The “Ask a Builder” series is dedicated to answering some of the many questions Fairbanks residents have about building, energy and the many other parts of home life.

FAIRBANKS — To begin 2015, we are running a series on Heat Recovery Ventilators (HRV) in this column. Indoor air quality is a perpetual concern in cold climate housing, and has become more so in recent years as homes are more airtight and better insulated than ever before. This means they use less fuel for space heating, but also that they require mechanical ventilation, such as an HRV, to keep the interior atmosphere healthy. Today’s column is the last in this series!

Energy Recovery Ventilators, or ERVs, provide balanced ventilation to buildings, exhausting stale air and bringing in fresh air. ERVs are connected to a duct system to move air through a home: exhaust air is usually pulled from the bathrooms and kitchen, and fresh air is delivered to the bedrooms and the living areas. Like heat recovery ventilators, or HRVs, they transfer heat from the outgoing exhaust air to the incoming fresh air in the winter, saving energy by reducing the amount of heat that leaves the home during ventilation. However, ERVs are also designed to transfer moisture from stale air to fresh air, which helps stabilize indoor humidity levels.

Most ERVs in the United States contain a central core that facilitates the heat and moisture exchange between incoming and outgoing air streams. The incoming and outgoing air streams pass through adjacent channels separated by membranes that absorb moisture from one air stream and transfer it to the other while preventing the air streams from mixing. Heat is also transferred across the membrane surfaces. Filters, which must be cleaned every few months, surround the core to prevent pollutants and dust from entering the core of the ERV and the home.

ERVs are used throughout the world to ventilate homes and improve comfort levels. In cold climates, they transfer heat and moisture from outgoing exhaust air, which is typically warm and humid, to incoming cold, dry fresh air. In warm humid climates, ERVs transfer heat and moisture from incoming air to the exhaust air stream to keep the house cool and dry. By recovering moisture from outgoing air in cold climates, ERVs help homes maintain desirable indoor humidity levels of 30 percent to 40 percent. These humidity levels promote occupant comfort by keeping skin and sinuses from becoming too dry, while preventing condensation that can promote the growth of mold, mildew, and rot in homes.

ERVs can save considerable energy by easing the load on the space heating system of the home. Because ERVs recover some of the heat and moisture that leaves the home when the pollutants are exhausted, less energy is lost than if the air was to leave the house through other means, such as from an exhaust fan in a bathroom.

There are a few extra considerations when operating an ERV in a cold climate. In the winter, frost protection is necessary to prevent the moisture in the core from freezing in low temperatures, as this causes problems such as blocked air flow and impaired heat transfer.

There are ERVs on the market that address this by pre-heating supply air, recirculating indoor air for a short time or diverting cold supply air around the core so as not to freeze it. Also, remember to check outdoor ducts regularly to ensure they are not blocked by snow or frost.

To learn more about ERVs and other ventilation strategies, please visit our website www.cchrc.org.

Ask a Builder articles promote home awareness for the Cold Climate Housing Research Center (CCHRC). If you have a question, contact us at info@cchrc.org or 457-3454.