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HOME Ventilation Guide Heat and energy recovery ventilators For continuous indoor air quality ventilation, a heat or energy recovery ventilator (HRV or ERV) should provide 0.35 air changes per hour. This calculation must consider the complete occupied volume of the house. This rate can be more easily calculated by allowing 5 CFM per 100 square feet of floor area. Total area of home (sq. ft.) Continuous ventilation rate 1,000 sq. feet 50 CFM 2,000 sq. feet 100 CFM 3,000 sq. feet 150 CFM In addition to this minimum, continuous ventilation rate, HRVs and ERVs often have additional capacity to provide higher ventilation rates to meet the occupants’ needs. Such needs could occur as a result of larger gatherings of people; smoking; a hobby or activity using paint, glues or other air-quality contaminants; or for any other reason that requires additional ventilation to improve the quality of indoor air. Local codes may require different continuous ventilation rates – always check with your building officials to determine the specific requirements for your area. Whole-house comfort ventilator HVI recommends that a whole-house comfort ventilator have a minimum capacity to provide approximately one complete air change every two minutes within the occupied area. This flow rate will be great enough to create a perceptible “breeze” through the house. The required flow rate can be calculated by multiplying the gross square footage of the entire house (including unoccupied areas like closets) by 3. Be sure to include the “upstairs” area of multilevel homes. This formula assumes an eight-foot ceiling and takes into account typical unoccupied areas. House area Capacity in CFM 1,000 sq. feet 3,000 CFM 2,000 sq. feet 6,000 CFM 3,000 sq. feet 9,000 CFM 32 H V G www.hvi.org circle 73 on reader service card
A smaller fan can effectively cool the mass of the house, relying on other fans such as “paddle-fans” to create the breeze needed to cool the people. This lower flow rate can be determined by multiplying the square footage by 0.4. 2,000 sq. feet 800 CFM 3,000 sq. feet 1,200 CFM For proper cooling and efficient operation, any whole-house comfort ventilator requires adequate, unobstructed outlets in the attic through soffit vents, grilles or louvers. To calculate the amount of attic exhaust area you will need, divide the fan’s capacity in CFM by 750. Fan capacity Exhaust area required 1,000 CFM 1.5 sq. feet 4,800 CFM 6.4 sq. feet NOTE: Large fans can put the house under significant negative pressure. At least one window should be open before the fan is operated. Powered attic ventilators – PAVs Powered attic ventilators should provide at least 10 air changes per hour. Multiplying the total square footage of the attic by 0.7 will provide the rate required. For particularly dark or steep roofs, we recommend a slightly higher rating. Attic area CFM required +15% for / in sq. feet dark/steep roofs 1,000 sq. feet 700 CFM 805 CFM 2,000 sq. feet 1,400 CFM 1,610 CFM 3,000 sq. feet 2,100 CFM 2,415 CFM The air being exhausted must be replaced by outside air drawn through vents under the eaves in the soffit. To calculate the total minimum soffit vent intake area in square inches, divide the CFM of the PAV by 300 and multiply the result by 144. CFM of PAV Soffit vent in net sq. in. 805 CFM 386 net sq. inches 1,610 CFM 773 net sq. inches 2,415 CFM 1,160 net sq. inches A minimum of one square foot of inlet area for every 300 CFM of HVI-Certified fan capacity is required for proper fan operation. • Use only soffit vents as fresh-air intakes for powered attic ventilators. • Do not use gable vents because it’s possible to draw rain and snow into the attic. Static attic ventilation In all seasons, the attic space is warmer than the outside air. This results in a constant upward air movement due to the buoyancy of warmer air. This characteristic of air may be used to help create a flow of air ventilating the attic. Placing exhaust vents in the roof, the gables or at the ridge of the roof and providing adequate air intake vents in the soffits best accomplishes this. HVI recommends vent selection and placement such that 60 percent of the vent net free area is provided by intake vents placed at the under-eave area and 40 percent of the vent net free area is provided by exhaust vents on the roof, at the ridge or high in the gable area. To determine the static ventilator net free area (NFA) needed for your attic space, determine the area of the attic in square feet. Divide that area by 150 to determine the square feet of attic ventilation needed. Since manufacturers of static ventilation products rate their products in square inches of NFA, it will be necessary to multiply that value by 144 to determine the square inches required. Attic area in sq. ft. Ventilation area in sq. ft. Net free area in sq. in. 1,000 sq. feet 6.67 sq. feet 960 sq. inches 2,000 sq. feet 13.3 sq. feet 1,920 sq. inches 3,000 sq. feet 20.0 sq. feet 2,880 sq. inches Static ventilation needs may be reduced if you have an uninterrupted ceiling vapor barrier installed with a rating of 0.1 perm or less. To calculate the ventilation required with such a vapor barrier, divide the square footage of the attic by 300 instead of 150. Attic area in sq. ft. Ventilation area in sq. ft. Net free area in sq. in. 1,000 sq. feet 3.33 sq. feet 480 sq. inches 2,000 sq. feet 6.67 sq. feet 960 sq. inches 3,000 sq. feet 10.0 sq. feet 1,440 sq. inches Use these numbers to select, proportion and place static ventilation products. ■ www.hvi.org H V G 33
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- Page 29 and 30: with low energy cost. These efficie
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HOME Ventilation<br />
Guide<br />
Heat and energy recovery ventilators<br />
For continuous indoor air quality ventilation, a<br />
heat or energy recovery ventilator (HRV or ERV)<br />
should provide 0.35 air changes per hour. This<br />
calculation must consider the complete occupied<br />
volume of the house.<br />
This rate can be more easily calculated by allowing<br />
5 CFM per 100 square feet of floor area.<br />
Total area of home (sq. ft.) Continuous ventilation<br />
rate<br />
1,000 sq. feet 50 CFM<br />
2,000 sq. feet 100 CFM<br />
3,000 sq. feet 150 CFM<br />
In addition to this minimum, continuous ventilation<br />
rate, HRVs and ERVs often have additional capacity<br />
to provide higher ventilation rates to meet the<br />
occupants’ needs. Such needs could occur as a result<br />
of larger gatherings of people; smoking; a hobby or<br />
activity using paint, glues or other air-quality contaminants;<br />
or for any other reason that requires additional<br />
ventilation to improve the quality of indoor air.<br />
Local codes may require different continuous<br />
ventilation rates – always check with your building<br />
officials to determine the specific requirements for<br />
your area.<br />
Whole-house comfort ventilator<br />
HVI recommends that a whole-house comfort ventilator<br />
have a minimum capacity to provide approximately<br />
one complete air change every two minutes<br />
within the occupied area. This flow rate will be great<br />
enough to create a perceptible “breeze” through the<br />
house. The required flow rate can be calculated by<br />
multiplying the gross square footage of the entire<br />
house (including unoccupied areas like closets) by 3.<br />
Be sure to include the “upstairs” area of multilevel<br />
homes. This formula assumes an eight-foot ceiling<br />
and takes into account typical unoccupied areas.<br />
House area<br />
Capacity in CFM<br />
1,000 sq. feet 3,000 CFM<br />
2,000 sq. feet 6,000 CFM<br />
3,000 sq. feet 9,000 CFM<br />
32<br />
H V G<br />
www.hvi.org<br />
circle 73 on reader service card
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