6.3 Infiltration, Ventilation & Internal Heat Gains

6.3 Infiltration, Ventilation & Internal Heat Gains

[!IMPORTANT] The Latent Load Challenge in Alabama: In hot, humid climates like Alabama (Climate Zones 2A and 3A), ambient air carries extraordinary moisture levels. Outdoor air brought into the home via infiltration and code-mandated mechanical ventilation introduces massive latent cooling loads. Sizing air conditioning systems without calculating latent loads separately leads to inadequate moisture removal, indoor relative humidity exceeding 60%, and severe indoor air quality hazards.

Total residential cooling load consists of two thermodynamic components: Sensible Heat Gain (heat that raises the dry-bulb temperature of the air) and Latent Heat Gain (heat energy in the form of airborne water vapor that must be condensed into liquid water at the evaporator coil). Quantifying infiltration, whole-house mechanical ventilation, and internal heat gains is the final step in establishing total cooling and heating demands.

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Infiltration Dynamics: Blower Door Testing & Natural Airflow

Infiltration is the uncontrolled leakage of outdoor air into a conditioned space through cracks, building seams, electrical penetrations, plumbing chases, and unsealed ductwork. It is driven by three natural pressure forces:

1. Wind Pressure: Positive pressure on the windward exterior wall forces outdoor air into the envelope, while negative pressure on the leeward side pulls indoor air out.
2. Stack Effect: Temperature and air density differences cause warm indoor air to rise and escape through upper attic ceiling leaks in winter, drawing cold outdoor air in through lower foundation cracks.
3. Mechanical Duct Leakage: Leaky supply ducts located in unconditioned attics force conditioned air outdoors, creating a powerful negative pressure inside the living space that pulls unconditioned, humid outdoor air in through every available envelope crevice.

+---------------------------------------------------------------------------------------------------+
|                             BLOWER DOOR TO NATURAL INFILTRATION CFM                               |
|                                                                                                   |
|   Blower Door Metric (ACH_50) ──> LBL N-Factor (15 to 20) ──> Natural Air Changes (ACH_nat)       |
|                                                                                                   |
|                                          ACH_50                                                   |
|                             ACH_nat = ─────────────                                               |
|                                            N                                                      |
|                                                                                                   |
|                                         Volume × ACH_nat                                          |
|                             CFM_inf = ───────────────────                                         |
|                                                60                                                 |
+---------------------------------------------------------------------------------------------------+

The Blower Door Metric ($ACH_{50}$)

Modern building codes require testing building airtightness using a calibrated blower door assembly installed in an exterior door frame. The fan depressurizes the house to 50 Pascals (Pa) relative to outdoors. The resulting metric is Air Changes per Hour at 50 Pascals ($ACH_{50}$):

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

• Under the IECC, modern homes must achieve an airtightness of $ACH_{50} \le 3.0$ (or $\le 5.0$ in jurisdictions adopting specific state amendments).

Converting $ACH_{50}$ to Natural Airflow ($ACH_{nat}$ & $\text{CFM}_{inf}$)

Under normal atmospheric conditions, houses experience pressure differentials of only 1 to 4 Pascals, not 50 Pascals. To convert $ACH_{50}$ into natural air changes per hour ($ACH_{nat}$), designers apply the LBL Infiltration Factor ($N$) developed by Lawrence Berkeley National Laboratory:

$ACH_{nat} = \frac{ACH_{50}}{N}$

For Alabama (low wind speeds, 1- to 2-story construction, well-shielded suburban lots), $N$ typically ranges from 15 to 20 (commonly 17.5).

$\text{CFM}_{inf} = \frac{\text{Conditioned Volume (cu ft)} \times ACH_{nat}}{60}$

Example: A 2,000 sq ft home with 9-foot ceilings has a volume of $18,000\text{ cu ft}$. If blower door testing yields $ACH_{50} = 3.5$ and $N = 17.5$: $ACH_{nat} = \frac{3.5}{17.5} = 0.20\text{ ACH}$ $\text{CFM}_{inf} = \frac{18,000 \times 0.20}{60} = 60\text{ CFM}$

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Whole-House Mechanical Ventilation: ASHRAE 62.2 & IRC M1505

Because modern construction produces airtight building envelopes ($ACH_{50} \le 3.0$), natural infiltration is no longer sufficient to dilute indoor air contaminants such as volatile organic compounds (VOCs), formaldehyde, carbon dioxide ($CO_2$), and radon. Modern building codes mandate continuous whole-house mechanical ventilation.

The Continuous Ventilation Equation

Under IRC Section M1505 and ASHRAE Standard 62.2, the minimum continuous mechanical ventilation airflow rate is calculated as:

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

Where:

• $A_{floor}$ = Conditioned floor area in square feet.
• $N_{bedrooms}$ = Number of bedrooms in the dwelling.
• $(N_{bedrooms} + 1)$ = Assumed baseline occupant count.

+---------------------------------------------------------------------------------------------------+
|                    MECHANICAL VENTILATION STRATEGIES IN ALABAMA                                   |
+-----------------------+----------------------------------+----------------------------------------+
| VENTILATION STRATEGY  | SYSTEM OPERATION                 | ALABAMA CLIMATE SUITABILITY            |
+-----------------------+----------------------------------+----------------------------------------+
| Exhaust-Only          | Continuous bath/laundry fan pulls| POOR: Induces negative pressure,       |
|                       | indoor air outdoors              | pulling humid outdoor air into walls   |
+-----------------------+----------------------------------+----------------------------------------+
| Supply-Only           | Dedicated duct introduces outdoor| ACCEPTABLE: Creates positive pressure, |
|                       | air into central return plenum   | but adds massive latent load to coil   |
+-----------------------+----------------------------------+----------------------------------------+
| Balanced / ERV        | Energy Recovery Ventilator pairs | EXCELLENT: Exchanger pre-cools and     |
| (Recommended)         | supply and exhaust airstreams    | pre-dehumidifies incoming fresh air    |
+-----------------------+----------------------------------+----------------------------------------+

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Psychrometric Formulas for Air Loads (Sensible & Latent)

Air introduced into a structure—whether through uncontrolled infiltration or mechanical ventilation—must be conditioned to indoor design conditions (75°F DB and 50% RH). Psychrometric equations govern this heat transfer:

1. Sensible Heat Gain / Loss Equation

Sensible heat transfer changes air temperature without changing moisture content:

$Q_{sensible} = 1.08 \times \text{CFM} \times \Delta T$

• Derivation: Standard air has a density of $\rho = 0.075\text{ lb/cu ft}$ and a specific heat capacity of $c_p = 0.24\text{ BTU}/(\text{lb}\cdot^\circ\text{F})$. Factoring 60 minutes per hour: $60 \times 0.075 \times 0.24 = 1.08$.
• $\Delta T$ is $(T_{outdoor} - T_{indoor})$ for cooling, or $(T_{indoor} - T_{outdoor})$ for heating.

2. Latent Heat Gain Equation

Latent heat transfer condenses or vaporizes airborne water vapor without altering dry-bulb temperature:

$Q_{latent} = 0.68 \times \text{CFM} \times \Delta W$

• Derivation: Standard air density ($0.075\text{ lb/cu ft}$) multiplied by the latent heat of vaporization of water ($h_{fg} \approx 1,061\text{ BTU/lb}$) divided by 7,000 grains per pound: $60 \times 0.075 \times (1061 / 7000) = 0.682 \approx 0.68$.
• $\Delta W$ is the humidity ratio difference in grains of moisture per pound of dry air ($W_{outdoor} - W_{indoor}$).

3. Total Heat Gain Equation

Total heat is the sum of sensible and latent heat, or enthalpy difference ($\Delta h$ in $\text{BTU/lb}$):

$Q_{total} = Q_{sensible} + Q_{latent} = 4.5 \times \text{CFM} \times \Delta h$

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Internal Heat Gains: Occupants, Appliances & Lighting

Heat generated inside the building envelope contributes directly to the cooling load. In modern airtight, heavily insulated homes, internal gains frequently represent 20% to 30% of the total sensible cooling demand.

Occupant Heat Dissipation Rules

Manual J standardizes human heat output based on a seated or light residential activity level:

+---------------------------------------------------------------------------------------------------+
|                             MANUAL J RESIDENTIAL OCCUPANCY LOADS                                  |
+--------------------+-------------------------+------------------------+---------------------------+
| LOAD COMPONENT     | HEAT RATE PER OCCUPANT  | RESIDENTIAL COUNT RULE | 3-BEDROOM HOME (4 PEOPLE) |
+--------------------+-------------------------+------------------------+---------------------------+
| Sensible Heat Gain | 230 BTU/hr per person   | Master Bedroom = 2     | 4 × 230 = 920 BTU/hr      |
| Latent Heat Gain   | 200 BTU/hr per person   | Each other bedroom = 1 | 4 × 200 = 800 BTU/hr      |
| TOTAL HEAT GAIN    | 430 BTU/hr per person   | N_people = N_br + 1    | 4 × 430 = 1,720 BTU/hr    |
+--------------------+-------------------------+------------------------+---------------------------+

[!NOTE] Occupancy Counting Rule: Manual J specifies that residential occupancy is calculated as Number of Bedrooms + 1. The master bedroom is always assigned two occupants, and each additional bedroom is assigned one occupant. A 4-bedroom home is sized for 5 people ($5 \times 230 = 1,150\text{ BTU/hr sensible}$ and $5 \times 200 = 1,000\text{ BTU/hr latent}$).

Appliance and Lighting Heat Loads

• Kitchen Default Allowance: ACCA Manual J assigns a flat default allowance of 1,200 BTU/hr sensible (and 0 BTU/hr latent) for standard residential kitchens, based on the assumption that range hoods and dishwashers are power-vented directly to the outdoors.
• Miscellaneous Appliances: Refrigerators (300 to 500 BTU/hr sensible), home entertainment systems, flat-panel displays, and computers (400 to 800 BTU/hr sensible).
• Lighting Loads: Electrical lighting converts 100% of consumed electrical wattage into thermal heat: $Q_{lighting} = \text{Total Watts} \times 3.412\text{ BTU/hr per Watt}$. Transitioning from legacy incandescent lighting to solid-state LED fixtures reduces residential lighting heat gains by over 80%.

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Sensible Heat Ratio (SHR) & ACCA Manual S Equipment Sizing

Total cooling load is the aggregate of all sensible and latent gains:

$Q_{total} = Q_{sensible} + Q_{latent}$

Sensible Heat Ratio (SHR)

The Sensible Heat Ratio defines the fraction of total cooling load that is sensible:

$\text{SHR} = \frac{Q_{sensible}}{Q_{total}} = \frac{Q_{sensible}}{Q_{sensible} + Q_{latent}}$

• In dry, arid climates (such as Phoenix, AZ), SHR is typically 0.90 to 0.95 (almost purely sensible cooling).
• In humid Alabama climates, design SHR typically ranges between 0.70 and 0.80. Between 20% and 30% of the cooling system's total thermodynamic work must be dedicated entirely to condensing airborne moisture.

+---------------------------------------------------------------------------------------------------+
|                         ACCA MANUAL S EQUIPMENT SELECTION CRITERIA                                |
+-----------------------------+---------------------------------------------------------------------+
| COOLING CAPACITY DIMENSION  | ACCA MANUAL S ALLOWABLE CAPACITY LIMITS                             |
+-----------------------------+---------------------------------------------------------------------+
| Total Cooling Capacity      | 95% to 115% of calculated Manual J Total Cooling Load                |
|                             | (Up to 125% for variable-speed / inverter multi-stage systems)       |
+-----------------------------+---------------------------------------------------------------------+
| Sensible Cooling Capacity   | 95% to 115% of calculated Manual J Sensible Cooling Load             |
+-----------------------------+---------------------------------------------------------------------+
| Latent Cooling Capacity     | MUST EQUAL OR EXCEED calculated Manual J Latent Cooling Load        |
|                             | (Capacity_latent ≥ Q_latent)                                        |
+-----------------------------+---------------------------------------------------------------------+
| Gas Heating Capacity        | 100% to 140% of calculated Manual J Heating Load (or next size)     |
+-----------------------------+---------------------------------------------------------------------+

Manual S Sizing Tolerances

Once Manual J establishes building loads, contractors must select equipment using ACCA Manual S (Residential Equipment Selection). Sizing must be evaluated using the manufacturer's expanded performance data tables at the specific indoor and outdoor design conditions (not just nominal AHRI ratings at 95°F outdoor / 80°F indoor):

1. Total Capacity Tolerance: Standard single-stage and two-stage air conditioners or heat pumps must have a total cooling capacity between 95% and 115% of the Manual J total load. Variable-capacity (inverter-driven) heat pumps may be sized up to 125% of total load because the compressor modulates down to match low loads without short-cycling.
2. Sensible Capacity Tolerance: Sensible capacity must be between 95% and 115% of the calculated sensible load.
3. Latent Capacity Mandate: The equipment's latent capacity at design conditions must be greater than or equal to the calculated latent load ($Capacity_{latent} \ge Q_{latent}$). If equipment latent capacity falls short, an auxiliary whole-house dehumidifier must be integrated into the mechanical system.

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Comprehensive Worked Calculation Example

Scenario: Size mechanical ventilation, evaluate air loads, and verify Manual S selection for a newly built 3-bedroom, 2,000 sq ft home in Mobile, Alabama:

• Outdoor Design (Zone 2A): $T_o = 93^\circ\text{F}$, $W_o = 118\text{ gr/lb}$.
• Indoor Design: $T_i = 75^\circ\text{F}$, $W_i = 65\text{ gr/lb}$ (50% RH).
• Manual J Envelope & Internal Loads: $Q_{s, env+int} = 22,500\text{ BTU/hr}$, $Q_{l, int} = 800\text{ BTU/hr}$ (4 people).

Step 1: Calculate Mechanical Ventilation Airflow (ASHRAE 62.2 / IRC M1505): $\text{CFM}_{vent} = (0.03 \times 2,000) + 7.5 \times (3 + 1) = 60 + 30 = 90\text{ CFM}$

Step 2: Calculate Ventilation Air Cooling Loads: $\Delta T = 93 - 75 = 18^\circ\text{F}, \quad \Delta W = 118 - 65 = 53\text{ grains/lb}$ $Q_{s, vent} = 1.08 \times 90\text{ CFM} \times 18^\circ\text{F} = 1,749.6\text{ BTU/hr}$ $Q_{l, vent} = 0.68 \times 90\text{ CFM} \times 53\text{ gr/lb} = 3,243.6\text{ BTU/hr}$

Notice that in Mobile, the ventilation latent load (3,244 BTU/hr) is nearly double the sensible load (1,750 BTU/hr)!

Step 3: Aggregate Total Building Cooling Loads: $Q_{sensible, total} = 22,500 + 1,749.6 = 24,249.6\text{ BTU/hr}$ $Q_{latent, total} = 800 + 3,243.6 = 4,043.6\text{ BTU/hr}$ $Q_{total} = 24,249.6 + 4,043.6 = 28,293.2\text{ BTU/hr} \quad (\approx 2.36\text{ tons})$ $\text{Sensible Heat Ratio (SHR)} = \frac{24,249.6}{28,293.2} = 0.857$

Step 4: Establish ACCA Manual S Equipment Sizing Range:

• Allowable Total Cooling Capacity: $0.95 \times 28,293 = 26,878\text{ BTU/hr}$ to $1.15 \times 28,293 = 32,537\text{ BTU/hr}$.
• Allowable Sensible Cooling Capacity: $0.95 \times 24,250 = 23,038\text{ BTU/hr}$ to $1.15 \times 24,250 = 27,888\text{ BTU/hr}$.
• Minimum Latent Capacity: $\ge 4,044\text{ BTU/hr}$.

A nominal 2.5-ton split system (30,000 BTU/hr nominal) delivering 28,800 BTU/hr total, 24,500 BTU/hr sensible, and 4,300 BTU/hr latent at Mobile design conditions satisfies all Manual S criteria.