LPG continuous flow heater and cooktop interaction
You are called to a rural lifestyle property with two 45 kg LPG cylinders and an automatic changeover regulator. The existing installation supplies a four-burner cooktop through 15 mm copper tube that runs 22 m from the regulator, including 6 m buried in concrete without a sleeve. The owner has added a 26 L/min continuous-flow water heater, connected by a tee off the same 15 mm line about 18 m from the regulator and 10 m from the cooktop. After commissioning, the cooktop flames drop sharply whenever the water heater ignites, and the water heater occasionally locks out. The installing gasfitter says the pipe is fine because the pressure at the regulator is 2.75 kPa and the appliances light. The cylinder contents are about 60% and the outdoor temperature is 8°C. The owner wants you to certify the installation so the retailer will release payment.
1. What is the most likely cause of the interaction, and what measurements would you take to confirm it?
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The symptom points to inadequate gas supply capacity under simultaneous demand, not a failure of either appliance. A 26 L/min continuous-flow heater has a very high gas input, likely over 200 MJ/h, while a four-burner cooktop adds roughly 20 MJ/h. A 15 mm copper line at 22 m is unlikely to carry that combined load at 2.75 kPa without excessive pressure drop. The line also has a tee arrangement that may favour one appliance.
I would measure operating pressure at the regulator outlet and at each appliance inlet while both appliances run, using a manometer and a suitable test point. I would check gas rate by clocking the meter or using the appliance data plate, verify the regulator capacity and changeover operation, and check cylinder vaporisation at 8°C with 60% contents. A 45 kg cylinder cannot supply unlimited vapour, especially when cooled. I would also inspect the buried copper for sleeving, corrosion and mechanical protection. The key evidence is pressure drop at the appliance inlet, not static pressure at the regulator.
2. Outline the compliant remediation and any safety issues you would immediately address.
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The installation should not be certified in its current form. The buried 6 m of copper without a sleeve is a safety and compliance defect: it cannot be inspected, may corrode, and may be damaged by movement or concrete. I would require the buried section to be exposed or replaced with a sleeved, protected run or an approved alternative.
The gas supply needs to be redesigned for the combined load: upsize the main and provide dedicated sized branches to the water heater and cooktop, or relocate the water heater closer to the regulator. The design should include pressure-drop calculations, regulator capacity, cylinder vaporisation and changeover performance. I would also check flue clearances, combustion air, condensate disposal and the water heater installation instructions.
Immediate safety action depends on the measured pressures and gas leakage. If the appliances are unsafe or spilling combustion products, I would isolate the supply and tag it. I would not rely on the gasfitter's statement that the appliances light; lighting is not proof of correct gas pressure or safe operation.
3. What certification and notification obligations apply, and how would you handle the payment pressure?
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Certification must follow verification, not pressure from the retailer or owner. I would complete a gas installation certificate only after the pipework, regulator, appliances, flueing and ventilation comply with the applicable gas standard and manufacturer instructions. The certificate should record the appliances, gas type, operating pressures, test results and any limitations.
I would put the non-conformance in writing to the owner and installer, listing the buried unsleeved pipe, the inadequate gas supply and any appliance defects. If the work is notifiable or requires council inspection, I would confirm that the correct notification has been made. Payment is a contractual matter between the owner, retailer and installer; it does not change the technical standard. I would explain that releasing payment before certification exposes the owner to risk and may leave the installer liable for rectification. If the installation is unsafe, I would isolate it and record that decision.