The problem:Replacing electric heating with gas.

The apprentice: Allen Charles Edwards (ACE), a first-year mechanic.

The technician: N. Arthur Thomas Egan (NATE), a15-year veteran.

As the work for the day is being set up, NATE and ACE are teamed together to install the piping for the installation of four gas-fired rooftop heating units. In setting up the materials listed for the job, NATE questions ACE to make sure he understands how the piping is to be installed and that the proper materials are loaded.

NATE: “Well, what do you think is the first thing we need to know, ACE?”

a) Just how many units will be installed.

b) The heating output of one unit.

c) The total heating output.

d) The total heating input.


dThe toal heating input.

NATE: “That’s not all we need to know. There are a number of factors that are needed, including the total input, which is the same as the maximum gas demand.”

The other factors the team needs to know:

  • The pressure loss in the pipe from the point of delivery to the equipment;
  • The length of the pipe and the number of fittings;
  • The specific gravity of the gas; and
  • The diversity factor.

  • NATE:

    “Well, there is this formula in the National Fuel Gas Code, but I have found that the code book also has tables that take into account all of the factors and make the determination much easier. For us to figure what we need for the job, I’ll give you a list of information that we will use and the chart we need to determine the pipe sizes. I’ll draw the piping layout and you will figure the pipe sizes. OK?”

    NATE: “I’ll give you a list of details, which I have paraphrased from the Fuel Code, that you need to follow to use the table.”

    1. Determine the gas demand for each appliance in cubic feet per hour.

    2. The system pressure, pressure loss, specific gravity and diversity factor are included.

    3. Measure the length of piping from the point of delivery to the most remote outlet.

    4. Select the column showing the measured length or the next longer length if the table does not give the exact length. This is the only length used in determining the size of the section of gas piping.

    5. Use the measured length vertical column to locate all gas demand figures for this piping system.

    6. Starting at the most remote outlet, find in the vertical column just selected, the gas demand for that outlet. If the exact demand is not shown then choose the next higher demand in the column.

    7. Opposite this demand figure, in the first column at the left, will be found the correct size of the gas piping. 8. Proceed in a similar manner for each outlet and each section of gas piping.

    NATE: “And ACE, don’t forget that the chart is in cubic feet per hour of the gas and you will need to convert the equipment Btu input. The gas company in this area generally supplies fuel that has approximately 1,000 Btu per cubic foot.”

    Publication date:09/24/2001