ACCA Manual S: Equipment Selection
Manual J tells you the load. Manual S tells you which box to hang. The trap is stopping at nameplate tonnage — a "3-ton" unit almost never puts out 36,000 BTU/hr at your design conditions. Here is how to select from OEM performance data the way the standard actually requires.
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In This Guide
Where Manual S Fits in the J-S-T-D Chain
ACCA's residential design manuals run in sequence, and each one feeds the next. Skip a step and the whole system drifts out of tune.
- Manual J — the room-by-room heating and cooling load, split into sensible and latent.
- Manual S — equipment selection that matches those loads using the manufacturer's performance data.
- Manual T — register and grille selection for throw, spread, and noise.
- Manual D — duct sizing to deliver the airflow the selected equipment needs.
Manual J and Manual S are not optional niceties. IRC Section M1401.3 and the 2009-and-newer IECC both require that equipment be sized in accordance with ACCA Manual S based on loads calculated per Manual J (or an equivalent). When an inspector asks for "your load calc and sizing," this is the pair of documents they mean.
Field reality: oversizing is the single most common residential installation error. A unit sized off square-footage rules of thumb almost always ends up too big, which is exactly the outcome Manual S exists to prevent.
Why the Nameplate Tonnage Misleads You
The tonnage on the data plate is an AHRI rating, measured at one fixed set of test conditions. AHRI 210/240 rates split cooling systems at a 95°F outdoor dry bulb, 80°F indoor entering dry bulb, 67°F indoor entering wet bulb, and 400 CFM per ton. Your customer's house on design day does not sit at those numbers.
A comfortable indoor setpoint of 75°F at roughly 50% relative humidity gives an entering wet bulb closer to 62–63°F, not 67°F. Total capacity climbs and falls with entering wet bulb and outdoor dry bulb, so the same box delivers a different number in the field than on the sticker. Manual S is built around this fact: you select from a corrected capacity, never the round nameplate figure.
Raises capacity
- Higher entering wet bulb (more latent)
- Lower outdoor dry bulb
- Higher airflow (within limits)
Lowers capacity
- Lower entering wet bulb (drier return)
- Higher outdoor dry bulb (hot design day)
- Lower airflow / dirty coils and filters
Notice the tension: a drier, more comfortable return lowers total capacity but shifts the mix toward sensible, while a humid climate demands more latent. A single tonnage number cannot express that split — the performance table can.
The Manual S Cooling Sizing Limits
Manual S sets acceptance limits so equipment is neither starved nor grossly oversized. For a straight-cool or heat pump in cooling mode, the widely applied fixed-capacity limits are:
Cooling acceptance limits (fixed-capacity)
- Total capacity: 90% to 115% of the total (sensible + latent) cooling load at design conditions.
- Sensible capacity: at least 90%, and no more than about 115%, of the sensible cooling load.
- Latent capacity: equal to or greater than the latent cooling load — you may not undersize latent in a humid climate.
Heat pumps get a little more room on total capacity — commonly up to 125% — because extra cooling capacity helps the heating side in cold climates and reduces strip-heat reliance. Variable-capacity (inverter) equipment is evaluated at its rated maximum and minimum steps rather than a single point; the intent is that the low stage can match part loads without short cycling.
Why the 115% ceiling matters
An oversized AC satisfies the thermostat on sensible temperature fast, then shuts off before it has run long enough to wring moisture out of the air. The house feels cold and clammy, the compressor short cycles, and efficiency and equipment life both suffer. That is the failure mode the upper limit guards against.
Reading OEM Expanded Performance Tables
Every manufacturer publishes an expanded performance table (sometimes called extended ratings) for each indoor/outdoor combination. You cannot do Manual S without it. To pull a capacity you need four inputs:
- Outdoor dry bulb — your 1% or 2% cooling design temperature (e.g. 95°F).
- Entering wet bulb (EWB) — from your indoor design point (≈63°F at 75°F / 50% RH).
- Entering dry bulb (EDB) — the return-air dry bulb, typically 75–80°F.
- Indoor airflow (CFM) — the blower setting you will actually commission.
The table returns total capacity and sensible capacity at that operating point; latent is simply total minus sensible. If your exact conditions fall between rows, interpolate — do not round to the nearest published line, because that can quietly push you past a sizing limit.
Airflow drives the sensible/latent split. Push more CFM per ton and sensible capacity rises while latent falls; pull airflow back and you dehumidify more. In a humid climate you might commission around 350 CFM/ton to gain latent removal; in a dry climate 425–450 CFM/ton favors sensible. Manual T and Manual D have to support whatever airflow you commit to here.
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Worked Example: Selecting a Condenser
The job:
A 2,000 sq ft home in a mixed-humid climate. The Manual J returns a total cooling load of 30,000 BTU/hr — 24,000 sensible and 6,000 latent (a sensible heat ratio of 0.80). Outdoor cooling design is 95°F; indoor design is 75°F at 50% RH, giving roughly a 63°F entering wet bulb. The candidate is a nominal 3-ton condenser with a matched coil.
Step 1: Start from the load, not the tonnage
Total load 30,000 BTU/hr; sensible 24,000; latent 6,000.
Step 2: Set the operating point
95°F outdoor DB, 63°F EWB, 75°F EDB, 1,150 CFM (≈383 CFM/ton).
Step 3: Pull corrected capacity from the OEM table
Total 34,200 BTU/hr; sensible 25,100; latent = 34,200 − 25,100 = 9,100.
Step 4: Check total capacity against the load
34,200 / 30,000 = 114% → within the 90–115% window ✓
Step 5: Check sensible capacity
25,100 / 24,000 = 105% → covers sensible load, under 115% ✓
Step 6: Check latent capacity
9,100 ≥ 6,000 latent load → adequate dehumidification ✓
| Metric | Load (Manual J) | Equipment (OEM, corrected) | Ratio / Result |
|---|---|---|---|
| Total capacity | 30,000 | 34,200 | 114% ✓ |
| Sensible capacity | 24,000 | 25,100 | 105% ✓ |
| Latent capacity | 6,000 | 9,100 | ≥ load ✓ |
| Nameplate (for contrast) | — | 36,000 (AHRI) | not used for sizing |
Result: this 3-ton passes Manual S
Every limit is satisfied at design conditions. Notice the corrected total (34,200) sits well below the 36,000 nameplate — if you had sized off the plate you would have read this as a 120% oversize and wrongly reached for a smaller unit that could not cover the sensible load.
The counter-case is just as important: had the corrected sensible come back at 22,000 (92% of the 24,000 sensible load) on the hottest design day, the box would fail to hold setpoint at peak, and you would step up a size and re-run the checks.
Heat Pumps and the Heating Side
For a heat pump you still size the cooling side first, then evaluate heating. Two numbers drive the heating decision:
- Thermal balance point — the outdoor temperature where the heat pump's declining heating capacity equals the building's rising heating load. Below it, supplemental heat carries the difference.
- Supplemental heat sizing — the strip or furnace capacity needed to cover the gap between heat pump output and the design heating load below the balance point.
Because heat pump heating capacity falls off as it gets colder, the cooling-based selection often leaves a heating shortfall in cold climates. Manual S allows oversizing cooling capacity up to about 125% partly to raise the balance point and shrink that shortfall — but you still verify heating output from the OEM heating performance table at your winter design temperature, never from the nominal tonnage.
Do not chase the heating load with cooling tonnage in a cold climate. Pushing an oversized compressor to cover heating wrecks summer humidity control. Size cooling correctly, then add supplemental heat for the winter gap.
Field Checklist and FAQ
Before you commit to a model number
- Have a real Manual J with sensible and latent loads split out.
- Use your local cooling and heating design temperatures, not 95°F everywhere by habit.
- Pull capacity from the specific indoor/outdoor combination's expanded table.
- Interpolate between table rows instead of rounding.
- Confirm total 90–115%, sensible ≥ load, latent ≥ load (cooling).
- For heat pumps, check balance point and size supplemental heat separately.
- Make sure your Manual D duct design can deliver the CFM the selection assumes.
Can I just size off square footage?
No. Rules of thumb like 500–600 sq ft per ton are for rough field estimates only and almost always oversize. Code requires Manual J plus Manual S, and an oversized system is the outcome those manuals exist to prevent.
Why is the corrected capacity lower than the AHRI rating?
Your indoor entering wet bulb (≈63°F) is lower than the 67°F AHRI test point, and your design day outdoor temperature is often higher than 95°F. Both pull total capacity below the nameplate.
What if only the total capacity passes but sensible fails?
Sensible governs whether the system holds temperature on the hottest day. If sensible falls short, raise airflow (CFM/ton) to shift the split toward sensible, or step up equipment size and re-check all three limits.
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