The Art and Science of Good Ventilation

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By recovering the energy to heat in the winter and cool in the summer, an ERV or HRV can provide clean fresh air continuously while helping to keep energy costs low. These units transfer the heating and cooling energy from the exhaust stream to the incoming fresh air. An ERV will also recover moisture from the exhaust stream when it is dry in the home.

An ERV/HRV is a good solution if the annual cost of operation is a consideration, although it may take a period of time to pay back its higher initial installation cost. The ERV/HRV selection process must consider if the air handler must run to distribute the air. A number of ERV/HRVs exist on the market that stand-alone and operate without the use of the air handler.

Because the ERV has an energy transfer (enthalpic) core, excessively low humidity is less likely to be a problem, thereby reducing the need for a separate humidifier and its considerable energy cost. HRVs and ERVs also provide balanced ventilation that avoids problems associated with the home's pressurization or depressurization.

The use of an HRV or ERV to provide low-level general, effective ventilation at an energy efficient rate must be supported with intermittent bath and kitchen ventilation for source control.

Continuous ventilation equipment must run whenever the house is occupied, especially during spring and fall. Windows may be opened, but unless there is a breeze to ensure ventilation, it is easiest and best to just let the unit run, even if windows are open. Incidental (or accidental) ventilation is driven by natural forces and they are least effective when the temperature difference between indoors and outdoors is the smallest. That is when mechanical ventilation is needed the most.

Moisture Can Be Critical

"We have come a long way in the past 10-15 years in our understanding and reduction of gaseous contaminants, and the impact of furniture, wood products and building materials that, historically, out-gassed VOCs," says David W. Wolbrink, former vice president of research and development for the nation's leading residential ventilation manufacturer.

"Excess moisture still exists in the home. In fact, the problem has grown more severe," Wolbrink says.

It is essential to understand moisture in designing and operating a home in a way that preserves good indoor air quality.

Swings in the moisture level-in either direction-can lead to serious problems.

Ideally, indoor relative humidity should be held within a range of 30-to-50 percent. Inhot humid climates that may be difficult, but relative humidity must absolutelynever be permitted to beabove 65%. Incold dry climates, relative humidity should beno lower than 20%.

Less than 30 percent relative humidity can cause wood to shrink, result in increased dust, cause respiratory, throat and skin irritations and create static electricity.

As the outside temperature drops, so does the recommended relative humidity inside the home, because during cold weather higher humidity will lead to excessive moisture and condensation, which can lead to mold and mildew.

Mold can grow on surfaces in any climate at humidity levels above 70 percent.

 

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Originally published in Architectural Record.
Originally published in December 2006

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