The 500 ft² per Ton Bid on a Tampa Retrofit
A Tampa homeowner with a 2,400 ft² single-story house built in 1998 (R-19 attic insulation, single-pane glass with western shade, 8 ft ceilings) is replacing a 20-year-old 3-ton, 10 SEER split with a leaking evaporator coil. Bid A proposes a 4-ton, 14.3 SEER2 single-stage condensing unit and reusing the existing R-6 flex duct system, reasoning that "at 500 ft² per ton and Tampa heat, 4 tons is right, and the extra capacity lets the house recover faster." The contractor's block Manual J, run at 95°F outdoor and 75°F indoor at 50% RH, gives 34,800 Btu/h total with 26,200 Btu/h sensible. You measure 0.72 in. w.c. external static at about 1,050 cfm on the existing system.
1. Using the load numbers, evaluate whether the 4-ton recommendation is defensible. What size and type of equipment should be selected, and what must be verified before committing?
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Start with the load, not the area.
- Design total = 34,800 Btu/h ≈ 2.9 tons; sensible = 26,200 Btu/h. System sensible heat ratio (SHR) = 26,200 / 34,800 = 0.75, which is typical of a Florida envelope but leaves a large latent share (~8,600 Btu/h) to be removed by the coil.
- A 4-ton nominal unit is roughly 48,000 Btu/h - about 38% oversized. The rule of thumb (500 ft²/ton) ignores envelope, glass, orientation, duct location and infiltration, and cannot substitute for the Manual J that was already run.
Why oversizing hurts in this climate: at part load the compressor satisfies the thermostat before the coil runs long enough to pull moisture out. The house then floats at 58–65% RH while the thermostat reads 75°F, and the blower keeps re-evaporating a wet coil on the "on" fan setting.
Selection: a nominal 3-ton system, confirmed through Manual S against the manufacturer's expanded performance table at 95°F outdoor / 75°F entering dry bulb, 63°F entering wet bulb, verifying that sensible capacity at the actual delivered airflow meets or exceeds 26,200 Btu/h. A two-stage or variable-speed unit is a defensible upgrade for latent control, not extra tonnage.
Verify before buying: blower can deliver ~1,200 cfm at ≤0.50 in. w.c. external static (you already measured 0.72 at only 1,050 cfm - the duct system is a problem), line set size/length against the new unit's requirements, electrical circuit vs. nameplate MCA/MOP, return air path, and attic duct insulation and leakage. Pull the permit and file the Manual J and S.
2. The homeowner later calls: it is 75°F inside at 62% RH with the 4-ton unit. Explain the mechanism and list remedies in priority order.
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Mechanism - a latent-capacity and runtime problem, not a charge problem (usually):
- Short runtimes: the oversized unit drops sensible temperature fast, the thermostat opens, and the coil never stays below the dew point long enough to condense and drain the moisture the house generates.
- Airflow too high: if the 4-ton is moving 1,500–1,600 cfm, coil temperature rises and moisture removal per cfm falls.
- Re-evaporation: fan set to "on" pushes air over a wet coil between cycles.
- Duct leakage in a hot attic: a negative return pressure pulls 130°F attic air (and humid outdoor air) straight into the airstream, adding both sensible and latent load.
Remedies, priority order:
- Confirm the sizing error with the Manual J/S. Correct sizing is the only real fix; everything else is a patch.
- Seal and measure ducts (total leakage and static); repair return-side leaks first - they are the biggest humidity penalty.
- Set the fan to auto, not "on."
- Reduce airflow toward ~350 cfm/ton only after verifying the coil's expanded ratings still meet sensible load at that airflow; do not blindly slow the blower.
- Verify charge by subcooling/superheat after the duct and coil work, and check for a wet, dirty, or partially blocked coil and a trap/drain problem.
- If the correct-size replacement is not immediate, a whole-house dehumidifier or reheat control is the honest interim answer - not more tonnage.
Document the airflow, static, ΔT, and RH readings with each step so the recommendation is defensible.
3. What information must the contractor give the homeowner in writing so they can judge Bid A versus a correctly sized bid?
Read the worked answer
Give a one-page comparison the homeowner can actually evaluate:
- Design load: 34,800 Btu/h total / 26,200 Btu/h sensible, with the design conditions (95°F/75°F, 50% RH) and the software used.
- Selected capacity at design conditions: the actual (not nominal) sensible and latent capacity from the manufacturer's table at the delivered cfm, and the resulting SHR.
- Airflow and static: required cfm, measured existing static (0.72 in. w.c.), and what duct work is included to reach ≤0.50 in. w.c.
- Scope boundaries: what is not included (attic duct replacement, return enlargement, electrical, dehumidification) and what it would cost.
- Latent/comfort statement: expected indoor RH, not just temperature.
- Warranty and permit: permit number to be pulled, warranty registration in the homeowner's name, and whether the existing line set is reused and why that is or is not acceptable.
Avoid leading with SEER2 numbers alone - a higher SEER2 unit that is oversized will perform worse than a correctly sized, correctly ducted lower-tier unit. Put that sentence in the proposal.