5.3 Infiltration, Duct Losses & Total Block Load Synthesis

Infiltration, Duct Losses & Total Block Load Synthesis

System Synthesis Rule: Total building cooling load is the summation of Total Sensible Heat Gain ($q_{\text{sensible}}$) and Total Latent Heat Gain ($q_{\text{latent}}$). Equipment capacity must satisfy both the sensible and latent load requirements at design conditions under ACCA Manual S selection criteria.

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Infiltration & Envelope Air Leakage

Infiltration is the uncontrolled entry of outdoor air into the conditioned envelope through cracks, gaps around windows and doors, penetrations for plumbing/wiring, sill plates, and recessed lighting fixtures. Infiltration is driven by:

1. Stack Effect: Warm air rising and escaping through upper ceiling penetrations in winter, pulling cold air in through lower foundation cracks.
2. Wind Pressure: Direct positive wind pressure pushing air through the windward facade and negative suction drawing conditioned air out the leeward facade.
3. Mechanical Pressurization / Depressurization: Exhaust fans or leaky return ductwork located in unconditioned spaces pulling outside air into the conditioned space.

Air Leakage Testing & $ACH_{50}$ Conversion

Under the North Carolina Energy Conservation Code (NCECC Section R402.4.1.2), new residential buildings must demonstrate an air leakage rate not exceeding $3.0\text{ Air Changes per Hour at 50 Pascals}$ ($3.0\text{ ACH}_{50}$) via a calibrated blower door depressurization test:

$\text{ACH}_{50} = \frac{\text{CFM}_{50} \times 60}{\text{Conditioned Building Volume (cu ft)}}$

To determine the natural infiltration rate under normal atmospheric pressures ($\text{ACH}_{\text{nat}}$), Manual J utilizes the LBL (Lawrence Berkeley Laboratory) $N$-Factor (typically $N = 15 - 20$ for North Carolina single-story/two-story homes with moderate shielding):

$\text{ACH}_{\text{nat}} = \frac{\text{ACH}_{50}}{N} \quad \Longleftrightarrow \quad \text{CFM}_{\text{inf}} = \frac{\text{Conditioned Volume} \times \text{ACH}_{\text{nat}}}{60}$

Manual J Infiltration Load Formulas

• Sensible Infiltration Heat Load ($q_{\text{sensible, inf}}$): $q_{\text{sensible, inf}} = 1.08 \times \text{CFM}_{\text{inf}} \times \Delta T$ Derivation of the constant $1.08$: $1.08 = 0.075\text{ lb/cu ft (air density)} \times 0.24\text{ BTU/(lb}\cdot^\circ\text{F) (specific heat)} \times 60\text{ min/hr}$

• Latent Infiltration Heat Load ($q_{\text{latent, inf}}$): $q_{\text{latent, inf}} = 0.68 \times \text{CFM}_{\text{inf}} \times \Delta W$ Derivation of the constant $0.68$: $0.68 = 0.075\text{ lb/cu ft} \times \frac{1{,}061\text{ BTU/lb (latent heat of water)}}{7{,}000\text{ grains/lb}} \times 60\text{ min/hr}$ where $\Delta W = W_{\text{outdoor}} - W_{\text{indoor}}$ in $\text{grains of moisture per pound of dry air}$.

• Total Infiltration Heat Load ($q_{\text{total, inf}}$): $q_{\text{total, inf}} = 4.5 \times \text{CFM}_{\text{inf}} \times \Delta h$ where $\Delta h = h_{\text{outdoor}} - h_{\text{indoor}}$ is the enthalpy difference in $\text{BTU per pound of dry air}$, and $4.5 = 0.075\text{ lb/cu ft} \times 60\text{ min/hr}$.

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Mechanical Ventilation (ASHRAE 62.2 / NCRC)

Modern airtight building envelopes require continuous mechanical ventilation to maintain indoor air quality. Under ASHRAE Standard 62.2 and NCRC Section M1505, continuous whole-house mechanical outdoor air ventilation rate ($\text{CFM}_{\text{vent}}$) is calculated as:

$\text{CFM}_{\text{vent}} = (0.03 \times A_{\text{floor}}) + 7.5 \times (N_{\text{bedrooms}} + 1)$

Example for a $2{,}400\text{ sq ft}$, 3-bedroom home: $\text{CFM}_{\text{vent}} = (0.03 \times 2{,}400) + 7.5 \times (3 + 1) = 72 + 30 = 102\text{ CFM}$

Ventilation Impact on Equipment Sizing

• Direct Outdoor Air Intake: Introducing $102\text{ CFM}$ of raw outdoor air at Raleigh design conditions ($92^\circ\text{F}\text{ DB}$, $53\text{ grains}\ \Delta W$) adds:
  • Sensible load: $1.08 \times 102 \times 17^\circ\text{F} = 1{,}873\text{ BTU/h}$
  • Latent load: $0.68 \times 102 \times 53 = 3{,}676\text{ BTU/h}$
  • Total ventilation load penalty = $5{,}549\text{ BTU/h}$ (almost half a ton of cooling capacity!).
• Energy Recovery Ventilators (ERV) / Heat Recovery Ventilators (HRV): An ERV transfers both sensible heat and moisture between outgoing exhaust air and incoming fresh air with an effectiveness of $65% - 75%$, reducing the ventilation load penalty by over two-thirds.

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Duct Heat Gain, Heat Loss & Leakage Multipliers

The location of air distribution ductwork dramatically affects system efficiency and equipment sizing:

Duct Placement Thermal Comparison
├── Ducts in Conditioned Space (Dropped ceilings, open-web floor trusses)
│   ├── Conduction Loss/Gain: 0 BTU/h (Heat exchange stays inside living envelope)
│   ├── Leakage Penalty: 0% capacity adder to outdoors
│   └── Result: Smallest equipment size, maximum comfort
└── Ducts in Unconditioned Attic (130°F - 140°F Summer Temp)
    ├── Conduction Heat Gain: Hot attic air heats supply air 3°F - 6°F
    ├── Duct Leakage: Supply air lost to attic; unconditioned air pulled into return
    └── Result: 20% - 35% equipment capacity adder required

NC Energy Code Duct Mandates

• Insulation: Supply and return ducts in unconditioned space and outdoors must be insulated to a minimum of $R-8$ under the North Carolina amendment to NCECC Section R403.3.1 — North Carolina does not use the lower $R-6$ tier that appears in the unamended IECC.
• Duct Tightness Mandate: Under the North Carolina amendment to NCECC Section R403.3.3, the system complies if total duct leakage $\le 5\text{ CFM}_{25}$ per $100\text{ sq ft}$ of conditioned floor area served, or duct leakage to the outside $\le 4\text{ CFM}_{25}$ per $100\text{ sq ft}$, tested at $25\text{ Pascals}$ ($0.10\text{ in. w.g.}$).
• Manual J Duct Multipliers: Duct heat gains and losses are applied using Duct Gain Factors (DGF) and Duct Loss Factors (DLF) based on duct surface area, insulation $R$-value, supply air temperature, and ambient attic/crawlspace temperature.

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Total Block Load vs. Room-by-Room Load Synthesis

ACCA Manual J produces two distinct calculations for every building:

1. Total Building Block Load (Manual S Selection)

The Block Load is the instantaneous peak heating and cooling load for the entire building structure treated as a single unified zone.

• Solar Diversity: East windows peak at 9:00 AM, while West windows peak at 4:00 PM. The Block Load accounts for this diversity by calculating the building-wide peak (typically occurring between 3:00 PM and 5:00 PM in North Carolina).
• Sensible Heat Ratio (SHR): $\text{SHR} = \frac{q_{\text{sensible, total}}}{q_{\text{sensible, total}} + q_{\text{latent, total}}} = \frac{q_{\text{sensible, total}}}{q_{\text{total}}}$ Typical residential SHR values in North Carolina range from $0.72\text{ to } 0.82$. In humid coastal regions (Wilmington, Morehead City), the latent load is larger, driving SHR down toward $0.70$.

ACCA Manual S Equipment Sizing Rules

• Air Conditioners & Heat Pumps (Cooling Mode): The total cooling capacity of the selected equipment must be between $95%$ and $115%$ of the calculated total cooling load ($95% \le \text{Capacity} \le 115%$) for standard single-speed systems, or up to $125%$ for heat pumps or multi-stage / variable-capacity inverter systems to satisfy heating capacity balance points.
• Latent Capacity Matching: The equipment selected must have a sensible capacity $\ge q_{\text{sensible}}$ and a latent capacity $\ge q_{\text{latent}}$. Selecting a unit based purely on total tonnage without checking latent capacity in humid North Carolina climates will cause indoor humidity control failure.

2. Room-by-Room Peak Loads (Manual D Airflow Distribution)

While the primary heating and cooling unit is sized to the unified Block Load, the individual supply duct branches must be sized to handle the individual peak room loads (which occur at different times of day):

$\text{CFM}_{\text{room}} = \frac{q_{\text{sensible, room}}}{1.08 \times (T_{\text{room}} - T_{\text{supply}})}$

Assuming standard cooling supply air temperature of $55^\circ\text{F}$ and room setpoint of $75^\circ\text{F}$ ($\Delta T_{\text{supply}} = 20^\circ\text{F}$):

$\text{CFM}_{\text{room}} = \frac{q_{\text{sensible, room}}}{1.08 \times 20} = \frac{q_{\text{sensible, room}}}{21.6}$

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Step-by-Step Synthesis: Total Block Load & Equipment Sizing

Problem Data Summary (Raleigh, NC Residence)

• Opaque Envelope Sensible Gain: $12{,}400\text{ BTU/h}$
• Fenestration Conduction Gain: $2{,}800\text{ BTU/h}$
• Fenestration Solar Gain (Diversified Block Peak): $5{,}600\text{ BTU/h}$
• Internal Sensible Gains (Occupants + Appliances + Lighting): $3{,}200\text{ BTU/h}$
• Infiltration Sensible Gain ($40\text{ CFM}_{\text{inf}}$ @ $\Delta T = 17^\circ\text{F}$): $1.08 \times 40 \times 17 = 734\text{ BTU/h}$
• Duct Sensible Heat Gain (R-8 in Attic): $3{,}200\text{ BTU/h}$
• Internal Latent Gains (4 Occupants): $800\text{ BTU/h}$
• Infiltration Latent Gain ($40\text{ CFM}_{\text{inf}}$ @ $\Delta W = 53\text{ gr/lb}$): $0.68 \times 40 \times 53 = 1{,}442\text{ BTU/h}$
• Duct Latent Leakage Gain: $650\text{ BTU/h}$

Step 1: Calculate Total Sensible Cooling Load ($q_{\text{sensible}}$)

$q_{\text{sensible}} = 12{,}400 + 2{,}800 + 5{,}600 + 3{,}200 + 734 + 3{,}200 = 27{,}934\text{ BTU/h}$

Step 2: Calculate Total Latent Cooling Load ($q_{\text{latent}}$)

$q_{\text{latent}} = 800 + 1{,}442 + 650 = 2{,}892\text{ BTU/h}$

Step 3: Calculate Total Cooling Load ($q_{\text{total}}$) & Sensible Heat Ratio (SHR)

$q_{\text{total}} = 27{,}934 + 2{,}892 = 30{,}826\text{ BTU/h} \quad (2.57\text{ Tons})$ $\text{SHR} = \frac{27{,}934}{30{,}826} = 0.906$

Step 4: ACCA Manual S Equipment Sizing Range

• Lower Limit ($95%$ of load): $0.95 \times 30{,}826 = 29{,}285\text{ BTU/h}$ ($2.44\text{ Tons}$)
• Upper Limit ($115%$ of load): $1.15 \times 30{,}826 = 35{,}450\text{ BTU/h}$ ($2.95\text{ Tons}$)
• Selected Equipment: A nominal $2.5\text{-Ton}$ ($30{,}000\text{ BTU/h}$) or $3.0\text{-Ton}$ multi-stage unit satisfies Manual S sizing criteria perfectly.

Step 5: Calculate Total System Cooling Airflow Requirement

$\text{Total CFM} = \frac{q_{\text{sensible}}}{1.08 \times \Delta T_{\text{supply}}} = \frac{27{,}934}{1.08 \times 20} = \frac{27{,}934}{21.6} = 1{,}293\text{ CFM}$ (Equates to approximately $431\text{ CFM per ton}$ of cooling capacity, consistent with the standard 400 CFM/ton rule for humid Southern climates).