Guest guest Posted February 23, 2006 Report Share Posted February 23, 2006 Dear Colleagues: After reading the comments about the tendency of people designing just to code as good enough, I want to comment on Bob's words about ASHRAE. The ASHRAE standards for ventilation of an office area is 20 cfm per person. This is designed to deal with odors from people and office equipment and furniture and good air quality and is based on years of studies of the reports of poor air quality and ventilation rates going back to the 1800s. As I understand it, ASHRAE Standard 62 lists minimum recommended ventilation rates to achieve acceptable indoor air quality. According to ASHRAE's own definition, acceptable IAQ means that a substantial majority, up to 80% of the people present will not be dissatisfied. This means, conversely, that up to 20% of the people present can be dissatisfied and the ASHRAE goal of acceptable IAQ has been met. IMHO, it is a stretch to call this good IAQ. I feel that achieving good IAQ requires the delivery of ventilation amounts in excess of ASHRAE's minimum recommended values to where the people are. I also have strong concerns about relying on building leaks to provide ventilation. This approach will over ventilate and waste energy when it is cold and windy, but fail to provide a healthy indoor environment during more moderate weather. My recommended solution is to have a very tight envelope but to have a dedicated pathway for outdoor air to easily enter the space and provide replacement air for the exhaust fans. Sincerely, W. Bearg, PE, CIH LIFE ENERGY ASSOCIATES www.LifeEnergyAssoc.com -------------- Original message ---------------------- > > Some thoughts to consider on house wrap and infiltration. > > The ASHRAE standards for ventilation of an office area is 20 cfm per > person. This is designed to deal with odors from people and office > equipment and furniture and good air quality and is based on years of > studies of the reports of poor air quality and ventilation rates going > back to the 1800s. > > However, for homes, we now add cooking odors, laundry odors, e.g. > fabric softeners (yech), perfumes, and those consumer products that add > VOCs to your home. The implications are that a home needs more > ventilation than an office. > > Consider the following minimum ventilation rate calculation for a > typical home. > > 20 cfm x 3 people x 60 min = 900 cfm. > > 1800 sq ft house x 8 = 14,400 cu. ft. > > = 0.25 ACH > > If a home needs more ventilation than an office to maintain good air > quality, then if Tyvek reduces this to less than 0.25 ACH is this a > problem? Maybe Tyvek needs to leak a little more air? > > A second dimension to this questions is the energy cost. If one has > to add an HRV because a home is too tight, there is an energy cost of > operating this unit which is substantial. > > The HVR unit requires 100 watts to operate and the furnace fan motor > requires another 3600 watts (0.33+ HP) to operate. So the overall > operating energy for the HRV is 4 kw/hr. If the furance operates 25% > of the time for heating, you can discount this 4 kw to 3 kw because the > fan energy was necessary to operate the furnace. > > In a 24 hr period, the energy necessary to operate the HRV could > require 72 kw. This converts roughly to 72 kw x 3,413 btu/kw = > 245,736 Btu/day. However, since this is electricity you need to > multiply this times 3, since electrical generation from fossil fuels is > only 33% efficient. This is equal to 737,208 BTU of natural gas or oil > or coal. Since there are about 1000 btu per cu ft of NG, this means > that 737 extra cu ft per day would be burned at the power plant to > operate the HRV. > > How much extra infiltration air could this 737 cu ft of Nat gas heat? > Air has a very low specific heat of 0.018 BTU per cubic foot. To raise > the temperature of air from 20 to 70 °F would require 50 x .018 = 0.9 > BTU per cubic foot. > > So 737,000 btu / 0.9 btu/cuft air = 663,000 cu ft of air. This is > equal to 253 cu. ft. per minute of infiltration air. This is almost > 1/3 of the minimum infiltration necessary. > > So if we were trying to save energy and green house gases, by reducing > infiltration, the saving are not are clear cut at they would seen. > Add to this the extra cost of the HRV and the energy to manufacture the > HRV to the potential cost of extra risk of water damage and mold growth > and the issue gets even grayer. > > Maybe Tyvek needs to leak a little more air? > > Bob > > > > > > Some thoughts to consider on house wrap and infiltration. The ASHRAE standards for ventilation of an office area is 20 cfm per person. This is designed to deal with odors from people and office equipment and furniture and good air quality and is based on years of studies of the reports of poor air quality and ventilation rates going back to the 1800s. However, for homes, we now add cooking odors, laundry odors, e.g. fabric softeners (yech), perfumes, and those consumer products that add VOCs to your home. The implications are that a home needs more ventilation than an office. Consider the following minimum ventilation rate calculation for a typical home. 20 cfm x 3 people x 60 min = 900 cfm. 1800 sq ft house x 8 = 14,400 cu. ft. = 0.25 ACH If a home needs more ventilation than an office to maintain good air quality, then if Tyvek reduces this to less than 0.25 ACH is this a problem? Maybe Tyvek needs to leak a little more air? A second dimension to this questions is the energy cost. If one has to add an HRV because a home is too tight, there is an energy cost of operating this unit which is substantial. The HVR unit requires 100 watts to operate and the furnace fan motor requires another 3600 watts (0.33+ HP) to operate. So the overall operating energy for the HRV is 4 kw/hr. If the furance operates 25% of the time for heating, you can discount this 4 kw to 3 kw because the fan energy was necessary to operate the furnace. In a 24 hr period, the energy necessary to operate the HRV could require 72 kw. This converts roughly to 72 kw x 3,413 btu/kw = 245,736 Btu/day. However, since this is electricity you need to multiply this times 3, since electrical generation from fossil fuels is only 33% efficient. This is equal to 737,208 BTU of natural gas or oil or coal. Since there are about 1000 btu per cu ft of NG, this means that 737 extra cu ft per day would be burned at the power plant to operate the HRV. How much extra infiltration air could this 737 cu ft of Nat gas heat? Air has a very low specific heat of 0.018 BTU per cubic foot. To raise the temperature of air from 20 to 70 °F would require 50 x .018 = 0.9 BTU per cubic foot. So 737,000 btu / 0.9 btu/cuft air = 663,000 cu ft of air. This is equal to 253 cu. ft. per minute of infiltration air. This is almost 1/3 of the minimum infiltration necessary. So if we were trying to save energy and green house gases, by reducing infiltration, the saving are not are clear cut at they would seen. Add to this the extra cost of the HRV and the energy to manufacture the HRV to the potential cost of extra risk of water damage and mold growth and the issue gets even grayer. Maybe Tyvek needs to leak a little more air? Bob Quote Link to comment Share on other sites More sharing options...
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