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Manual S Equipment Selection — ACCA-procedure right-sizing in 30 seconds.

Match Manual J loads to AHRI-certified equipment with sensible and latent capacity at your design conditions. Built on ACCA Manual S 2nd Edition procedures. Free, no signup.

Worksheet / Manual S · #BS-2026-MS-0001 · Example project

Project2,000 sqft single-family
LocationHouston, TX 77002
Climate zoneIECC 2A · hot-humidASHRAE 169
Date2026-05-15
Prepared byBuildSolver

ACCA MJ8 · Table 1A — Design conditions

Outdoor DB (cooling)95 °FMS § A4 ¹
Outdoor DB (heating)28 °FMJ8 T.1A
Indoor DB (cooling)75 °F
Indoor RH50 %

Manual J output — load from prior step

Total cooling load35,500 BTU/hMJ8 § 6.4
Sensible cooling25,000 BTU/h
Latent cooling10,500 BTU/h
Sensible heat ratio0.704
Total heating load24,000 BTU/hMJ8 § 7.5

ACCA Manual S § 3-1 — Equipment selection (Zone 2A · 115% cap)

Recommended unit3-ton heat pumpMS § 3-1
Rated capacity (47 °F)36,000 BTU/hAHRI H1
Total cooling @ 95 °F36,000 BTU/h
Sensible cooling @ 95 °F25,488 BTU/h
Heating capacity @ 28 °F29,880 BTU/h
Cooling load coverage36,000 ≥ 35,500 ✓ · ≤ 40,825 (115%) ✓
Sensible coverage25,488 ≥ 25,000 ✓ (no reallocation)
Heating coverage29,880 ≥ 24,000 ✓

Equipment · ACCA Manual S § 3-1

3 tonheat pump(36,000 BTU/h @ 47 °F)
[+] Show your work

Heat-pump bin set (47 °F rating): 18 · 24 · 30 · 36 · 48 · 60 kBTU/h (no 42K size in residential HP catalog)

Cooling derate @ 95 °F (AHRI A-test rating point) = 1.000 · rated SHR = 0.708

Heating derate @ 28 °F (interp between 35 °F / 0.90 and 17 °F / 0.72) = 0.830

3-ton total: 36,000 × 1.000 = 36,000 ≥ 35,500 ✓ · ≤ 35,500 × 1.15 = 40,825 ✓

3-ton sensible: 36,000 × 0.708 = 25,488 ≥ 25,000 ✓

3-ton heating @ 28 °F: 36,000 × 0.830 = 29,880 ≥ 24,000 ✓

Next size down (2.5-ton / 30,000): sensible = 30,000 × 0.708 = 21,240< 25,000 ✗ — undersized on sensible.

Next size up (4-ton / 48,000, the next HP bin): 48,000 > 40,825 (115% cap) ✗ — oversized total for cooling-dominated zone 2A.

¹ Cooling outdoor 95 °F is the AHRI A-test rating point, used here for instructional clarity. Actual Houston 1% cooling design dry-bulb is closer to 96 °F per ACCA Manual J Table 1A; 1 °F changes total derate by <1%. Heating 28 °F is the Houston 99% heating design dry-bulb per the same table.

Numbers chosen so the equipment passes both the 115% total-cooling cap and the sensible-coverage check at rated SHR — a clean example where ACCA Manual S §“½ excess latent → sensible” reallocation is not needed. Most real Houston residential loads sit close to this profile.

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01 · What is Manual S

Manual J gives you the load. Manual S gives you the equipment.

ACCA Manual S is the second half of correct residential HVAC sizing. After Manual J tells you the building needs, say, 35,500 BTU/h of cooling, Manual S forces you to verify that the specific condenser/coil combination you're going to install actually delivers that capacity at your real-world design conditions — not at the AHRI test conditions printed on the brochure.

That distinction matters because manufacturer nominal capacity is published at AHRI A-test (95 °F outdoor dry-bulb, 80 °F indoor dry-bulb, 67 °F indoor wet-bulb). At a Phoenix design day of 109 °F outdoor, a 3-ton unit delivers closer to 2.6 tons. At a Denver altitude of 5,300 feet, the same unit loses another ~16%. At a Houston 96 °F design with 50% RH indoors, the sensible/latent split shifts in ways that catalog tonnage doesn't reveal. Manual S is the procedure that surfaces all of this before the install.

02 · Why Manual S after Manual J

Three checks Manual J doesn't make.

Manual J output is the input to Manual S, not its replacement. A Manual J that says "35,500 BTU/h" doesn't tell you whether a 3-ton or 3.5-ton unit is the right answer at your design conditions. Manual S forces three checks Manual J skips:

  1. Total capacity at design — not nominal. Apply the outdoor-DB derate factor (Cutler 2013 model, validated against expanded performance tables) to the AHRI-rated capacity. The result must cover Manual J total cooling load and not exceed it by more than 115% in cooling-dominated zones (IECC 1–3) or 125% in heating-dominated zones (IECC 4+) per ACCA Manual S § 3-1.
  2. Sensible capacity at design. Most catalog data hides the sensible/latent split. At rated SHR 0.708 (industry average for residential single-stage AC), a 36,000 BTU/h unit delivers about 25,488 BTU/h of sensible cooling at AHRI conditions. If your sensible load is 26,000, this unit is undersized on sensible even though total capacity matches.
  3. Heating capacity at design — for heat pumps. Heat-pump capacity drops as outdoor temperature falls. A 3-ton standard split heat pump rated at 36,000 BTU/h at 47 °F delivers about 29,880 BTU/h at 28 °F (Houston heating design) and only 25,920 BTU/h at 17 °F (Atlanta heating design). Manual S verifies the unit covers heating load at your specific heating design dry-bulb.

BuildSolver runs all three checks every time the select_hvac_equipment_manual_s tool fires. Each check is a separate line in the audit trail you can show your customer or a reviewing PE.

03 · Common oversizing mistakes

Three field mistakes Manual S would catch.

Rounding up to the next bin "to be safe"

Manual J says 35,500 BTU/h, contractor installs a 4-ton (48,000 BTU/h). That's 135% of load — violates Manual S § 3-1 in every climate zone. Symptoms: short-cycle, poor humidity control in hot-humid climates, compressor failure 5–7 years earlier than expected. Energy Vanguard and Bryan Orr at HVAC School have published the post-mortems on hundreds of these installs.

Matching cooling tonnage to replaced unit

"The old one was 4-ton, so the new one is 4-ton." If the original was oversized 12 years ago, the replacement perpetuates the error and adds another 12 years of short-cycling. Manual S forces you to start from Manual J, not from the nameplate of the unit you're replacing.

Ignoring sensible/latent split in hot-humid climates

Houston, Miami, Mobile, New Orleans — climates where latent load regularly hits 25–30% of total. A unit with high SHR (sensible capacity dominant) at design conditions removes heat but not moisture. The thermostat satisfies on temperature, the house sits at 65% indoor RH, and the homeowner calls back about mold within the first cooling season. Manual S sensible/latent check catches this before install.

04 · AHRI Coil Match-Up

Match-Up: the only capacity number Manual S accepts.

ACCA Manual S § A4 requires the rated capacity input come from an AHRI-certified Coil Match-Up — a specific outdoor condenser paired with a specific indoor coil, tested as a combination by AHRI. The published nominal tonnage on a condenser is meaningless until you also specify which coil is installed: the same condenser with a larger coil delivers different total and sensible capacity.

The AHRI Certified Directory lives at ahridirectory.org. Search the condenser model and the directory returns every certified Match-Up: AHRI reference number, rated total cooling at 95 °F, sensible cooling, SEER2, heating at 47 °F and 17 °F (for heat pumps), and EER. Those are the numbers Manual S expects.

Carrier, Trane, Lennox, Goodman, Rheem, and the rest publish expanded performance data internally — multiple outdoor temperatures, multiple indoor wet-bulbs, multiple airflows. AHRI ratings are a single point on those curves (the A-test). Manual S uses the A-test rating plus a derate model to interpolate to your design conditions. BuildSolver's derate tables are validated within ±5% of expanded performance data between 85 °F and 115 °F.

05 · Sensible vs Latent

SHR is what separates good comfort from bad.

Sensible Heat Ratio (SHR) = sensible cooling load divided by total cooling load. A residential SHR of 0.70 means 70% of the cooling energy goes to dropping temperature, 30% to removing moisture. A warehouse with low occupancy and no cooking has SHR closer to 0.85; a kitchen-heavy household in Houston might drop to 0.65.

The industry rated SHR for single-stage residential AC at AHRI A-test conditions is 0.708 — a published value used in EnergyPlus, ResStock, and ASHRAE 90.1 reference models. BuildSolver uses this rated value (plus a 0.002 per °F adjustment for outdoor DB) as the sensible capacity derate when running Manual S sensible check.

Where contractors get into trouble: they pick a unit whose total capacity covers Manual J total load, but never check that the sensible capacity at design conditions covers Manual J sensible load. In our worked example above, a 3-ton unit delivers 25,488 BTU/h sensible at 95 °F (36,000 × 0.708), which just barely covers the 25,000 BTU/h sensible load. A 2.5-ton unit would deliver only 21,240 BTU/h sensible — undersized on sensible despite reasonable total capacity.

06 · The ½-excess-latent rule

When sensible falls short, half the excess latent moves over.

ACCA Manual S includes a specific allowance — almost nobody outside ACCA training discusses it openly — that resolves the common case where total capacity passes but sensible capacity falls just short. The rule: half of the excess latent capacity may be reallocated to sensible.

Worked example. Suppose Manual J output is 35,500 / 26,000 sensible / 9,500 latent (SHR = 0.732, slightly drier than our hero example). A 3-ton unit delivers 36,000 total / 25,488 sensible / 10,512 latent at design.

  • Total check: 36,000 ≥ 35,500 ✓ · ≤ 40,825 (115%) ✓
  • Sensible check (raw): 25,488 < 26,000 ✗ — fails by 512 BTU/h
  • Excess latent: 10,512 (capacity) − 9,500 (load) = 1,012 BTU/h
  • ½ excess latent → sensible: 1,012 / 2 = 506 BTU/h
  • Adjusted sensible: 25,488 + 506 = 25,994 ≈ 26,000 ✓ (passes by the rule)

The physical justification: when the unit operates at a real-world indoor wet-bulb below the AHRI rating point (drier-than-rated indoor air), part of the catalog "latent capacity" actually delivers as sensible — the coil doesn't reach dew point as effectively when the air is already drier. ACCA permits crediting half of the unused latent capacity to the sensible budget. Our hero example doesn't trigger this rule, but many Houston, Atlanta, and Tampa residential projects do.

Straight answers

Questions contractors ask about Manual S.

Do I need Manual S if I already did Manual J?
Yes. Manual J tells you the load. Manual S tells you which specific equipment will deliver that load at your design conditions, accounting for outdoor temperature derate, altitude, and the sensible/latent split. Skipping Manual S is the #1 cause of oversized residential HVAC — the contractor rounds up to the next tonnage to be safe, the unit short-cycles, and dehumidification suffers in hot-humid climates.
What is AHRI Coil Match-Up and why does it matter?
AHRI (Air-Conditioning, Heating, and Refrigeration Institute) certifies condenser/coil combinations through independent testing. Each certified pair gets a Match-Up reference number you can look up in the AHRI Directory at ahridirectory.org. ACCA Manual S requires you to use AHRI-rated capacity, not manufacturer-published nominals, because catalog tonnage often corresponds to a different coil than what's actually installed in the field. The sensible/latent split you see in the AHRI data is what the unit will actually deliver.
Why is my AC oversized and what happens?
Common causes: Manual J was skipped or done by rule-of-thumb (400 sqft per ton); Manual J was done but Manual S was skipped, so the contractor rounded up; existing equipment was oversized and replacement matched the old size; total capacity matched the load but sensible capacity wasn't checked. Result: short cycling, poor dehumidification in hot-humid climates (mold risk), higher energy bills, compressor failure in 7–9 years instead of 12–15. The cost premium over the equipment's life is roughly $2,400 for picking 4-ton when 3-ton was correct.
What's the difference between 115% and 125% oversizing limits?
ACCA Manual S §3-1 caps cooling oversizing at 115% of Manual J total cooling load for cooling-dominated climates (IECC zones 1, 2, 3). For heat-pump heating in cold climates (IECC zone 4 and colder), Manual S allows up to 125% on the cooling side because the equipment is dual-purpose — sizing for heating capacity at low ambient can push you above 115% cooling, and that trade-off is acceptable. Your climate zone determines which rule applies. Houston (Zone 2A) is 115%. Minneapolis (Zone 6A) is 125%.
Can BuildSolver replace Manual S for permit submission?
No. BuildSolver is sales-phase preliminary sizing. For permit submittal in jurisdictions requiring ACCA-approved software, use Wrightsoft Right-S or Cool Calc (both ACCA-approved). BuildSolver is what you use during the sales call to give a fast, defensible number to the customer with a citation-ready output. For the permit packet, the engineer of record uses ACCA-approved software.
What design conditions does BuildSolver use for derating?
BuildSolver pulls design conditions from ASHRAE Standard 169-2021: 1% cooling dry-bulb and 99% heating dry-bulb for the project location (city or ZIP). The cooling derate combines the Cutler 2013 biquadratic capacity model (industry-standard, used in EnergyPlus and ResStock) with a published rated SHR of 0.708 at AHRI A-test conditions. Heating derate uses NEEP CCASHP database 2024 averages for standard split heat pumps and cold-climate heat pumps separately.
How do I get AHRI-certified equipment data?
Look up your candidate condenser/coil pair in the AHRI Certified Directory at ahridirectory.org. Search by manufacturer model number. The directory returns the certified Match-Up reference, AHRI-rated total cooling at 95°F outdoor, sensible cooling, SEER2, and heating capacity at 47°F and 17°F for heat pumps. Use those numbers as the input to Manual S — they're what the equipment actually delivers, not what the marketing brochure claims.
What if my design outdoor temperature is above 115°F?
BuildSolver's derate tables are validated within ±5% of manufacturer expanded performance tables between 85°F and 115°F outdoor. Above 115°F (extreme Phoenix or Las Vegas summer design days), the validation residual rises to ±10%. We emit a warning above 115°F. For projects with design dry-bulb above 115°F we recommend cross-checking against manufacturer expanded performance data for the specific unit, because the biquadratic model loses accuracy in extreme heat.

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