High Space Humidity After an Economizer Actuator Replacement
A 10,000 cfm central AHU serves a 12,000 ft² office and is designed for 20% outdoor air. Design conditions are 95°F DB/78°F WB outdoors and 75°F DB/50% RH indoors, with the coil leaving 55°F DB and a coil SHR of 0.67. In July the building sits at 78°F and 62% RH, the coil valve is 100% open, and chilled water is 44°F supply/56°F return, exactly as designed. A calibrated meter in the mixed-air plenum reads 84°F DB/71°F WB; the return grille reads 75°F DB. Economizer actuators were replaced last fall. The owner's engineer wants approval to buy a larger coil.
1. Using the mixed-air readings, determine the actual outdoor-air fraction and state what it implies about the coil.
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Two independent estimates of the outdoor-air fraction:
- Temperature balance: (84 − 75)/(95 − 75) = 0.45
- Moisture balance: W_mix ≈ 0.0134, W_return ≈ 0.0092, W_outdoor ≈ 0.0185 → (0.0134 − 0.0092)/(0.0185 − 0.0092) = 0.45
The two methods agree, so the meters are believable and the mixed-air state is real, not a sensor artifact. The mixed-air plenum is upstream of the coil, so the coil cannot be the cause of this condition. If the coil were the fault, the mixed air would sit at its design point (79°F DB, W ≈ 0.0111) and the leaving air would be warm and wet. Instead, the coil is being handed roughly 2.25× its design outdoor-air load. The fault is in the outdoor-air/economizer dampers: a stuck linkage, an actuator that was stroked or rotation-limited incorrectly, or a minimum-position potentiometer reset to about 45%. Confirm by stroking the actuator through its full range and observing the damper blade position and the mixed-air response.
2. Quantify the coil overload and predict the symptoms you would see at the coil and at the plant.
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Enthalpies: h_outdoor ≈ 41.3, h_return ≈ 28.2 Btu/lb.
- Design mixed air: 0.2(41.3) + 0.8(28.2) = 30.8 Btu/lb
- Actual mixed air: 0.45(41.3) + 0.55(28.2) = 34.1 Btu/lb
- Δh = 3.3 Btu/lb → Q_total,extra = 4.5 × 10,000 × 3.3 ≈ 148,000 Btu/h ≈ 12.4 tons
- ΔW = 0.0134 − 0.0111 = 0.0023 → Q_latent,extra = 4,840 × 10,000 × 0.0023 ≈ 111,000 Btu/h ≈ 9.3 tons
The design coil total is 4.5 × 10,000 × (30.8 − 22.2) ≈ 387,000 Btu/h (32.3 tons), so the coil is being asked for about 38% more than it can deliver, and most of the excess is latent. Expect the leaving air to drift above 55°F, the space to lose both temperature and humidity control, the chilled-water valve to sit wide open, and the plant to be short about 12 tons. The 12°F chilled-water ΔT holding at design actually supports this: the coil is clean and is absorbing the extra load, so the plant, not the coil, will show the shortfall first.
3. Recommend the corrective action and describe how you would verify the outdoor-air rate without relying on a single sensor.
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Corrective action
- Do not approve a larger coil. The coil meets its rating at its rated entering condition.
- Reset the minimum outdoor-air position to 2,000 cfm (20%) and verify the actuator stroke, rotation limit, and blade linkage; a replaced actuator is the prime suspect.
- Re-check the economizer changeover logic (enthalpy or dry-bulb), the changeover setpoint, and damper leakage at the closed position.
Verification without trusting one sensor
- Compute the fraction two ways and require agreement within about 0.05: temperature balance (T_mix − T_return)/(T_outdoor − T_return) and moisture balance (W_mix − W_return)/(W_outdoor − W_return). A blocked or wetted sensor in one of the three streams shows up immediately as disagreement.
- Cross-check against the ASHRAE 62.1 ventilation requirement for the space; 2,000 cfm of outdoor air should also be confirmed by a pitot traverse of the outdoor-air duct when the damper is at minimum.
- Trend T_mix for 24 hours. A damper that closes under command but drifts open at low load indicates a failed spring return or a leaking blade seal, which temperature readings at a single instant will miss.
- Re-verify with the coil valve at 100% after the fix: the leaving air should return to 55°F DB and the space to 50% RH.