Condensing Furnace Tripping High Limit on High Fire
A 60,000 Btu/h, 96% AFUE two-stage condensing furnace in a second-floor closet trips its high-limit switch after 8 to 10 minutes on high fire. Low fire runs continuously. The return air passes through a louvered closet door and a 20x25x1 filter that is new. The furnace is rated for 0.50 in. w.c. external static pressure. You measure 0.95 in. w.c. total external static with the coil dry, and the supply plenum temperature is 138°F with 70°F return. The nameplate temperature rise range is 30-60°F. High-fire manifold pressure is 3.5 in. w.c. natural gas. A combustion analyzer reads 5.5% O2, 45 ppm CO air-free, and 118°F flue temperature. The blower is on high speed and the amp draw is within nameplate. The closet door has 120 square inches of free area; the return duct is 16 in. flexible duct with two 90-degree turns. The condensate trap is clear. The installer says the limit switch is defective and wants to replace it.
1. Calculate the temperature rise. Is the limit switch defective? Explain.
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Temperature rise = supply - return = 138°F - 70°F = 68°F. The nameplate range is 30-60°F, so the furnace is operating 8°F above the maximum. The limit switch is opening because the furnace is overheating, not because the limit is defective. Replacing the limit would mask a real problem and could create a hazardous condition. High fire input should be checked, but the 3.5 in. w.c. manifold pressure is normal for natural gas and the analyzer readings (5.5% O2, 45 ppm CO air-free) do not indicate gross overfiring. The primary suspect is low airflow: 0.95 in. w.c. external static is nearly double the 0.50 in. w.c. rating. A high static pressure with a new filter and clean coil points to duct restriction, undersized return, or a blower tap/performance issue.
2. Describe the static-pressure and blower-table tests you would run to isolate the cause.
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Measure static pressure at four points: return air before the filter, after the filter and before the blower, after the blower and before the evaporator coil, and after the coil in the supply plenum. Subtract to get pressure drops for filter, blower, coil, and supply/return ducts. Compare each to the manufacturer's allowable drop. Measure return-duct pressure drop with the closet door open and closed; if it drops sharply with the door open, the louvered door or closet return is undersized. Compare blower speed and motor amp draw with the blower performance table at 0.95 in. w.c. If the blower is on high speed and still delivering low cfm, the duct system is the limit. Check the 16 in. flexible return for crushed sections and the two 90-degree turns; flex duct with sharp bends can lose significant capacity.
3. What corrective actions are acceptable, and what should you not do?
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Acceptable: increase return-air capacity by adding a return duct or transfer grille, enlarging the closet louver to the manufacturer's free area, replacing sharp flex bends with metal elbows, and correcting the blower tap only if the performance table shows the correct cfm. After airflow is corrected, recheck temperature rise, limit operation, and combustion. If the duct system cannot be corrected, the manufacturer may permit a lower firing rate or a different blower setting, but only per written instructions. Do not replace the limit switch, do not reduce manifold pressure to hide the symptom, and do not remove the limit from the circuit. Also verify the heat exchanger and venting are intact before returning the furnace to service.