High-Limit Trips on a 90,000 Btu/h Furnace
A technician is called to a two-story house where the 90,000 Btu/h, 80% induced-draft furnace runs for about 12 minutes, trips the high-limit switch, and restarts after the blower cools the heat exchanger. The nameplate temperature rise range is 35-65°F. Measured supply air is 145°F and return air is 63°F, a rise of 82°F. Manifold gas pressure is 3.5 in. w.c., matching the nameplate for natural gas, and the burners show a steady blue flame. The filter is new, the blower is already on the highest speed tap, and total external static pressure measures 0.85 in. w.c. against a rated maximum of 0.50 in. w.c. The evaporator coil is in the same cabinet and has not been cleaned in at least three years. The homeowner asks the technician to lower the manifold pressure to 3.0 in. w.c. "so it stops overheating."
1. Using the temperature rise and static pressure, determine the root cause and show the supporting calculation.
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The furnace is overheating because it is moving too little air. At 80% efficiency, a 90,000 Btu/h input delivers about 72,000 Btu/h. A 1,200-1,400 cfm blower would produce a rise near 50-55°F, but the measured 82°F implies roughly 800-820 cfm using cfm = output Btu/h ÷ (1.08 × temperature rise). Total external static pressure of 0.85 in. w.c. is far above the 0.50 in. w.c. rating and confirms a severe airflow restriction. The likely culprits are a clogged evaporator coil, undersized or crushed ductwork, or an over-restrictive filter grille. The burners are clean and gas pressure is correct, so combustion is not the cause, and the high-limit switch is doing exactly what it was designed to do.
2. Why is lowering manifold pressure to 3.0 in. w.c. the wrong response, and what risks does it create?
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Lowering manifold pressure reduces input, which can hide the high-limit trip by making less heat, but it does not fix the airflow restriction and it takes the appliance out of its listed operating condition. Derating a furnace below nameplate input can cause incomplete combustion, elevated CO, flame impingement, and nuisance lockouts, and it voids the manufacturer's ratings. It also leaves the homeowner with a furnace that produces less heat than the load requires on design days, while the real problem, high static pressure, continues to strain the blower motor and could eventually cause it to overheat or fail. A technician who adjusts gas pressure to cover a duct defect is creating a combustion safety issue and a liability exposure.
3. What diagnostic and repair steps should the technician recommend, and how should the fix be verified?
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Inspect and clean the evaporator coil, then measure static pressure before and after to quantify improvement. Check the filter grille and return duct for undersizing; a single small return on a 90,000 Btu/h furnace is common. Measure static pressure across the coil and in the supply plenum to locate the restriction, and compare the blower performance table against the required 1,200-1,400 cfm. If ductwork is undersized, the repair may be duct enlargement or adding a return rather than a new blower. After repairs, verify the temperature rise is within 35-65°F, static pressure is at or below 0.50 in. w.c., and perform combustion analysis to confirm CO and draft are normal. Document before-and-after readings for the homeowner.