Zoned Duct System With Bypass Damper and High Fan Energy
A 5-ton constant-speed split system serves a two-zone house through a bypass damper. When both zones call, total airflow is 1,750 CFM at 0.60 in. w.c. external static. When only the small zone calls, the zone damper closes, the bypass damper opens, and total blower airflow rises to 1,900 CFM at 0.95 in. w.c. Measured airflow is 900 CFM to the active zone and 1,000 CFM through the bypass; the inactive zone duct is closed. Return air is 76°F dry bulb/63°F wet bulb; supply air is 55°F. The blower motor draws 8.8 A at 230 V single-phase with a 0.85 power factor, versus 7.0 A when both zones are open. The bypass damper is a barometric type and does not modulate smoothly. The homeowner complains that the small zone is noisy and does not dehumidify well.
1. Calculate the fraction of total airflow bypassed in the small-zone condition and explain why this is an efficiency and comfort problem.
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Bypass fraction = 1,000 ÷ 1,900 = 52.6%. More than half the blower airflow is being sent back to the return instead of to the occupied zone. The active zone receives only 900 CFM, so its supply velocity and noise increase while its actual delivered capacity is limited. The coil sees 1,900 CFM, but much of that air is already-cooled bypass air returning at a lower temperature, which reduces the coil's entering-air enthalpy difference and can reduce moisture removal. The blower is also operating at 0.95 in. w.c. instead of 0.60 in. w.c., so fan energy rises. This is an efficiency problem because fan power is wasted recirculating air, and a comfort problem because the zone gets less airflow and the house has poorer dehumidification.
2. Use fan laws to estimate the change in blower power if the bypass is eliminated and total airflow is reduced to 1,750 CFM at 0.60 in. w.c., assuming the same fan efficiency. Compare with the measured electrical input.
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Measured small-zone blower input = 230 × 8.8 × 0.85 = 1,720 W. Measured both-zones input = 230 × 7.0 × 0.85 = 1,369 W. Using the fan-law approximation P2/P1 = (SP2/SP1)^1.5 with SP1 = 0.95 and SP2 = 0.60: P2/P1 = (0.60 ÷ 0.95)^1.5 = 0.502. Estimated power at 0.60 in. w.c. = 1,720 × 0.502 ≈ 863 W. That is about 857 W less than the small-zone measured input, or roughly half. This is an estimate because the system resistance curve changes when the bypass is removed and airflow is rebalanced, and fan efficiency is not constant. Still, it shows that the bypass condition is imposing a large fan-energy penalty and that correcting the duct/control design should reduce blower power substantially.
3. Recommend duct and control changes and describe how to verify improvement.
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Replace the barometric bypass with a control strategy that maintains minimum airflow without dumping supply air into the return, such as a variable-speed blower with a static-pressure setpoint or modulating zone dampers. If a bypass is retained, size and control it to modulate smoothly and limit it to the minimum airflow needed. Improve the small-zone duct run by reducing equivalent length, using smooth-radius fittings, and verifying the zone damper and terminal airflow. Commission with both zones open and with the small zone alone. Record external static, total and zone CFM, bypass CFM if any, blower watts, supply/return temperatures, and indoor RH. Verify that the small zone receives its required airflow without excessive static and that fan power is lower than the 1,720 W bypass condition.