35 kW Boiler Run Fails the Permitted Pressure Drop
A new 35 kW commercial boiler is fed by 22 m of 28 mm copper tube with 12 elbows, 4 tees and 2 full-bore isolation valves. The meter outlet pressure is 21 mbar. With the boiler at full rate, the pressure at the boiler inlet is 18.6 mbar, giving a 2.4 mbar drop. The manufacturer requires a minimum inlet pressure of 17 mbar and a maximum allowable pipework drop of 1.0 mbar. A tightness test on the new section shows a 0.6 mbar drop over 5 minutes at 20 mbar test pressure; the installation volume is 0.04 m³. You cannot find the meter installation's test volume or the pipework's equivalent-length calculation. The builder wants to board over the pipe route tomorrow.
1. Evaluate the pipe sizing and pressure drop. What action is required?
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The measured drop of 2.4 mbar exceeds the manufacturer's 1.0 mbar maximum, so the pipework is not acceptable even though inlet pressure is above 17 mbar. The equivalent length is more than the 22 m actual: 12 elbows and 4 tees add significant resistance, so the run is roughly 30 m or more. 28 mm copper at 35 kW over that length is likely undersized. Upsize to 35 mm or reroute to shorten the run, then recalculate and retest.
2. Can the 0.6 mbar tightness-test drop be accepted? What information is missing?
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A 0.6 mbar drop over 5 minutes at 20 mbar may or may not be acceptable; it cannot be judged without the test volume, stabilization period, temperature correction and the permissible drop in the applicable standard. The 0.04 m³ is the new section only-confirm whether the meter and existing pipework are included. Check let-by first. If the allowable drop is exceeded, the test fails and the leak must be found.
3. What should you do before the builder boards over the pipe route?
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Do not allow the route to be boarded. Record the non-compliant pressure drop, complete the sizing calculation, upsize or re-route, and repeat strength and tightness tests. Protect and label the pipe, provide as-built drawings and test certificates, and only then close the route. If the builder boards it, insist on access panels or a revised route so joints remain accessible.