High Temperature Rise After a Blower Replacement
A 100,000 BTU/h, 80% AFUE upflow furnace has a rated temperature rise of 35-65°F and a rated external static pressure of 0.50 in. w.c. A replacement 1/2 hp four-speed blower is set on medium-low. At steady state, return air is 68°F, supply air is 142°F, and external static pressure is 0.82 in. w.c. The filter is new, the evaporator coil is clean, and the duct system is original with long runs and undersized returns. The high-limit switch trips after about 12 minutes of operation. The technician wants to move the blower to high speed and leave, saying the higher airflow will solve the problem. A combustion test shows O2 5.8%, CO2 9.1%, CO 12 ppm, and manifold pressure 3.5 in. w.c.
1. Calculate the delivered airflow using the temperature-rise formula and determine whether the furnace is within its rated rise.
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Output is 100,000 x 0.80 = 80,000 BTU/h. Temperature rise is 142 - 68 = 74°F. Using CFM = output / (1.08 x temperature rise), CFM = 80,000 / (1.08 x 74) = 80,000 / 79.92 = about 1,001 CFM. The rated rise is 35-65°F, so 74°F is above the maximum. To stay at 65°F, the minimum airflow would be 80,000 / (1.08 x 65) = about 1,140 CFM. The furnace is therefore about 140 CFM short of the airflow needed at rated input. The 0.82 in. w.c. external static pressure also exceeds the 0.50 in. w.c. rating, confirming that the duct system is restricting airflow.
2. Evaluate the technician's plan to use high speed. What else must be checked, including combustion and heat exchanger?
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Moving to high speed may increase airflow, but the duct system is already at 0.82 in. w.c., well above the rated 0.50 in. w.c. The blower curve may not deliver 1,140 CFM at that static, and the motor may run at high amps or trip on thermal overload. A better approach is to reduce system static by improving the filter, cleaning the coil, and correcting undersized returns or restrictive duct fittings, then select a speed that delivers at least 1,140 CFM within the rated static. Combustion readings are acceptable but must be rechecked after airflow changes; verify input by clocking the meter. Inspect the heat exchanger for cracks or overheating, test the limit switch, and confirm the temperature rise after correction. High speed alone is not a repair if the duct system cannot support the airflow.
3. What would you tell the customer about the cause and the risk of leaving it as is?
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The replacement blower is moving too little air for the furnace's heat input, so the temperature rise is 74°F instead of the rated 35-65°F. The high limit is cycling because the furnace is overheating. Repeated overheating stresses the heat exchanger and can lead to cracks, which can allow carbon monoxide into the supply air. Simply raising blower speed may not solve the problem because the duct system is already restrictive, and it may increase noise and motor load. The correct repair is to reduce static pressure and set airflow to at least 1,140 CFM, then verify temperature rise, limit operation, combustion, and CO. Leaving the furnace as is risks heat exchanger failure and unsafe operation, so it should not be returned to normal service until corrected.