3.2 ACCA Manual S Equipment Selection and Manual D Duct Design Principles

Once peak heating and cooling loads have been determined using ACCA Manual J, the HVAC contractor must select appropriate equipment using ACCA Manual S and design the air distribution system using ACCA Manual D. Oversizing equipment or improperly sizing ductwork creates excessive noise, uneven room temperatures, inadequate humidity removal, and high operating costs.

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1. ACCA Manual S Equipment Sizing Limits

ACCA Manual S establishes maximum allowable sizing limits for residential cooling and heating equipment. Sizing equipment based solely on AHRI nominal ratings (e.g., calling a unit a "3-ton system") is illegal under Texas mechanical codes because nominal capacity drops significantly under actual Texas design temperatures.

Cooling Capacity Sizing Limits (Single-Speed Systems)

• Minimum Allowable Capacity: Total equipment cooling capacity must not be less than 95% of the Manual J total cooling load.
• Maximum Allowable Capacity: Total equipment cooling capacity must not exceed 115% of the Manual J total cooling load.
• Heat Pump & Variable-Speed Exception: Heat pumps and variable-capacity (inverter-driven) air conditioners may be sized up to 125% to 140% of the Manual J total cooling load to satisfy winter heating requirements or operate efficiently across wide modulation ranges.

Consequences of System Oversizing

• Short-Cycling: Oversized equipment satisfies indoor dry-bulb temperature settings rapidly ($5\text{--}10\text{ minutes}$ per cycle). The system shuts off before the evaporator coil reaches saturation temperature long enough to condense moisture, leaving indoor relative humidity elevated above 60%.
• Compressor Wear & Energy Spikes: Inrush starting current occurs frequently, increasing electrical bills and accelerating mechanical wear on contactors, capacitors, and compressor windings.

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2. Latent vs. Sensible Equipment Capacity Matching

Equipment selection requires evaluating expanded performance tables at actual design conditions:

$\text{Sensible Heat Ratio } (SHR) = \frac{\text{Sensible Cooling Load } (Q_s)}{\text{Total Cooling Load } (Q_t)}$

1. Calculate Building SHR: Divide Manual J sensible load by Manual J total load. In humid regions, building SHR may range between $0.70$ and $0.80$.
2. Obtain Expanded Performance Data: Select equipment performance tables provided by the manufacturer at:
   • Local Outdoor Design Dry-Bulb Temperature (e.g., $100^\circ\text{F}$)
   • Indoor Entering Air Wet-Bulb Temperature (EWB, typically $62^\circ\text{F}\text{--}63^\circ\text{F}$ corresponding to $75^\circ\text{F}$ DB / 50% RH)
   • Design Airflow CFM (typically $350\text{--}400 \text{ CFM/ton}$)
3. Verify Sensible and Latent Capacities:
   • Equipment Sensible Capacity ($S\text{-CAP}$) must meet or exceed Manual J Sensible Load ($S\text{-LOAD}$).
   • Equipment Latent Capacity ($L\text{-CAP}$) must meet or exceed Manual J Latent Load ($L\text{-LOAD}$).

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3. ACCA Manual D Duct Sizing Procedure

ACCA Manual D outlines the equal friction method for sizing supply and return duct systems. Sizing ductwork requires calculating the system's Friction Rate (FR).

Step 1: Total External Static Pressure (TESP)

Determine the total external static pressure ($TESP$) delivered by the selected blower at design CFM from the manufacturer's fan performance table (typically $0.50 \text{ in. w.c.}$ for standard residential air handlers).

Step 2: Component Static Pressure Drops

Sum all internal and external device pressure losses within the air path:

• Evaporator Coil ($DP_{\text{coil}}$): e.g., $0.20 \text{ in. w.c.}$
• High-Efficiency Air Filter ($DP_{\text{filter}}$): e.g., $0.12 \text{ in. w.c.}$
• Supply Registers & Return Grilles ($DP_{\text{grilles}}$): e.g., $0.06 \text{ in. w.c.}$
• Balancing Dampers / Electric Heater ($DP_{\text{aux}}$): e.g., $0.04 \text{ in. w.c.}$

$\text{Total Component Losses} = 0.20 + 0.12 + 0.06 + 0.04 = 0.42 \text{ in. w.c.}$

Step 3: Available Static Pressure (ASP)

Subtract total component pressure losses from blower $TESP$:

$ASP = TESP - \text{Total Component Losses}$

$ASP = 0.50 - 0.42 = 0.08 \text{ in. w.c.}$

Step 4: Total Effective Length (TEL)

Measure the physical length of the longest (most restrictive) duct run from the air handler outlet to the furthest supply register, plus the return path. Add equivalent length values for all fittings, turns, elbows, tees, and boots along that critical path:

$TEL = \text{Measured Straight Feet} + \text{Equivalent Feet of Fittings}$

Example: $80\text{ ft straight duct} + 120\text{ ft equivalent fitting losses} = 200 \text{ ft TEL}$.

Step 5: Calculate Friction Rate (FR)

Calculate the friction rate per 100 feet of ductwork:

$FR = \frac{ASP \times 100}{TEL}$

Using $ASP = 0.08 \text{ in. w.c.}$ and $TEL = 200 \text{ ft}$:

$FR = \frac{0.08 \times 100}{200} = \frac{8.0}{200} = 0.040 \text{ in. w.c. per 100 ft}$

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4. Duct Velocity Limits and Air Balancing

Manual D specifies maximum recommended air velocities across residential supply and return ductwork to maintain acoustic comfort and prevent static pressure drops:

Duct Sizing Reference Table (typical residential practice, roughly $FR \approx 0.08\text{--}0.10 \text{ in. w.c. / 100 ft}$)

Read this as a field rule of thumb, not as a Manual D answer. These are the diameters commonly run in residential work. At a true design friction rate of 0.05 in. w.c./100 ft the ducts come out noticeably larger — 100 CFM needs about a 7-inch metal round and 200 CFM about a 9-inch. On the exam, size from the actual friction chart or the ADI Duct Size Calculator at the friction rate the question gives you.