Field Conversion to Propane with the Wrong Orifices
A 150,000 BTU/h cast-iron boiler in a basement was field-converted from natural gas to propane with a manufacturer's conversion kit. The installer replaced the gas valve spring but left the original natural-gas main burner orifices in place. The new nameplate states propane input 150,000 BTU/h, manifold pressure 10 in w.c., and a combustion-air requirement of 50 ft³ per 1,000 BTU/h. The technician measures 3.5 in w.c. manifold pressure, 7.0 in w.c. at the meter, and 6.8 in w.c. at the valve inlet. A combustion test shows 900 ppm CO (air-free), 6.5% CO2, 10.0% O2, and a yellow, lifting flame. The boiler has a 6 in outside-air duct and a 6 in B-vent, 15 ft vertical with a cap. The homeowner reports headaches and soot on the draft hood. The propane tank is new and the regulator is set to 11 in w.c. at the tank.
1. Explain why the boiler is producing high CO and what the combustion readings indicate. Include the role of the orifices and manifold pressure.
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The boiler is not in its certified propane configuration. Leaving natural-gas orifices in place and operating at 3.5 in w.c. changes the orifice area, gas velocity, and primary air entrainment. Propane has a higher heating value per cubic foot and different density, so the burner must have the propane orifices and the specified 10 in w.c. manifold pressure. The readings show 10% O2 and 6.5% CO2, which indicates a lot of excess air, yet CO is 900 ppm air-free. That combination points to incomplete combustion from flame quenching or impingement, not simply a rich mixture. The yellow, lifting flame confirms poor primary aeration. Possible causes: wrong orifice, low manifold pressure, blocked primary air ports, or a draft problem. The soot and headaches show the flue products are not safely contained. The appliance should be shut down.
2. The technician proposes to simply adjust the manifold pressure to 10 in w.c. without changing orifices. Explain why this is unsafe and what could happen to the input rate and combustion.
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Simply raising manifold pressure to 10 in w.c. with natural-gas orifices would overfire the boiler. The natural-gas orifices are larger than the propane orifices; at propane pressure the gas flow could be well above the rated input. Overfiring can lift flames, overheat the heat exchanger, produce CO and soot, and damage the boiler. It also leaves the appliance unlisted for the installation. The correct procedure is to install the propane orifices from the conversion kit, confirm the orifice size against the data plate, set the manifold pressure to 10 in w.c. with the burner operating, and verify the supply pressure and regulator lockup. Then check input by orifice table or by clocking a propane meter if available.
3. List the corrective actions and the tests required before returning the boiler to service.
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Shut off the boiler and tag it. Install the correct propane orifices and any other kit components. Set the manifold pressure to 10 in w.c. while running, and confirm the inlet pressure remains within the appliance tolerance. Inspect and clean the burners, primary air openings, and heat exchanger. Check the draft hood and B-vent for spillage, obstructions, and proper rise. Perform combustion analysis: CO air-free, CO2, O2, and stack temperature; adjust air shutters only if the manufacturer allows. Measure ambient CO near the boiler and in the living space. Leak-test the gas train. Replace the CO alarm if expired, document the conversion, and notify the owner and the authority having jurisdiction as required. Do not return the boiler to service until it passes the manufacturer's combustion and draft criteria.