Meter Clocking Reveals a 33% Overfire
On a service call for a 60,000 Btu/h natural-gas upflow furnace, the homeowner reports a roaring burner and short cycling. The rating plate calls for a 3.5 in. w.c. manifold pressure and lists a #45 burner orifice. The technician observes a 1/2 ft³ test dial on the gas meter. With all other appliances off, the dial makes two complete revolutions in 45 seconds. Supply pressure is 7.0 in. w.c. static and 6.4 in. w.c. while the furnace is running. Manifold pressure is 4.5 in. w.c. The gas heating value is approximately 1,000 Btu/ft³ (37.5 MJ/m³). The furnace is in a basement with a dedicated combustion-air duct, and the vent is a properly sized Category I chimney. The technician has not yet inspected the burner orifices.
1. Calculate the furnace input rate from the meter data and compare it with the rating plate. State whether the unit is acceptable as found.
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Using the test dial: 0.5 ft³/rev × 2 rev = 1 ft³ in 45 s. Convert time to hours: 45/3,600 = 0.0125 h. Input volume = 1 ft³ / 0.0125 h = 80 ft³/h. At 1,000 Btu/ft³, input = 80 × 1,000 = 80,000 Btu/h. The rating plate is 60,000 Btu/h, so the appliance is firing at 80,000/60,000 = 1.33, or about 33% above rating. That is a real overfire condition, not a small timing error. An overfired furnace can overheat the heat exchanger, damage the flue, and produce elevated CO. Before condemning the appliance, confirm the meter dial factor, confirm all other gas appliances are off, and verify the rating plate input and gas type. As found, the unit is not acceptable.
2. The technician suggests simply reducing manifold pressure from 4.5 to 3.5 in. w.c. and leaving the furnace. Evaluate whether that will correct the overfire.
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Reducing pressure will reduce flow, but not enough. For a fixed orifice, flow varies approximately with the square root of pressure: 80,000 × sqrt(3.5/4.5) = 80,000 × 0.882 = about 70,600 Btu/h. That is still roughly 18% above the 60,000 Btu/h rating. The excess manifold pressure alone accounts for sqrt(4.5/3.5) = 1.134, only about 13% more flow than design. Since the measured overfire is 33%, pressure error cannot explain it all. The likely causes are an orifice larger than the listed #45, a wrong gas valve regulator, a gas conversion error, or a mismatched rating plate. Setting pressure to 3.5 in. w.c. without checking orifice size would leave the furnace overfired and unsafe.
3. List the next diagnostic and corrective steps before returning the furnace to service.
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Shut the furnace off and tag it out. Verify the gas type and confirm no LP conversion kit is installed incorrectly. Remove the burners and inspect the orifice stamp against the rating plate; measure or replace any orifice that is not the listed size. Inspect the gas valve regulator and verify it can hold the called-for manifold pressure. Set manifold pressure with a calibrated manometer at 3.5 in. w.c. with the burner operating. Re-clock the meter to confirm input is at or below rating. Perform combustion analysis: check CO, CO2, and stack temperature, and verify the temperature rise is within the rating plate range. Check flame appearance, burner alignment, and heat exchanger condition. Document all readings and the corrected orifice size before returning the appliance to service.