ACCA Manual S Equipment Selection Reference

Why this matters

ACCA Manual S is the procedure for matching specific HVAC equipment to the loads calculated in Manual J. It accounts for the difference between nameplate capacity at AHRI rating conditions and actual capacity at your customer's design conditions - a difference that's often 10-25%. Skipping Manual S produces equipment that looks right by tonnage but underperforms in the field. Together with Manual J (load) and Manual D (ducts), it's the foundation of right-sized installation.

What Manual S does

Manual J calculates the LOAD - what the building needs (heating BTU/hr and cooling BTU/hr at design conditions).

Manual S selects EQUIPMENT - finds a specific model that delivers the calculated load at the actual operating conditions (not AHRI test conditions).

The two outputs interact:

• A 3-ton AC at AHRI 95°F outdoor / 80°F indoor DB / 67°F indoor WB delivers 36,000 BTU/hr total (rated)
• The same unit at 105°F outdoor / 75°F indoor delivers less (~32,000 BTU/hr or less)
• A heat pump at 47°F outdoor delivers rated capacity; at 17°F outdoor it might be 65% of rated; at 5°F it might be 50%
• Manual S accounts for this and selects appropriately

Cooling equipment selection

Required Manual J inputs:

• Total cooling load (sensible + latent)
• Sensible cooling load (just temperature, no moisture)
• Latent cooling load (moisture removal)
• Indoor design conditions (dry-bulb, wet-bulb, RH)
• Outdoor design conditions (1% dry-bulb, mean coincident wet-bulb)

Equipment match criteria:

1. Total capacity at design conditions ≥ Manual J total load
2. Sensible capacity at design conditions ≥ Manual J sensible load (within 100-115%)
3. Latent capacity at design conditions ≥ Manual J latent load
4. Sensible Heat Ratio (SHR) of equipment ≥ ratio of sensible / total load

AHRI 210/240 standard rating conditions:

• Outdoor: 95°F DB
• Indoor: 80°F DB, 67°F WB
• Indoor airflow: 350-400 CFM/ton standard

Manufacturer extended ratings:

• Performance at 82°F, 95°F, 105°F outdoor
• Performance at various indoor wet-bulbs (62-72°F)
• Indoor airflow variations (lower CFM for higher latent, higher CFM for higher sensible)

A Manual S calc uses the manufacturer's expanded rating tables to find the model + airflow setting that matches the load.

Heat pump heating selection

This is where Manual S gets critical.

Heat pump capacity drops as outdoor temperature drops. A nominally 3-ton heat pump:

• 47°F outdoor: ~36,000 BTU/hr heat output (rated)
• 30°F outdoor: ~26,000 BTU/hr (about 72% of rated)
• 17°F outdoor: ~20,000 BTU/hr (about 55%)
• 5°F outdoor: ~14,000 BTU/hr (about 39%)
• -10°F outdoor: ~9,000 BTU/hr (about 25%)

(Numbers approximate; vary by specific model. Cold-climate heat pumps like Mitsubishi Hyper-Heat maintain capacity better.)

Balance point: the outdoor temperature where heat pump capacity exactly matches building heating load. Above this, heat pump alone is enough. Below, supplemental heat is needed.

A Manual S calc for heat pumps must:

1. Identify the design heating temperature (99% outdoor)
2. Look up the heat pump's capacity AT that temperature (not at 47°F)
3. Verify capacity ≥ heating load
4. Calculate the balance point (typically 25-40°F for standard heat pumps; lower for cold-climate)
5. Size supplemental heat (electric strips or backup furnace) to cover the gap

Cold-climate heat pump selection

Standard heat pumps lose capacity sharply below 30°F. Cold-climate variants (Mitsubishi Hyper-Heat, Carrier Greenspeed, Daikin Aurora):

• Maintain 80%+ of rated capacity at 17°F
• Operate efficiently down to -13°F or lower
• Variable-speed compressors
• Enhanced vapor injection
• More expensive but eliminate balance-point challenges

For climates with 99% design < 20°F, cold-climate heat pumps often beat standard heat pump + electric strips on operating cost.

Equipment over-sizing penalty

The Manual S procedure penalizes oversized equipment for cooling:

• 100-115% of sensible load = acceptable
• 115-125% = caution (humidity removal compromised)
• 125%+ = unacceptable (short cycling, comfort failure)

In humid climates, this is critical - oversized AC produces clammy comfort failures. ACCA Manual S enforces "right size, slightly small for sensible, latent-capable" in humid markets.

Sensible Heat Ratio (SHR) considerations

Equipment SHR varies:

• AHRI rating SHR: typically 0.75-0.78 for standard residential
• Low-SHR equipment (high latent removal): 0.65-0.72; designed for humid climates
• High-SHR equipment: 0.85-0.95; dry climates

In humid climates (gulf coast, southeast), choose low-SHR equipment so latent capacity matches the high latent load.

In dry climates (Phoenix, Denver), high-SHR is fine.

Heating equipment sizing - furnace

Gas furnaces are simpler than heat pumps:

• Capacity is BTU input × AFUE = output
• Output is steady regardless of outdoor temp (above design)
• Match to Manual J heating load within reasonable margin

Furnace sizing rule (per ACCA Manual S):

• Output at 100% of heat load = ideal (often impractical with standard models)
• 100-125% of heat load = acceptable
• 125-150% = some oversizing tolerance (short-cycling on mild days but acceptable)
• 150%+ = excessive oversizing (cycling damage, customer discomfort)

A 60,000 BTU/hr load on standard 80% AFUE furnace → 60,000 / 0.80 = 75,000 BTU input minimum. Most close-to model: 80,000 BTU. Acceptable.

Two-stage and variable-speed considerations

Single-stage (on/off):

• Either at full capacity or off
• Match closely to load
• Short-cycles when oversized

Two-stage:

• Low-fire (60-70% of high) for mild conditions
• High-fire for design days
• Better humidity control
• More tolerance for slight oversizing

Variable-speed / modulating:

• 30-100% modulation
• Longest run times, best comfort
• Match high-stage capacity to design load
• Low-stage handles mild weather

For homes in mixed climates with frequent mild days, two-stage or variable produces better customer satisfaction than single-stage.

Manual S worksheet (residential)

A typical Manual S worksheet captures:

Project data:

• Address
• Design temperatures (heating + cooling)
• Indoor design conditions

Loads from Manual J:

• Total cooling
• Sensible cooling
• Latent cooling
• Total heating

Equipment selection:

• Manufacturer + model number
• Rated capacity (AHRI)
• Capacity at customer's design conditions
• Sensible capacity at design
• Indoor airflow setting (per Manual S airflow procedure)

Verification:

• Equipment total capacity vs Manual J total load
• Equipment sensible capacity vs Manual J sensible load
• Equipment latent capacity vs Manual J latent load
• Equipment SHR vs required SHR

References

• ACCA Manual S (Residential Equipment Selection)
• ACCA Manual J (load calculation)
• ACCA Manual D (duct design)
• AHRI 210/240 (cooling rating conditions)
• AHRI 380 (heat pump rating conditions)
• Manufacturer expanded rating tables (Carrier, Trane, Lennox, Goodman, Mitsubishi, Daikin)
• DOE efficiency standards (SEER2, HSPF2, EER2)