6.3 Indoor Air Quality (IAQ): Ventilation, Filtration, Humidity Control, and ASHRAE 62.2

ASHRAE Standard 62.2 Residential Mechanical Ventilation Requirements

Modern residential construction techniques create highly airtight building envelopes to meet energy conservation codes. While tight building construction reduces heating and cooling energy waste, it traps indoor air pollutants—including volatile organic compounds (VOCs) off-gassing from building materials, carbon dioxide, formaldehyde, radon, pet dander, and excess moisture.

To maintain healthy indoor air quality (IAQ), ASHRAE Standard 62.2 ("Ventilation and Acceptable Indoor Air Quality in Residential Buildings") mandates continuous mechanical outdoor air ventilation in single-family homes and multi-family structures.

ASHRAE 62.2 Continuous Ventilation Airflow Formula

The total required continuous mechanical ventilation airflow rate ($Q_{\text{fan}}$) in cubic feet per minute (CFM) is calculated using the standard ASHRAE 62.2-2016/2019 equation:

$Q_{\text{fan}} = (0.03 \times A_{\text{floor}}) + [7.5 \times (N_{\text{br}} + 1)]$

Where:

• $Q_{\text{fan}}$ = Continuous mechanical ventilation airflow rate in CFM.
• $A_{\text{floor}}$ = Total conditioned floor area of the dwelling in square feet (sq ft).
• $N_{\text{br}}$ = Number of bedrooms (must not be less than 1).
• $(N_{\text{br}} + 1)$ = Assumed number of occupants in the dwelling.

Sample Calculation:

Calculate the required continuous mechanical ventilation airflow for a 4-bedroom home with 3,000 sq ft of conditioned floor area:

$Q_{\text{fan}} = (0.03 \times 3000) + [7.5 \times (4 + 1)] = 90 + (7.5 \times 5) = 90 + 37.5 = 127.5\text{ CFM (round to 128 CFM)}$

Intermittent Ventilation Adjustment

If the mechanical ventilation system operates intermittently (controlled by an automated timer cycling fan operation on and off each hour) rather than 24/7 continuously, the fan airflow rate during the active run cycle must be increased proportionately using the ASHRAE 62.2 fractional schedule multiplier so that the total volume of fresh air delivered per day remains equivalent to continuous ventilation.

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Mechanical Ventilation Systems: ERV vs HRV Mechanics

Mechanical outdoor ventilation air can be introduced using three core system configurations: Exhaust-Only (continuous bath exhaust fan), Supply-Only (fresh air intake tied to return plenum with motorized damper), or Balanced Ventilation using energy recovery equipment.

Heat Recovery Ventilator (HRV) Mechanics

A Heat Recovery Ventilator (HRV) contains a fixed air-to-air heat exchanger core (constructed of aluminum or synthetic plates). Outgoing warm indoor exhaust air passes through alternating channels beside incoming cold outdoor air without mixing streams.

• Sensible Energy Exchange: HRVs transfer sensible heat only between air streams.
• Climate Application: HRVs excel in cold, dry northern winter climates where the goal is to recover sensible heat from exhaust air without transferring interior moisture outside. HRVs are unsuitable for hot, humid Texas summers because they do not extract latent humidity from incoming outdoor fresh air.

Energy Recovery Ventilator (ERV) Mechanics

An Energy Recovery Ventilator (ERV) utilizes a permeable desiccant-treated core or rotating enthalpy wheel that allows air streams to transfer both sensible heat and latent moisture.

• Total Enthalpy Exchange: ERVs transfer both sensible heat AND latent moisture.
• Summer Operation in Texas: Hot, humid outdoor air entering the ERV passes across the desiccant core. Heat and moisture pass from incoming outdoor air into the cool, dry outgoing exhaust air stream. This pre-cools and pre-dehumidifies incoming outdoor ventilation air before it enters the living space.
• Climate Recommendation: ERVs are strongly recommended across Texas Climate Zones 2 and 3 to prevent fresh air ventilation requirements from overloading central cooling equipment with excess humidity.

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Air Filtration Standards and MERV Rating Classifications

Air filtration protects HVAC equipment coils and improves indoor air quality by removing airborne particulates. Filters are rated under ASHRAE Standard 52.2 using the Minimum Efficiency Reporting Value (MERV) scale, ranging from MERV 1 to MERV 16.

Comprehensive MERV Rating Table

Critical Pressure Drop Hazard: Replacing a standard MERV 4 fiberglass filter with a high-resistance 1-inch MERV 13 filter inside a narrow filter grille can add $0.35\text{ in. w.g.}$ of static pressure drop, restricting blower airflow. To install MERV 13+ filtration without penalizing fan performance, contractors must install 4-inch or 5-inch deep media filter cabinets to expand filter surface area and keep face velocity low.

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Indoor Humidity Control and Dehumidification Strategies

Indoor relative humidity (RH) directly influences human thermal comfort, health, and building durability. ASHRAE Standard 55 recommends maintaining indoor relative humidity between 30% and 60%.

Hazards of High Humidity ($> 60%$ RH)

• Accelerates dust mite reproduction and fungal/mold spore growth on drywall and duct surfaces.
• Causes occupant discomfort by inhibiting evaporative cooling from human skin, prompting occupants to lower thermostat setpoints needlessly.

Sensible Heat Ratio (SHR) and Latent Load Challenges

The Sensible Heat Ratio (SHR) represents the proportion of sensible cooling capacity relative to total cooling capacity:

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

In modern insulated Texas homes, high-performance windows and high-R insulation reduce sensible heat gain while latent moisture gains (from occupants, showers, cooking, and ventilation) remain high. This lowers the required building SHR down to 0.65-0.70. Standard air conditioners operating at 400 CFM/ton have an SHR of ~0.75-0.80, which can satisfy sensible thermostat setpoints before extracting sufficient latent moisture.

Field Dehumidification Methods

1. Variable-Speed Low Airflow Fan Mode: Modern thermostats command variable-speed blowers to drop airflow to 325 - 350 CFM per ton during high-humidity calls. Lower airflow drops evaporator coil saturation temperature, increasing moisture condensation rate off the air stream.
2. Whole-House Dedicated Dehumidifiers: Installed parallel to the central HVAC return system, dedicated dehumidifiers extract 70-130 pints of water per day independently of cooling calls. They run during humid spring and fall shoulder seasons when cooling load is low but ambient humidity is high.
3. Hot Gas Reheat Coils: Used in commercial HVAC packages. Air is sub-cooled across an evaporator coil to remove moisture, then passed across a downstream hot gas reheat coil (fed by hot compressor discharge vapor) to raise air temperature back to neutral room delivery temperature.

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Carbon Monoxide (CO) Hazards and Pollutant Source Control

Carbon monoxide (CO) is an odorless, colorless, toxic gas produced by incomplete combustion of fossil fuels (natural gas, LP, fuel oil). When inhaled, CO binds to blood hemoglobin with an affinity approximately 200 times greater than oxygen, forming carboxyhemoglobin and preventing cellular oxygen transfer.

CO Exposure Thresholds and Standards

• EPA Outdoor Ambient Limit: 9 ppm over an 8-hour period.
• OSHA 8-Hour Permissible Exposure Limit (PEL): 50 ppm.
• Indoor Action Threshold: If ambient air testing inside a home reveals CO concentrations exceeding 35 ppm, the technician must immediately turn off all fuel-burning appliances, evacuate occupants, ventilate the space, and locate the combustion leak source.

IAQ Source Control and Combustion Safety Rules

1. Combustion Air Provisions (IFGC Chapter 3): Ensure fuel-burning furnaces and water heaters receive adequate combustion air (minimum 50 cu ft of room volume per 1,000 BTU/hr aggregate input rating for unconfined spaces).
2. Heat Exchanger Integrity Inspections: Inspect gas furnace heat exchangers annually for stress cracks or rust perforations that permit toxic flue gases to bleed directly into supply air distribution ducts.
3. Garages and Attached Structures: Maintain positive pressure in conditioned spaces relative to attached garages to prevent motor vehicle exhaust fumes from infiltrating living areas.