Clearing the Air

Referring to the example from the previous “Clearing the Air- Part 1” blog post, if we decide to install a single, continuously operating high-efficiency fan in the main bathroom to provide local exhaust and mechanical ventilation for the entire main level, the other two bathrooms will require local exhaust fans to meet the bathroom local exhaust requirements outlined above. However, these fans will only need to operate intermittently.

To meet kitchen local exhaust requirements, a local exhaust fan must be used above the range. The code however allows an exception, where the fan does not vent air outside but recirculating the air drawn through the fan, filtering it, capturing a portion of the grease and particulates produced in cooking processes. Though code compliant, recirculating fans fail to effectively remove contaminants and are unpopular among cooks. A vented to outdoor-range hood is more effective as it expels smoke, grease, and pollutants including carbon monoxide. Many cooktop manufacturers require range hoods, vented to outside, with specific CFM ratings over gas and larger electric ranges and cooktops. The code requires all equipment and appliances to be installed in accordance with the manufacturer’s installation instructions and listings. Further, IRC section M1503.6 specifies any home exhaust fan that exceeds 400 CFM requires make-up air.

Make-up Air

Knowing there is a make-up air requirement when fans exceed 400 CFM, what happens if the home is constructed much tighter than the typical home? The home will obviously leak air at a rate of less than 400 CFM. Research shows homes meeting minimum air sealing standards allow about 400 CFM of replacement air, but that same research also shows homes can be so effectively sealed that make-up is required for all air exhausted.  Whenever conditions require make-up air, IRC M1503.4 provides the requirements for make-up air.

If a home is air-sealed beyond minimum code standards, it will be too tight for Supply or Exhaust Fan options for whole-house ventilation. Common ‘above’ code practices include using closed cell spray foam in walls and roofs or building with ICFs or SIPs. In these super-tight homes, a balanced ventilation system like an HRV (Heat Recovery Ventilator) or ERV (Energy Recovery Ventilator) is necessary. These systems have two fans: one exhausts a specific cfm, while the other supplies make-up air, fresh outdoor air at the same rate, recovering heat or energy from the outgoing air to temper incoming air. For more, check out HRV and ERV on YouTube and stay tuned for future classes or blogs.

Combustion Air

Combustion air, the air required for fuel combustion, draft dilution, and equipment ventilation is essential for the safe burning of natural or propane gas. The full detailed set of code requirements for gas appliance installations, including piping, venting and combustion air are found in IRC Chapter 24 – Fuel Gas.

When installing fuel gas appliances indoors, it is vital to understand air movement into and out of the home where the appliances are being installed. Building professionals must prioritize combustion air and integrate it into building design, as they work on reducing building leakage. Insufficient combustion air can lead to carbon monoxide poisoning, which can be fatal.  Fuel gas appliances are manufactured in two basic designs with specific requirements for sizing and installing combustion air ducts:

• • • Direct vent gas appliances: These sealed, fan-forced units draw combustion air from outside and vent exhaust through PVC or stainless-steel pipes. They are typically the safest and most efficient appliances when properly installed.
    • Open combustion gas appliances: These need an external duct near the appliance open to the outside to provide necessary combustion air. Without the external duct, risks include inefficient combustion, carbon monoxide spillage, and fire hazards. The energy code now requires open combustion appliances with open combustion air ducts to be enclosed, separating them from the rest of the home (IRC N1102.4.4).

Other Considerations

Clothes Dryers: These appliances use a fan to blow warm air through clothes, removing moisture and discharging it, along with lint, outside. Dryers in closets need a transfer grill for adequate airflow replacement into the closet.

Steam rooms, saunas, hot tubs, craft rooms, or other uses requiring exhaust.  In any home requiring the removal of excess moisture, heat, fumes, odors or other contaminants, exhaust fans must be installed in compliance with the codes discussed. These codes have been developed to protect the safety, health, and general welfare of occupants.

Modern homes built to current codes are safer due to advances in technology and construction. It’s essential to follow codes and best practices for safety and efficiency when integrating these technologies into home design and construction.       

– Dr. Energy