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Ventilation Questions Posed by Home Energy Magazine(It should be noted that whole-house ventilation is discussed here, not source ventilation for kitchens and baths.)1. What is the purpose of installing a mechanical ventilation system in a home?In short, the primary purpose of a whole-house mechanical ventilation system in a home is to provide, throughout the habitable and conditioned space, a controlled amount of unpolluted outside air for indoor pollutant dilution and removal, and for the sensory satisfaction of occupants. Since my definition considers excessive moisture to be a pollutant, a corollary to the stated purpose would be, to control interior moisture. An important secondary purpose is to control interior pressure, with respect to outside, to maximize building durability, combustion safety, and indoor air quality. 2. What are the qualities of a good ventilation system? That is; what should a contractor strive for (and a homeowner expect from) a good ventilation system?First of all, a good ventilation system should achieve the purpose stated in response to question 1. In more specific terms, the system should:
3. Should mechanical ventilation be standard in homes built today? What is the one most important consideration in making this decision?Most people like their homes better when they are not expensive, drafty, stuffy, odoriferous, moldy and wet. On the other hand, they like their houses to be affordable, comfortable, healthy and durable. For homebuilders who want to accomplish that, random natural infiltration should be minimized and controlled mechanical ventilation should be employed. Natural infiltration occurs when ever-changing forces of wind and stack (due to inside to outside temperature difference) move outside air through randomly distributed, unintentional holes in the building enclosure. Properly designed and installed, mechanical ventilation provides a consistent and uniform building air change for pollutant dilution and removal, and for the sensory satisfaction of occupants. The most important consideration in making the decision for or against mechanical ventilation is, therefore, "How lucky do you feel?" 4. What minimum criteria would determine if a new home needs mechanical ventilation? In what cases would you retrofit a home for mechanical ventilation?For new construction, a minimum criteria could be demonstration of the greater of 40 ft3/min and 10 ft3/min per person, or the equivalent volume over a two hour period, of unpolluted outside air distributed uniformly throughout the habitable and conditioned space 24 hours a day while the home is occupied. A retrofit criteria for existing construction is much harder to determine, or imagine in the real world. A first criteria, based on common business sense, would be if the homeowner wanted to pay for it. A second criteria, based purely on my personal observation and satisfaction in many homes that I have tested with fan pressurization and tracer gas, might be homes that test less than somewhere between 4 and 5 air changes at 50 Pa pressure difference. 5. Out of the three main types of mechanical ventilation (a sidebar will briefly describe exhaust, supply, and balanced systems), certain types work better in certain climates and markets. What type of system would you recommend for "typical" homes in the following locations: Phoenix, Arizona; Seattle, Washington; Fargo, North Dakota; Tampa, Florida?I will give my recommendations with respect to two different markets in new construction- production homes (very first-cost sensitive) and custom homes (not first-cost sensitive within the range of any ventilation system). Phoenix, Arizona; Production homes: My recommendation for production homes in the Phoenix climate is, without a doubt, central-fan-integrated supply ventilation. This system would typically use a 5 to 8 inch insulated outside air duct, located to maximize the quality of ventilation air, connected to the return side of the central air distribution system, at a location that will provide sufficient potential to draw in the design quantity of outside air. A balancing damper should be installed in the outside air duct to field adjust the air flow upon system startup, and provisions should be made to filter the outside air prior to entering the central fan. See the example schematic in Figure 1. As in any good home, the air distribution ducts should be substantially airtight (sealed at every joint with approved mastic or tape), or, better yet, inside conditioned space. The minimum fan run time should be controlled by a fan recycling system, which cycles the fan after a preset period of inactivity. An upgrade would be to include a motorized damper in the outside air duct, also controlled by the fan recycling system to limit the introduction of outside air to the design level. This type of central-fan-integrated supply ventilation system is an energy efficiency strategy that utilizes the normal cycling of the fan, as the fan operates to distribute conditioned air in response to calls from the thermostat, to also distribute ventilation air at the same time. So, for the amount of time that the air handler operates for heating or cooling, ventilation air is distributed for free. Hourly simulations have shown that the cost to periodically operate the air handler (33% duty cycle), when there is no thermostat demand, is less than $50 per year across the U.S. An added benefit of this system, which can be very important to builders and customers alike, is enhanced temperature and humidity comfort control in conditioned spaces. Thermostats are typically located in a central area and are expected to serve an entire zone that usually includes closed rooms, and often, more than one floor level. Temperature conditions can vary widely between the thermostat location and extremities of the space the thermostat serves. This has been demonstrated in a recently published survey of 1000 homeowners by a major controls manufacturer. The top complaint was inconsistent temperature between rooms, levels, or at different times of day. The central-fan-integrated supply ventilation system will serve to smooth out temperature, humidity, and air quality conditions in the house by circulating and mixing indoor air. Phoenix, Arizona; Custom homes: My recommendation for custom homes in the Phoenix climate would be a heat recovery ventilation (HRV) unit combined with a fan recycling system on the central air handler. The purpose of the HRV unit would be to lower the cost of conditioning ventilation air. The HRV would be ducted such that it pulled inside air from the master bedroom and delivered the ventilation air to the central area. When heating and cooling is active, the ventilation air would be distributed throughout the house via the central fan, the central fan recycling system would make sure that the ventilation air was distributed when there was no demand for heating or cooling. This strategy would be more effective overall, considering the mixing benefits described above, and cost less, than installing a complete separate air distribution system for the HRV. Keep in mind that most HRV/ERV manufacturers recommend continuous operation of the central air handler with operation of the HRV/ERV; the fan recycling system produces the same effect with lower operating cost.  Figure 1 Schematic of central-fan-integrated supply ventilation used in Phoenix Seattle, Washington: My recommendations for homes in the Seattle climate are the same as for Phoenix. Referring to Figure 2, the outdoor dewpoint remains moderately below the indoor comfort conditions throughout the year and outside air is useful for interior moisture dilution. For extended mild periods in Seattle, when no space conditioning is needed and windows are used for ventilation, the central fan recycling system may be turned off. Fargo, North Dakota; Production homes: My recommendation for production homes in the Fargo climate, is a continuous exhaust fan pulling air from the central area combined with a lower level of central-fan-integrated supply ventilation with fan recycling. This system provides a base rate of exhaust ventilation with intermittent supply ventilation and mixing. While the central fan is operating, the ventilation system will be balanced, and the source of outside air is known and can be filtered. The continuous exhaust allows the outside air fraction of the central-fan-integrated system to be less than would be used for warmer climates. An upgrade to this system would be to pull the exhaust air from two or three locations in the house (central area, master bedroom, family bath) instead of the central area alone. This will improve ventilation air distribution when the central fan is not operating, and, if all duct connections are made in the same mechanical room, it can be relatively easy to convert to an ERV/HRV system (see recommendation for Custom home below). The creative "ERV/HRV ready" concept was first suggested by collaborators in Minnesota. Fargo, North Dakota; Custom homes: My recommendation for custom homes in the Fargo climate would be an energy recovery ventilation (ERV) unit combined with a fan recycling system on the central air handler. The purpose of the ERV unit would be to lower the cost of conditioning ventilation air, and, in winter, to recover back some interior generated moisture to keep the house from being too dry. The ERV would be ducted such that it pulled inside air from the master bedroom and delivered the ventilation air to the central area. When heating and cooling is active, the ventilation air would be distributed throughout the house via the central fan, the central fan recycling system would make sure that the ventilation air was distributed when there was no demand for heating or cooling. This strategy would be more effective overall, considering the mixing benefits described above, and cost less, than installing a complete separate air distribution system for the ERV. Keep in mind that most ERV/HRV manufacturers recommend continuous operation of the central air handler with operation of the ERV/HRV; the fan recycling system produces the same effect with lower operating cost. ERV's as opposed to HRV's are most useful when the average outdoor dewpoint temperature is a lot below the indoor dewpoint during winter and above the interior dewpoint temperature in summer. The closer the outdoor dewpoint is to the indoor dewpoint, the lower the ERV benefit. Where the outdoor dewpoint is a lot below the interior dewpoint, the ERV benefit is in recovering back some of the interior generated moisture to keep the house from being too dry. Where the outdoor dewpoint is above the interior dewpoint, the ERV benefit is to reduce the incoming moisture load in the outside air to reduce cooling/dehumidification energy consumption and to potentially keep the house dryer. Referring to Figure 2, it can be seen that an ERV can be most effective in Fargo, ND and Tampa, FL. However, contrary to a common misconception, the ERV cannot be used as a dehumidifier. In Tampa, FL, over extended periods with no cooling system operation, an ERV will not keep interior moisture below the interior dewpoint threshold shown. Table 1 shows the corresponding interior drybulb temperature and relative humidity used to calculate the interior dewpoint threshold for each season.  Figure 2 Mean monthly outdoor dewpoint temperature for various locations, and interior dewpoint threshold temperature
Table 1 Typical interior conditions by
season: drybulb temperature Tampa, Florida; Production homes: My recommendation for production homes in the Tampa climate is central-fan-integrated supply ventilation, with central fan recycling, and a stand-alone dehumidifier. The dehumidifier and the air handler can be installed in the same mechanical closet and use the same condensate drain line. The dehumidifier is not a penalty resulting from the ventilation system, it is needed in that climate anyway, with or without mechanical ventilation, to maintain good humidity control throughout the year. Throughout much of the year, even during summertime at night, humidity control cannot be maintained with the cooling system alone. The cooling system may not operate for long periods because the sensible heat load is low, and the thermostat, which only responds to temperature, not humidity, is satisfied. Exterior moisture continues to enter the house through infiltration or ventilation and interior generated moisture is added to that. During those times, a separate dehumidifier is required to maintain relative humidity below a healthy 50% to 60%. Supply ventilation will tend to pressurize an interior space relative to the outdoors, causing inside air to be forced out through leakage openings located randomly throughout the building enclosure. This strategy is advantageous in warm-humid climates to minimize moisture entry into the building structure from outdoors. In mechanically cooled buildings in hot-humid climates, if interior negative pressure causes moisture laden outdoor air to enter the building enclosure, moisture can condense on cool surfaces, and, if restricted from drying to the inside, material durability and indoor air quality problems can result. Tampa, Florida; Custom homes: My recommendation for custom homes in the Tampa climate is a high-efficiency dehumidifier that filters and pre-conditions a mixture of inside and outside air and delivers the ventilation air to the central area, with central fan recycling for whole-house circulation and mixing. The dehumidifier can be energized by a ventilation timer and an indoor dehumidistat. This is a premium system for the hot-humid climate, and will provide the highest quality of ventilation and indoor humidity control. 6. Other than climate, what else must be considered in terms of choosing a system? What are the pros and cons of each system?The source of outside air must be considered. The source is unknown for exhaust ventilation systems. Exhaust ventilation systems draw outside air from leakage openings and pathways located randomly throughout the building enclosure, thus, it is not possible to treat the outside air before it enters the living space. The "ventilation" air could be fresh and healthy, or it could be coming from locations with high pollutant concentrations. For example, ventilation air drawn from a garage, crawl space, basement sump, or from underneath a concrete slab may induce entry of fuel vapors, combustion gases, insecticides, radon gas, excessive water vapor, and fungal or mold spores. The source of outside air is known for supply and balanced ventilation systems. The ventilation air can be treated before distribution to the living space (i.e. one or more of: heated, cooled, dehumidified, filtered, cleaned). Marketability must be considered. While many energy purists would like to see systems installed that have the absolute lowest operating cost, even if it amounts to a relatively small dollar value, getting off ground zero with first-cost sensitive production homebuilders is a very important step in the process of change in the, understandably so, conservative and evolutionary, not revolutionary, homebuilding industry. Ease of use and maintenance should be considered. An advantage of central-fan-integrated ventilation is that the same air handler that distributes air for heating and cooling is used to distribute ventilation air, and that fan and filter system is likely to be kept in working condition and be used. Other ventilation systems may not be repaired or replaced if broken, and regular maintenance may not be as obvious. 7. Exhaust systems are the least expensive and most common type of mechanical ventilation system, and the most frequently installed in homes. How severe are the hazards associated with the house depressurization caused by exhaust ventilation? What is the best way to mitigate such hazards?Building depressurization in hot-humid climates, other than due to normal intermittent operation of bathroom fans, kitchen range hood, and dryer, is hazardous. Use of low permeability interior finishes increases the problem greatly. Humid outdoor air will randomly enter the building enclosure, even following circuitous paths within partitions and interstitial spaces, where it comes in contact with surfaces that are below the air dewpoint temperature. If those materials stay moist long enough, mold will result. Combustion appliances inside houses should be at least power vented, and preferably sealed combustion. In such houses, it would be unlikely for whole-house exhaust ventilation systems alone to cause a problem with combustion safety. Whole-house exhaust ventilation along with other exhaust devices- clothes dryer, kitchen range hood or down-draft, bathroom fans, power vented gas hot water heater and furnace- can cause more severe depressurization requiring an intended make-up air source. |
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Building Science Corporation |