Mismatched Split System With Low Airflow and High Indoor Humidity
A 4-ton R-410A condenser is paired with a 3-ton-rated air handler in a 2,200 ft² house. The homeowner reports long runtimes, 58% indoor RH at 76°F setpoint, and uneven temperatures. Measured airflow by flow hood is 1,050 CFM total; external static is 0.60 in. w.c. (return 0.22, supply 0.38), and the air handler is on high speed. Return air is 78°F dry bulb/66°F wet bulb; supply air is 56°F dry bulb, giving a 22°F delta T. Suction pressure is 125 psig, liquid pressure 310 psig, superheat 12°F, subcooling 9°F. The filter is new MERV 11, the condenser coil is clean, and the blower wheel is clean. Ductwork has six supplies and three returns; the longest supply run is 30 ft of flex duct with two tight elbows. The customer wants to know whether adding refrigerant or replacing the condenser will fix the comfort problem.
1. Calculate the sensible capacity implied by the measured airflow and delta T, and explain what it does and does not tell you about total capacity.
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Sensible capacity is estimated with Qs = 1.08 × CFM × ΔT. Here Qs = 1.08 × 1,050 × 22 = 24,948 Btu/h. This is sensible capacity only; it does not include latent removal. The measured 22°F delta T looks large, but it is occurring at low airflow. The nominal 4-ton condenser represents about 48,000 Btu/h total capacity at rating conditions, so the system is delivering roughly half of nominal total capacity in sensible terms. The 58% indoor RH confirms that latent capacity is inadequate. A high delta T at low airflow can mislead a technician into thinking capacity is fine, because the coil is cooling a small amount of air intensely while the house load is not being met.
2. Assess the 4-ton condenser/3-ton air handler mismatch and 1,050 CFM airflow. What efficiency and humidity consequences follow?
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A 4-ton system typically needs about 1,600 CFM, while a 3-ton air handler is rated near 1,200 CFM. The measured 1,050 CFM is only about 66% of the 4-ton design airflow and about 88% of the air-handler rating. The mismatch means the evaporator cannot absorb the condenser's capacity at design conditions, so the unit runs longer and removes less moisture per hour. Low airflow also raises the risk of a cold coil, low superheat, and icing if latent load is high. The static pressure is not extreme, but the long flex run with tight elbows and the high-speed tap suggest the duct system is limiting delivery. The correct efficiency response is not to add refrigerant; it is to correct airflow and verify the charge only after airflow is within the air-handler's rated range.
3. Recommend a corrective plan with measurement verification and customer reporting.
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First, measure static pressure across the filter, coil, and each duct branch. Reduce restrictions where possible: replace the tight-elbow flex run with smooth-radius fittings or larger duct, verify all dampers are open, and confirm the air handler is on the correct speed tap. If airflow cannot be raised enough for the 4-ton condenser, the durable fix is a matched 4-ton air handler with a variable-speed blower and duct upgrades. After airflow is corrected, verify charge by subcooling and superheat at stable conditions. Report before/after CFM, external static, delta T, indoor RH, and compressor amps. Explain to the customer that adding refrigerant would raise pressure and cost without fixing the airflow mismatch, and that replacing only the condenser would not solve the low-airflow problem.