Gas Pipe Sizing and how it affects the functionality of your 45

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Determining The Required Meter and Regulator size

Find the BTU requirement of each appliance in the home. In our example above, we have the following: 199,900 BTU Tankless, a 75,000 BTU Furnace, a 55,000 BTU Range, a 25,000 BTU Dryer, and a 40,000 BTU Gas Log Fireplace. The sum of these appliances is 394,900 BTU. Since most gas regulators and meters are rated in Cubic Feet per Hour, we need to convert the BTU calculation to ensure they are sized correctly. Divide total BTU by 1,024 to get the estimated Cubic Feet per Hour (CFH) requirement for the meter and regulator; 386 CFH in our example.

If the water heater was a typical tank type at 40,000 BTU, then the overall system requirement would have been just 235,000 BTU with a meter and regulator rated at 235 Cubic Feet per Hour. A typical household meter and regulator is commonly rated at 250 Cubic Feet per Hour. As you can see that in the example above, when you change the water heater to a tankless, the existing regulator and meter would be potentially undersized. It is important to have a properly sized meter and regulator on the system; otherwise, the appliances on the system could experience operational issues. The local gas utility can provide more information on upgrading the meter and regulator for the home.

Hybrid pressure systems, with a 2 psi static pressure with regulators at each appliance, are sized differently than in this example. Consult your local gas supplier or the National Fuel Gas Code in regards to these type systems.

Pipe Sizing Methods

There are two basic pipe sizing methods: longest length and branch length. Proper sizing will allow the system to maintain the required minimum pressure drop.

In the longest length method, the pipe size of each section should be determined by using the longest length of piping from the point of delivery, the gas meter or regulator, to the most remote outlet and the load of the section.

In the branch length method, the pipe size of each section of the longest pipe run, from the point of delivery to the most remote outlet, should be determined by the longest run of piping and the load of the section. The pipe size of each section of branch piping should be determined using the length of piping from the point of delivery to the most remote outlet in each branch and the load of the section. Branch length sizing is the most common method.

Determining Pipe Size by Length and Capacity

We will need to calculate the total load of the system and each branch. In our sample system, Figure 1, measure and add the lengths of pipes at each section. Total the BTU of the appliances for each branch line and the main trunk line back to the gas meter. Select the appropriate sized gas line based on length, BTU capacity, and pressure drop from Table 2, Table 3, or Table 4.

You can see that, in a typical gas system, a tankless water heater with a capacity of 199,900 BTU will require a 1-inch pipe size for a 20 ft branch length (based on the 0.3 in w.c. pressure drop in Table 2). The same appliance would require just a ½” pipe size based on Table 4 the 3.0 in w.c. pressure drop.

A branch line is a pipe off the main line that feeds a group of appliances. In our example, we have two branch lines. The pipe size of the main pipe on the branch must be sized based on the total BTU of all the appliances on that branch line and pipe length.

The trunk line pipe is the main pipe from the meter/regulator that feeds the different branches. The trunk line must be sized based on the total BTU from each branch-line system or the sum of the total BTU of all the appliances on the system and pipe length.

Items such as elbows, tees, and valves are not included in these sample calculations. Their equivalent pipe length should be included when sizing gas systems. It is recommend that a licensed gas tradesman size, design, and install the gas system.

Pipe Sizing Formula and Factors

You can calculate the required inside diameter of the piping required for a specific appliance/system capacity and length.

Calculate Q by dividing the BTU capacity of the appliance(s) by 1,024.

To determine the allowable pressure drop, find the system static input gas pressure using a Manometer. Then, find the highest minimum gas pressure from all the appliances, usually listed on the appliances rating label. Subtract the highest minimum gas pressure from the static input gas pressure to get the difference. For example, the input static pressure is 7 in. w.c.; the highest minimum pressure is 6 in. w.c.; leaving a difference of 1 in. w.c. In this example the system can have a .5 in. w.c. pressure drop based on Table 3. If the input pressure was 9 in. w.c., in this example, then a 3.0 in. w.c. pressure drop based on Table 4 would
be allowable.

Gas Pipe Capacity Charts

 

The information in this article is meant for educational purpose only; it is not meant to be an engineering guide or supplement any national or local code. All national and local codes must be followed. Refer to the National Fuel Gas Code or your local gas supplier or code official for information. Gas systems should be designed, installed, and inspected by a certified and licensed gas fitter, engineer, or tradesman. 

Eccotemp Systems, LLC.

315-A Industrial Rd.

Summerville SC, 29483

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