Replacement Sizing After an Envelope Retrofit
A 1978 ranch in Atlanta, 2,000 ft² with 8 ft ceilings. The owner has replaced the windows, air-sealed the attic floor, and blown R-38 over existing R-11; the duct system is still R-4 flex lying on the attic floor of a vented attic that reaches 135°F in August. A block Manual J (92°F DB/74°F WB outdoor, 75°F/50% RH indoor, duct gain included) returns 27,000 Btu/h total, 21,000 Btu/h sensible, and 6,000 Btu/h latent. Duct leakage to outdoors measures 180 CFM25. The existing 3-ton, 10 SEER split still keeps up on all but the hottest afternoons, but the house feels clammy. The owner wants a like-for-like replacement and asks whether a 2-ton unit would 'save money.'
1. What nominal capacity do you select, and what must you complete before you commit to a tonnage?
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Start from the load, not from the old nameplate. The 3-ton unit was sized for a leaky 1978 envelope and is now roughly 33% oversized against a 27,000 Btu/h design load.
Check the latent side first. 6,000 Btu/h latent ÷ 1,070 Btu/lb ≈ 5.6 lb/h of condensate, about 0.67 gal/h. Required SHR = 21,000/27,000 = 0.78. A 2-ton match rated 24,000 Btu/h at ARI 80/67 entering delivers about 18,000 sensible / 6,000 latent. Our actual entering air is 75°F DB / 63°F WB, which derates capacity roughly 5–8%, so real sensible output lands near 16,500–17,000 - short of the 21,000 sensible load. A 2-ton is not automatically wrong; it is wrong as long as the ducts stay where they are.
Three steps before committing:
- Pull manufacturer expanded ratings at 75 DB/63 WB entering and 95°F outdoor for the specific condenser/coil/air-handler match (Manual S), not the ARI headline number.
- Run Manual T so the air handler is not undersized for the room-by-room airflow the new envelope still needs.
- Price the duct remediation as part of the job. Sealing and encapsulating (or relocating) the attic ducts removes duct gain and the 180 CFM25 of outdoor leakage - typically 3,000–5,000 Btu/h of sensible load here.
Decision: If the ducts are fixed, a 2-ton match, set at 350–400 CFM/ton with a variable-speed air handler and dehumidification control, covers the revised load. If the owner refuses the duct work, select 2.5 tons with a matched expanded rating that shows at least 21,000 Btu/h sensible at actual entering conditions. Do not install another 3-ton.
2. The current system 'keeps up' but the house is clammy. Explain the mechanism and why oversizing makes it worse.
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- A 36,000 Btu/h unit on a 27,000 Btu/h load is running near 75% of capacity at design, so it satisfies the thermostat quickly and cycles. In partial-load operation the coil's sensible heat ratio rises - it removes fewer Btu of moisture per Btu of sensible cooling - and the entering wet-bulb at the coil drops as indoor humidity climbs.
- Meanwhile the attic ducts are pulling 135°F air in through 180 CFM25 of leakage and adding sensible gain, so the unit is working on dry-bulb load while the latent load (6,000 Btu/h, driven by the Atlanta design wet bulb) goes unaddressed.
- Going to 2 tons without fixing the ducts would fix nothing: the coil would run longer but the load would still exceed capacity on design day.
- Complaints of clamminess with short runtime are the classic signature of oversizing plus duct gain, not of a refrigerant problem. Confirm by logging runtime and indoor RH before condemning the equipment.
3. What measurements do you require at start-up to accept the job?
Read the worked answer
- External static pressure versus the air handler's rated ESP; compare to the value used in the expanded rating table.
- Airflow: verify within ±10% of design at 350–400 CFM/ton (a lower blower setting is a legitimate moisture-control lever in this climate - document whichever setting is used).
- Temperature split across the coil, 18–22°F at steady state, with the filter clean and all registers open.
- Superheat/subcooling per the manufacturer's charging chart for the actual indoor and outdoor conditions.
- Duct leakage after remediation, targeted at 10% or less of fan flow (or the adopted energy code's CFM25 per 100 ft² threshold) rather than the 180 CFM25 found today.
- Room-by-room temperature verification, plus indoor RH maintained near 50% at design - the metric the owner actually complained about.