Pressure Drop on a 32 m Run to a New Cooktop
A gasfitter is extending a natural gas installation to a new 12 MJ/h cooktop in a renovated kitchen. The meter and regulator are at the front of the house. The existing 20 mm copper line serves a 25 MJ/h ducted heater and a 20 MJ/h hot water unit. The new run to the cooktop is 32 m of 15 mm copper with six elbows and one tee. The gasfitter measures 1.10 kPa at the meter outlet under no-flow conditions and 0.85 kPa at the heater inlet with the heater and hot water running. The cooktop's manufacturer requires 1.00 kPa at the appliance inlet under full flow. The gasfitter proposes to tee off the existing 20 mm line near the meter.
1. Assess whether the proposed arrangement will deliver adequate pressure to the cooktop and explain the reasoning.
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The question is not just whether gas arrives, but whether every appliance receives its required pressure when all appliances run simultaneously. That is the design case.
Total connected load: 25 + 20 + 12 = 57 MJ/h.
Available pressure: 1.10 kPa at the meter with no flow. The measured 0.85 kPa at the heater with only two appliances running already shows a 0.25 kPa loss through the existing 20 mm line. Adding a 12 MJ/h load increases flow through that same line, so the loss will grow.
New run: 32 m of 15 mm copper with six elbows and a tee. Each fitting adds equivalent length, so the effective length is well beyond 32 m. A 15 mm line at this length carrying 12 MJ/h will develop a significant pressure drop, particularly since the cooktop needs 1.00 kPa - only 0.10 kPa below the standing meter pressure.
The margin is far too thin. Even if the cooktop lights, it will be starved when the heater and hot water run together, giving poor flame quality and possible flame failure on low settings.
Conclusion: the proposal is unlikely to comply. The gasfitter should either upgrade the main run, tee off closer to the meter with a larger branch, or resize the new run using the standard's pipe sizing tables with the full connected load and equivalent lengths. The design must be verified by calculation and then by a pressure test at the appliance under full flow.
2. What testing should follow installation before the appliance is handed over?
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Testing has two distinct purposes: proving the pipework is sound, and proving the appliance receives adequate pressure in service.
1. Strength and tightness test. Isolate the new section, pressurise with air or inert gas to the test pressure required by the standard for the pipe size and material, and hold for the specified period. A drop indicates a leak. Never use gas for a strength test on new pipework, and never use a naked flame to find leaks.
2. Purge. Purge air from the new line to a safe location before introducing gas to the appliance.
3. Operating pressure test. With all appliances connected and running at maximum, measure pressure at the cooktop inlet test point. It must be at or above the manufacturer's 1.00 kPa. Also measure at the heater and hot water unit to confirm they are still within tolerance.
4. Appliance checks. Confirm correct injectors, stable flame, no lift or yellow tipping, and that the appliance's own regulator (if fitted) is set correctly.
5. Documentation. Record all measured pressures, the test pressure and duration, and the result on the compliance certificate.