A 120 kW boiler that locks out at peak
A restaurant has a 120 kW condensing boiler and a 40 kW range. The meter outlet reads 21 mbar. From the meter, 18 m of 28 mm steel runs to a tee; 6 m of 28 mm continues to the boiler and 12 m of 22 mm runs to the range. At full rate the boiler inlet is 16 mbar and the range inlet is 17.5 mbar. The boiler locks out on ignition when the range is in use and runs normally when the range is isolated. The installer has twice adjusted the meter governor to raise outlet pressure to 23 mbar; the boiler still locks out. A U16 meter feeds the installation. The boiler manufacturer requires 20 mbar at the inlet, with a minimum of 17 mbar. The range is rated at 20 mbar. The pipework is all low level and accessible; there is no corrosion.
1. Assess the pressure readings and identify the most likely cause. Show your reasoning.
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At 21 mbar at the meter, 16 mbar at the boiler is a 5 mbar loss, and 17.5 mbar at the range is a 3.5 mbar loss. For low-pressure natural gas, the normal design allowance from meter to appliance is around 1 mbar; these drops are far too high. The boiler manufacturer's minimum is 17 mbar, so 16 mbar explains the lockout.
The fact that the boiler runs with the range isolated points to interaction through the common 28 mm section, not a boiler fault. Total load is about 160 kW, roughly 15 m3/h, through 18 m of 28 mm before it splits; that common section is undersized and the 22 mm range branch adds to the loss. The U16 meter is also close to its capacity at about 15 m3/h, so meter capacity must be checked. Raising the governor to 23 mbar masks the problem and can overpressure the range and boiler.
2. Propose a pipework remedy and explain how you would verify it.
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Re-size from the meter. A 35 mm common run to the tee, with 28 mm to the boiler and 28 mm to the range, would be a reasonable starting point; confirm with the sizing tables for the actual load, length and fittings. Replace the 22 mm range branch. Keep the run accessible and supported. Do not simply increase the governor.
After modification, isolate, strength and tightness test the new pipework, purge, then measure inlet pressure at each appliance at full rate and with the other appliance operating. Check the gas rate against the meter and boiler data, and complete combustion analysis. Verify the meter is adequate for the total load; if not, request a meter upgrade from the gas network operator. Record all readings.
3. Why is raising the meter governor pressure not an acceptable solution, and what else must be checked?
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The governor at the meter is set to provide a stable 20-21 mbar outlet, not to compensate for pipe loss. Raising it to 23 mbar means the appliance nearest the meter may see over 23 mbar when the boiler is off, which can affect flame shape, gas rate and controls; it also leaves the fundamental pressure loss unchanged.
The correct checks are: meter outlet pressure under load, pressure drop across the meter, appliance inlet pressure at full and part rate, gas rate, and the boiler's minimum inlet requirement. Also check for a partially closed valve, debris, a collapsed liner or a distorted pipe, but the readings point to sizing. Any change must be recorded and the installation re-tested. The boiler should not be left in service with 16 mbar.