Mechanical Ventilation Wins Fan Support

July 3, 2000
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MINNEAPOLIS, MN — It’s a topic that some homebuilders would probably rather not address. And it’s going to take some getting used to. But mechanical ventilation for homes is an idea whose time has come, especially since today’s tightly built homes are even more likely to have ventilation problems than their drafty counterparts.

Residential IAQ was the subject of a symposium at ASHRAE’s summer meeting in Minneapolis. David Grimsrud of the University of Minnesota chaired the symposium. He told The News: “In Minnesota, the need for ventilation became clearer in new homes with tight construction where there were moisture problems. This came from clothes washing, cooking, and other normal activities.”

In a study of lower-middle-class and upper-middle-class new homes, problems abounded, he said. About half of some 200 people responding to a survey reported moisture problems in their houses. These “weren’t just trivial condensation problems on windows,” Grimsrud said, noting that typically the homes involved were only two to four years old.

Altered States

Minnesota has been at the forefront (along with the state of Washington) in requiring increased ventilation in new residential construction. Ontario, Canada, and the Scandinavian countries have also paid this subject more attention, he said, because their harsher climates require it. A new energy code that took effect in Minnesota on April 15 requires some form of mechanical ventilation in every new home.

“Indoor Air Quality and Your Home,” a publication of the New York State Energy Research and Development Authority, asks, “Should houses be tightly built?” The answer is yes, with the following qualifier: “Many people are reducing home heating and cooling costs by weatherizing or tightening up their homes. When combined with source control and mechanical ventilation, tight homes provide the greatest opportunity for comfort, economy, and good health. However, if homes are not equipped for controlling and removing indoor air pollutants, indoor air quality problems may result.”

The California Energy Effi-ciency Standards for Residential Buildings (Title 24, Part 6), which provides a credit for tight construction, went into effect in July of 1999. If credit is taken for construction tighter than 3 specific leakage area (SLA), it states that mechanical ventilation is required. If the air tightness of the home is measured and is found to be below 1.5 SLA, then mechanical supply ventilation is required.

If the builder intends to comply with Title 24 through the performance standards approach, then the total power consumption of any mechanical ventilation must be included as part of the total energy analysis for the home. Builders who wish to understand how to obtain Title 24 credit for tight construction and the requirements for mechanical ventilation credit should refer to the Residential Manual published by the Califor-nia Energy Commission.

ASHRAE Standard 62.2, while still not final, will most likely require some form of mechanical ventilation in new homes. Proponents of mechanical ventilation systems hope to improve indoor air quality, while also helping to limit other related problems such as high humidity — which in turn can lead to mold growth.

Supply-fan Approach

Meeting the new ASHRAE standard may be as simple as mandating an additional fan, such as a bathroom or kitchen fan.

The preferred method for continuous mechanical ventilation is for the fan to be installed to push air into the house — called the “supply-fan approach.” This method is preferred because it directly controls the source of the ventilation air, and because it will not depressurize the house, which could cause back-drafting of fireplaces and/or vented combustion appliances.

In areas of high moisture generation (such as kitchens and showers) exhaust fans should be used in addition to the continuous supply-fan ventilation to remove the moisture generated in these areas from the house. In cold climates (such as Climate Zone 16 in California), care should be taken with the supply-fan approach to ensure that moisture does not accumulate in the envelope. If large, unvented sources of moisture are present (such as large fish tanks, waterfalls, saunas, spas) the exhaust-fan approach is preferred.

If the supply-fan approach is used, the fan should not be placed in a bathroom (due to possible distribution of polluted air). The outlet should be placed in a more central location, preferably away from bedrooms (due to possible noise concerns). The fan should be easily accessible for maintenance. To avoid contamination of fresh air, the outside air intake should be ducted directly from the outside, and located away from flues, chimneys, vents, or other pollutant sources. The intake should also have a rodent/insect screen.

If the occupants are likely to be sensitive to allergens or other pollutants in outside air, it may be advantageous to provide filtration for the supply ventilation air. Filters should be accessible for maintenance and be located upstream of the fan.

Fan Sizing

But how do you know what size fan to use? Consider the following:

Central mechanical ventilation shall be sized to provide 0.047 cubic feet per minute/square foot of house conditioned area (size the fan upwards).

For typical size fans, there are two ways to determine cubic feet per minute (cfm). The first method is to use the fan rating from manufacturer’s specifications with duct sized according to manufacturer’s specifications. Fan airflow (in cfm) shall be calculated to account for pressure loss due to ducting and fittings. System capacity shall be based on actual installation rather than a simple rating based on airflow at atmospheric or other simplified conditions. Refer to manufacturer’s data for maximum duct lengths.

The alternative is to use the fan rating value at 0.25-in. wg.

Size the minimum main trunk duct diameter for maximum air velocity of 700 fpm. Branch ducts should be sized based on the cfm serving each branch.

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