Short-Cycling Furnace After Duct Modification
A 90,000 Btu/h input, 80% AFUE upflow furnace in a basement serves a 1,800 sq ft single-story house. The homeowner reports the furnace runs for 4 minutes, shuts off, and restarts after about 6 minutes; supply registers feel warm but weak. A contractor recently replaced 40 feet of rigid trunk with 25 feet of flex duct and added two supply runs to a finished basement. Static pressure measured at the supply plenum is 0.85 in. w.c. and return static is 0.55 in. w.c. The furnace nameplate lists a maximum external static pressure of 0.50 in. w.c. at the installed blower speed. The high-limit switch trips on every cycle. The furnace is in a closet with a louvered door and the return is drawn from the closet.
1. Diagnose the short-cycling, quantifying the airflow problem and explaining the role of each measured value.
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The measured external static pressure is 0.85 + 0.55 = 1.40 in. w.c., which is 0.90 in. w.c. above the nameplate maximum of 0.50 in. w.c. at the installed blower speed. That is a severe restriction: the blower is operating far off its fan curve, so delivered cfm is well below the design airflow for a 90,000 Btu/h input. At roughly 80% AFUE the output is about 72,000 Btu/h; with low airflow the temperature rise climbs above the nameplate range and the high-limit switch opens, producing the 4-minute-on/6-minute-off pattern.
The flex duct replacement is the likely primary cause - 25 feet of flex with two added runs increases friction and, if the flex is compressed or has tight bends, adds far more resistance than the rigid trunk it replaced. The basement runs also changed the system balance. The 0.55 in. w.c. return static is abnormal for a closet return and suggests the louvered door or return path is undersized.
Diagnosis: insufficient airflow from added duct resistance plus a restricted return, causing limit trip. The fix is not a larger limit or a higher blower speed alone; it is reducing system resistance and verifying temperature rise and static within nameplate limits.
2. The contractor proposes increasing blower speed to the next tap to 'push more air.' Assess this and state what must be verified before the system is accepted.
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Increasing blower speed can raise cfm, but it is only acceptable if the resulting external static pressure remains within the nameplate maximum for that speed and the temperature rise falls within the nameplate range. On a system already at 1.40 in. w.c., moving to a higher tap shifts the operating point along the fan curve and may add some airflow, but it also increases velocity and noise and may push the motor beyond its rated horsepower. It does not remove the underlying restriction.
Before acceptance, verify: (1) external static pressure at the new speed is at or below the nameplate maximum; (2) supply and return static are measured separately and the return path (louvered door free area, closet return) is adequate; (3) temperature rise is within the nameplate range at steady state; (4) the flex duct is fully extended, supported, and not compressed, with bends meeting the manufacturer's minimum radius; (5) the added basement runs are balanced so the main house is not starved. If static cannot be brought within limits, duct and return modifications are required regardless of blower tap.