Mechanical Ventilation


As defined by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), ventilation is “the process of supplying outdoor air to or removing indoor air from a dwelling by natural or mechanical means”.

Pollutants, Source Control, and the Need to Ventilate

Numerous pollutants exist within buildings and homes that can negatively impact indoor air quality and occupant health. Some of the most common pollutants are the building and finishing materials used in construction. Many builders are doing a good job of limiting the use of products that contain volatile organic compounds (VOCs). VOC’s are carbon-based compounds that have been used in the construction or chemical composition of many building material products like compressed wood cabinets, paints, glues, adhesives, and carpets. These VOC’s can evaporate and “off gas” as part of the natural part of aging and curing in new homes.

The Benefits Of Mechanical Ventilation Are:

  • Better Indoor Air Quality. Indoor air can be many times more polluted that outdoor air.
  • More Control. Mechanical ventilation systems provide proper fresh air flow along with appropriate locations for intake and exhaust.
  • Improved Comfort. Mechanical ventilation systems allow a constant flow of outside air into the home and can also provide filtration, dehumidification, and conditioning of the incoming outside air.

How Ventilation Systems Work

A variety of mechanical ventilation systems are available to select from, based on local climate and the home’s heating and cooling system. In addition to one of the primary systems described below, “spot” ventilation fans should also be provided for kitchens and baths to remove the concentrated moisture and odors that can occur in these rooms. The typical systems and recommended climates are described below:

  • Supply Ventilation Systems—Hot or Mixed Climates. Fresh air is drawn in through an air “intake” vent and distributed to many rooms by a fan and duct system. The benefit of connecting to the return air duct is that outdoor air can be air conditioned or dehumidified before it is introduced into the home. Because supply systems continually introduce outdoor air, a home can become slightly pressurized. As a result, these systems are typically not appropriate for cold climates.
  • Exhaust Ventilation Systems—Cold Climates. Indoor air is continuously exhausted to the outdoors with one or more fans often located in bathrooms. Because indoor air is continually drawn out, the home become slightly depressurized.
  • Balanced Ventilation Systems—All Climates. With these systems, equal quantities of air are brought into and sent out of the home. This is usually achieved using two fans—one to bring fresh air in and another to send indoor air out.

Mechanical Ventilation Types

There are three types of whole house ventilation strategies: exhaust only, supply only, and balanced.

  • Exhaust only – Although generally the most affordable form of ventilation this strategy is not recommended for many climates due to the resulting negative air pressure. Negative air pressure will cause the home to suck in air from other sources in an effort to balance the pressure (the 2nd law of Thermodynamics can be credited for this process). Generally this air will come from unwanted sources such as attics, crawl spaces, and garages where dirty, humid air exists.
  • Supply only – Opposite of exhaust systems, supply only ventilation pressurizes the home or building by pumping fresh air into the space and removing it through dedicated exhausts and natural leakage. Supply ventilation allows for the outdoor air to be filtered before entering the home and is a better design for hot or mixed climates.
  • Balanced – A balanced ventilation system neither pressurizes or depressurizes the home but instead brings in and exhausts the same amount (slightly positive) of air through dedicated duct systems. Balanced systems are commonly referred to as air exchange systems and can be equipped with heat recovery (HRV) or energy recovery (ERV) components.

Essentials of Mechanical Ventilation for Homeowners/Residents

With the increasing tightness of homes, mechanical ventilation is a necessary feature for building design to ensure acceptable indoor air quality and maintain occupant comfort. However, mechanical ventilation is affected by many factors that determine its efficiency and effectiveness, including building tightness, duct systems, and outside air conditions. Understanding these factors is beneficial for homeowners and residential occupants to maintain a healthy and energy efficient space.

All of the above factors impact effective ventilation.