Rule-of-Thumb Sizing in a Humid Climate
A 2,000 sq ft single-story house built in 1994 near Huntsville, Alabama. Design conditions are 95°F DB / 75°F WB summer and 22°F winter. The homeowner had a 4-ton system installed last spring by a contractor who "sized it at 500 square feet per ton like we always do." A block load calculation on the house gives 32,000 Btu/h total cooling, 25,000 Btu/h sensible and 7,000 Btu/h latent at 75°F DB / 63°F WB indoor. Complaints: the house holds 74°F on a 72°F setpoint but the owner says it "feels sticky," a hygrometer shows 58% RH indoors, and the system runs only 6–8 minutes at a time. Ductwork is in a vented attic and the blower is set to deliver 1,600 CFM.
1. Assess the original sizing decision using the load numbers rather than the square-footage rule.
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- Required capacity: 32,000 Btu/h = 32,000 ÷ 12,000 = 2.67 tons.
- Implied square footage per ton for this house: 2,000 ÷ 2.67 ≈ 750 sq ft/ton, not 500.
- Installed: 4 tons = 48,000 Btu/h, about 50% larger than the block load.
The 500 sq ft/ton rule fails here because the house is not the average house the rule was calibrated on - the load calculation, not the area, is the governing document. A reasonable selection would be a 3-ton nominal unit with a sensible capacity of about 25,000 Btu/h and a latent capacity of at least 7,000 Btu/h at the design indoor wet bulb, or a 2.5-ton two-stage/variable unit that matches the load at low stage. The selection must be made from expanded ratings at 95°F/75°F WB with 75/63 entering air, not from nominal tonnage alone.
2. Explain mechanically how the oversized unit produces 74°F air at 58% RH and short runtimes, and what that means for the latent load.
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- A 4-ton coil at part load reaches setpoint quickly. Run time of 6–8 minutes means the coil is wet for only a few minutes per cycle and then sits idle while the blower (often on AUTO, but duct leakage and cabinet leakage still matter) lets moisture migrate back into the space.
- The coil only removes latent heat while it is below the entering-air dew point. Short cycles spend a larger fraction of the run time just pulling the coil down to temperature; the steady-state dehumidification window is small.
- The system's Sensible Heat Ratio at full load is roughly 25,000 ÷ 32,000 = 0.78 for the load, but the oversized unit at full load may show an SHR of 0.80–0.85 while delivering 48,000 Btu/h. At part load the effective sensible fraction is even higher, so the 7,000 Btu/h latent load is chronically under-served.
- Result: sensible temperature is met (or nearly met) while moisture accumulates - 58% RH at 74°F, which is uncomfortable and can support mold growth on cool surfaces.
The problem is not a refrigerant shortage or a thermostat setting; it is capacity mismatch plus short cycling.
3. Before recommending replacement, what corrective measures would you try, and what verification would you require afterward?
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Low-cost/commissioning steps, in order:
- Verify actual airflow. 1,600 CFM is 400 CFM/ton at 4 tons - acceptable for sensible, but if static is high the real CFM may be far lower. Measure TESP and component drops.
- Reduce blower to about 350 CFM/ton (roughly 1,400 CFM on this system) using the manufacturer's speed taps. This lowers the coil temperature, increases latent removal per minute of run time, and lengthens run time modestly. Do not go below about 300–325 CFM/ton or the coil risks icing.
- Confirm the thermostat fan is on AUTO, not ON. Continuous fan re-evaporates coil moisture and can raise indoor RH several points.
- Seal and insulate ducts in the attic. Return leakage in a vented attic pulls in hot, humid air that the coil must then remove; this is often a bigger RH driver than the equipment.
- Verify charge and check for any coil bypass around the evaporator (missing block-off plates), which raises effective SHR.
Verification: measure supply and return dry bulb and wet bulb, compute the actual ΔT and ΔW, confirm indoor RH drops (target under ~55% at 75°F), and document run times during a design-day test. If RH still cannot be held with correct airflow and sealed ducts, then recommend a properly sized two-stage or variable-capacity replacement with a matched coil and, if needed, dedicated dehumidification - and support the recommendation with the load calculation, not a square-footage rule.