Copper Supply Line Pressure Loss After a Renovation
During a kitchen renovation, a plumber has extended an existing copper cold-water line to serve a new sink and dishwasher. The original line is 15 mm copper fed from a 20 mm main at 350 kPa static pressure. The extension adds 6 m of 15 mm copper with four elbows and two isolation valves. After commissioning, the client complains that the kitchen tap has noticeably lower flow than before, and the dishwasher sometimes fails to fill within its allowed time. The plumber measures 330 kPa static at the kitchen tap with all other fixtures closed, and 180 kPa when the nearby laundry tap is opened. Other fixtures in the house perform as before. No leaks are visible. The plumber has a pressure gauge, a flow cup, and the manufacturer's dishwasher specification requiring a minimum 100 kPa dynamic pressure and 8 L/min flow.
1. Analyse whether the problem is a pressure issue, a flow issue, or both, and identify the most likely causes.
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The static reading of 330 kPa versus 350 kPa at the main is a small loss consistent with elevation and minor losses, so static pressure is not the primary fault. The significant clue is the drop to 180 kPa when the laundry tap opens, and the dishwasher's failure to meet its 8 L/min / 100 kPa requirement.
Likely causes:
- Excessive equivalent length and fittings on a 15 mm line - four elbows and two valves add considerable minor losses relative to a 15 mm bore.
- Undersized extension - a 15 mm line feeding a sink plus dishwasher may be marginal; the original line may already have been near its limit.
- Partially closed or restrictive isolation valves - check full-bore versus reduced-bore valves.
- Debris or solder in the line from the renovation, restricting flow.
Static pressure alone cannot diagnose this; dynamic pressure and flow must be measured at the fixture.
2. Recommend a compliant remedy and explain how you would verify it.
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Remedy options, in order of preference:
- Confirm valve status and flush the line to rule out debris or a partly closed valve - cheapest first step.
- Upgrade the extension to 20 mm copper from the main, or tee the dishwasher off separately upstream, reducing equivalent length and velocity.
- Replace reduced-bore isolation valves with full-bore types and minimise elbows by rerouting.
Verification:
- Measure dynamic pressure at the kitchen tap with the dishwasher filling and confirm it stays above 100 kPa.
- Use a flow cup to confirm at least 8 L/min at the tap and at the dishwasher inlet.
- Repeat with the laundry tap open to confirm the system tolerates simultaneous demand.
- Record results and provide to the client.
If dynamic pressure still fails, the main may need upgrading - document and advise.