[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

In water and air systems I am used to sizing pipes and ducts with velocity limits. For natural gas installations in residential and small commercial buildings, the gas codes and utility guidelines size pipes on an allowable pressure drop from the meter to the appliance instead.

• What is the physical reason for this?
• What is the general method, step by step?
• Is velocity still checked anywhere?

I am asking about the method only; I will use the tables or formula required by our local gas utility.

[MEP Center Engineering]

Because a low-pressure gas system has very little pressure to spend: every appliance needs a minimum inlet pressure to burn correctly and safely, so the pipe is sized to keep the total pressure loss from the meter regulator to the farthest appliance within the small allowance set by the gas code or utility. Velocity is at most a secondary check.

Why pressure drop governs

• Domestic natural gas is commonly supplied at about 20-21 mbar in Europe and Turkey (about 7 in. w.c. in the US). The appliance burner and its gas valve are adjusted for a nominal inlet pressure; below the minimum, flame stability, ignition and combustion quality deteriorate, and CO can rise.
• The allowable drop is only a small fraction of the supply pressure. The US fuel gas code tables, for example, are built on a specified drop such as 0.5 in. w.c. (about 1.2 mbar) for low-pressure systems. Your local code or utility sets the figure for your project.
• Gas is compressible and its density depends on pressure, so a velocity limit alone does not guarantee the delivered pressure.

The method

1. List appliances with their nominal heat input (gross or net, as the code requires) from the data plates.
2. Convert to gas flow: flow (m³/h) = heat input (kW) / calorific value (kWh/m³), using the value the utility specifies.
3. Apply a diversity (simultaneity) factor only where the code allows it, for example for several dwellings on one riser.
4. Measure lengths for each section, including vertical runs, and add fitting allowances as the code requires.
5. Size section by section with the code’s tables or formula so that the cumulative drop to the farthest appliance stays within the allowance.
6. Apply height correction if the code requires it.

Worked example (flow and sensitivity only)

A flat has a 24 kW combi boiler and a 7 kW cooker. With a calorific value of 9.5 kWh/m³ (use your utility’s value):

• Boiler: 24 / 9.5 = 2.53 m³/h; total at full simultaneous use: 31 / 9.5 = 3.26 m³/h.
• In turbulent flow, pressure drop rises roughly with flow to the power 1.8 and falls with internal diameter to the power of about 4.8. Doubling the flow therefore gives about 2^1.82 = 3.5 times the drop.
• Going from DN20 to DN25 steel (internal diameter about 21.6 mm to 27.2 mm) cuts the drop to roughly (21.6 / 27.2)^4.82 = 0.33 of the previous value.
• Natural gas is lighter than air, so it gains roughly 0.04 mbar per metre of rise; LPG is heavier and loses pressure going up. Many codes specify whether to include this.

The actual pipe sizes must come from the utility’s approved tables or formula, not from these ratios.

Velocity

Some codes and utilities also cap velocity, mainly for noise, dust carry-over and meter or regulator protection, especially in medium-pressure service lines. Treat it as an additional check, not the basis.

Common mistakes

• Sizing each section for the drop allowance instead of the whole route.
• Using appliance output instead of heat input.
• Ignoring future appliances the client has mentioned.
• Not checking pressure at the appliance during commissioning.

Related: Natural gas pipe sizing basics. Always follow the gas utility’s approval procedure and the local gas code for sizing, materials and testing.

Check the edition of the code that applies to your project.

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