This article was originally published in the March/April 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online March/April 1996
Mechanical Ventilation for the Home
by Don Stevens
Don Stevens is a ventilation consultant in Keyport, Washington.
A ventilation system gives occupants control over a home's air change rate and thermal comfort. This article discusses the options available today for good mechanical ventilation.
All houses and apartments need an efficient way to exhaust stale, moist indoor air and introduce outdoor air. Good ventilation makes a house healthier for occupants and protects the building structure from moisture damage (see Fundamentals of Moisture in Houses, HE Nov/Dec '95, p.11).
Traditionally, home builders have relied on natural ventilation and infiltration provided by random gaps and cracks in the building shell. The occupants can control ventilation in these houses only by operating the windows and doors. Natural ventilation depends on wind speed and temperature differences between indoors and outdoors. A house that breathes on its own often has too much ventilation in the winter, resulting in high heating bills and uncomfortable drafts. On mild or calm days in other seasons, the house often has too little ventilation.
Houses built today are tighter than houses of 50 years ago, while the number of pollutants in the home has grown (although smoking is less prevalent). Choosing less toxic building materials at the design stage can limit pollutants in a new home, but this option isn't available once the home is built. Fireplaces, wood stoves, and other combustion appliances increase pollutant levels. Moisture, pets, furnishings, cigarette smoke, aerosol sprays, household cleaners, and hobby supplies add to the mix.
A well-designed mechanical ventilation system with good controls is important to provide consistent ventilation through all seasons of the year for all housing. Fans exhaust stale air. Leaks, passive inlets, or another fan draws in outdoor air. Maintaining good indoor air quality in a house requires reducing pollution sources and providing adequate ventilation to remove and dilute pollutants.
In warm climates, good indoor air quality may be achieved with supply ventilation, but most systems described here deal with exhaust ventilation for temperate and cold climates.
Most building and ventilation codes and utility programs in the United States are based on ASHRAE 62. (ASHRAE develops guidelines and standards for engineering design, building practice, and building codes throughout the United States.) ASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air Quality, requires mechanical spot ventilation for specific sources of indoor pollutants. It also requires general ventilation that operates whenever the house is occupied to maintain indoor air quality. The number of occupants determines the amount of general ventilation required. Let's look at these two types of ventilation.
General indoor air quality (IAQ) ventilation removes contaminants and delivers outdoor air to the occupied rooms of a house. The current ASHRAE standard recommends a minimum of 0.35 air changes per hour (ACH)-that is, in one hour 35% of the home's air is exhausted and replaced with outdoor air. A proposed revision to the ASHRAE 62 standard uses 5 CFM per 100 ft2 of household area as a simpler calculation method. In either case, the ventilation rate must be not less than 15 CFM for each occupant, assuming two occupants for the master bedroom and one for each additional bedroom. I recommend that the ventilation installer use 0.5-0.6 ACH for an upper limit on ventilation rate.
The ASHRAE standard does not require mechanical ventilation, but we know from blower-door and tracer-gas testing that only mechanical systems provide outdoor air reliably through all seasons.
The fan should be quiet, with a sound rating of 1.5 sones or less. Most bath fans are rated at 3 to 4 sones and are too noisy for the extended operation needed for a general IAQ ventilation fan (see Table 3 for a list of quiet, low energy fans). Panasonic has set the quiet fan standard, with several 0.5- and 1.0-sone fans. Other manufacturers are introducing new fans to try to catch up.
Dedicated Fan A centrally located, quiet ceiling fan acts as the general IAQ ventilation fan. Often it is located in the hall or at the top of the stairs. All spot ventilation is handled by other fans. A 24-hour clock timer is wired to the fan so that occupants can schedule ventilation times.
The dedicated fan should be quiet enough not to disturb occupants, especially if it is located near bedrooms. Generally a sound rating of 1.5 sones will be acceptable, but the quieter the fan, the less likely it is that residents will complain. Fans rated at 0.5, 0.8, and 1.0 sones should be considered, since people aren't used to having a fan run for extended periods.
Although most places in the United States are not cold enough or do not have utility rates high enough to make them cost-effective, heat exchangers have other advantages. They filter fresh air coming in and temper it for comfort, and they better balance the flow of incoming fresh air and exhausted stale air.
A traditional heat recovery ventilator (HRV) or energy recovery ventilator (ERV) generally uses a fixed plastic or aluminum core or a rotating plastic wheel to move heat from the stale exhaust air to the incoming fresh air. An HRV transfers mostly sensible heat, while an ERV can recover additional heat, due to the latent heat of moisture transfer. The colder the climate, the more energy is saved by warming the incoming ventilation air. The HRV can also be used in warm climates to precool outdoor air, to reduce the air conditioning load. However, the designer should make sure that the heat exchanger has the ability to accommodate moist air without causing condensation problems. Exhaust ducts run from pickup points in bathrooms and in the living space to the HRV unit. Supply ducts to the main living space and the bedrooms or to the furnace ductwork deliver the tempered outdoor air to the occupied spaces. Supply and exhaust ducts are run from the HRV unit to the outside. Generally two fans operate within the HRV cabinet to bring in outdoor air and exhaust stale indoor air. Look at and compare the efficiency ratings by the Home Ventilating Institute (HVI) when considering HRV equipment.
An HRV unit is usually either controlled by a 24-hour timer or operated continuously. Some units have two or more ventilation speeds, and most offer defrost capability to keep surfaces from frosting up in cold weather. A ducted range hood provides spot ventilation in the kitchen, since grease from cooking should not be pulled into the HRV ducts.
A dehumidifying ventilator uses a small compressor to dehumidify and reheat the indoor air while introducing outdoor air to blend with it. This supply-side ventilator can be operated either as a positive-pressure system by itself in warm climates or as a balanced-flow system in cold climates if a quiet, low-energy fan is used to exhaust a like amount of indoor air. It includes a high-efficiency air filter to clean the blended air to near-HEPA quality. (HEPA stands for High Efficiency Particulate Arrestor. It is a filter capable of filtering out 99% of particles 1 micron or larger. The filter on the dehumidifying ventilator removes 95% of particles 1 micron or larger, so it is called near HEPA.) This type of ventilation can be used in areas of medium to high temperatures and high humidity to dehumidify and clean the indoor air efficiently, while introducing outdoor air. When it is relatively cool but humid, as at night, it will dehumidify the air and improve comfort even when the air conditioner does not get a call for cooling. Dehumidifying ventilators are available from American ALDES and DEC Therma-Stor.
Wall or window vents must allow air to flow at low pressures and should be tested according to the HVI test standard. (The HVI is the same organization that certifies the air flow of fans and the efficiency of HRVs.) Field testing shows that only a small portion of the outdoor air that comes into the structure actually passes through these vents. The use of the vents, however, ensures that at least some of the outdoor air is introduced into the spaces that most need fresh air, such as bedrooms.
Wall vents are generally made of plastic and/or metal and include a rain cap for the outside, an insect screen, and a cleanable filter. They also include a flow control mechanism to limit or stop air flow, a liner to pass through the wall cavity, and a device to direct the airstream away from people in the room. Wall vents should be located high on the wall and away from seating areas and beds, to minimize complaints about drafts. European products manufactured for this purpose have been used throughout the United States for several years. They are available from American ALDES (Airlet models 100 and 200) and DEC Therma-Stor (FRESH 80 and FRESH 100).
Window vents are generally located in the upper sash area of the window. They usually have a rain shield and an insect screen, but no filter. Occupants can control the air flow by opening and closing the vent. The design may be as simple as drilled holes in the vinyl frame with a sliding shutter. Window vents vary in size, but they all must be tested in accordance with the HVI procedure. Depending on the manufacturer, the upper sight line of the window may be affected, since the vent can make the frame taller. Many vinyl and aluminum and some wood window manufacturers now offer vented windows. Window vents are manufactured by Titon Incorporated and American ALDES.
Instead of wall or window vents, some systems use an outdoor air connection to a furnace return air plenum. The connection should be between 6 and 8 inches in diameter to reduce pressure drop, depending on the flow required. In the Pacific Northwest, we require that the duct be a minimum of 6 inches for up to 80 CFM, 7 inches for 80-120 CFM, and 8 inches for 120 CFM or more, assuming up to 20 ft of run from the outside. For best operation, a motorized damper should be used to limit the intake of outdoor air to periods when the exhaust fan is being operated for general IAQ ventilation. Otherwise the furnace will pressurize the house if the IAQ fan is off. The furnace air handler must also be turned on for circulation and to draw in the outdoor air. Honeywell and DuroZone both make controllers that turn on the air handler and the exhaust fan and open the motorized damper.
Although connecting to the furnace is relatively simple, using a 400W air handler fan motor to pull in outdoor air and circulate it wastes a lot of energy compared to a low-energy fan, such as Panasonic's 15W 70 CFM model. The furnace fan strategy would increase electrical consumption by over 1,000 kWh per year. (Using a variable-speed furnace fan would reduce the fan motor's energy use, but the pressure in the return air plenum-necessary to bring in fresh air-would be reduced even more, making it ineffective.)
Most actual duct installations are longer than 5 ft and/or have elbows, which makes the resistance greater than the HVI test assumes. To compensate for duct resistance, many utility programs and building codes require that the fan's air flow be rated at a higher static pressure than 0.1 wg, or that the fan be increased in size (flow rate). For example, the State of Washington's ventilation code requires that the fan's flow be certified at 0.25 wg. It's best to plan on using the flow at 0.25 wg to meet required flow, since this is more likely to be the typical static pressure when fans are installed.
Terminal Devices Ducts need to be sized correctly to deliver the desired air-flow rate. Flexible duct has about twice as much resistance to air flow as smooth metal duct, but either type can usually be used for bath fans. When using flex duct, keep runs short. Avoid droops that increase resistance and create a place for moisture to condense. Avoid using plastic dryer duct connectors. Use at least 4-inch ducts for fans of less than 80 CFM and 5-inch ducts for fans up to 120 CFM. Flex duct is acceptable, but smooth duct provides better air flow. For kitchen range hoods, only smooth metal duct meets the Uniform Mechanical Code and other model codes. All duct joints, fittings, and adjustable elbows need to be sealed with duct mastic or high-quality duct tape.
All fan ducts need to terminate outside the building shell-not in an attic or a soffit, or under the house. With a solid duct connection, moisture is carried outside the building shell so that it cannot condense inside a wall or on the bottom of the roof. Exhaust ducts should connect directly to a collar that extends through the wall or ceiling to a termination designed for exhaust air flow.
Other Controls For effective ventilation, the general ventilation fan needs to operate when contaminants are present or are being produced. The ASHRAE standard recommends ventilation whenever the house is occupied. Generally, a good compromise in practice is to ventilate 8 to 12 hours a day. Ventilation can be scheduled by using a 24-hour timer set for at least two periods. Part of the ventilation time should be scheduled during the night, when people are sleeping. For instance, a fan could be scheduled for 6 pm-12 midnight and 6 am-10 am for a household with nobody home during the day, while a parent home with preschool children might want to ventilate 24 hours a day.
Controls are now available that perform several functions. Honeywell has an integrated comfort setter that combines a thermostat with a ventilation control, a humidity sensor, and an outdoor thermometer. Called the PC8900, it allows the installer to program the ventilation run time and remembers if the air handler ran in the last half hour. American ALDES and Tamarack Technologies offer a combination timer and speed control called the Airetrak. This device allows the use of a larger fan that can be set to run at a lower and quieter speed for general ventilation; it includes a push-button override to full speed for 20 minutes, and a programmable duty cycle for general ventilation. The Airetrack was developed to be used with the new condenser motor in a Panasonic fan; it is the only speed control that Panasonic allows to be used with its fans.
Additional ventilation time may be needed during large parties or when someone is working on hobbies or other activities that produce noxious odors or extra moisture.
ASHRAE 62-Ventilation for Acceptable Indoor Air Quality, ASHRAE, 1791 Tullie Circle NE, Atlanta, GA, 30329-2305. Tel:(404)636-8400; Fax:(404)321-5478.
Understanding Ventilation: How to Design, Select, and Install Residential Ventilation Systems, by John Bower, The Healthy House Institute, 430 N. Sewell Rd., Bloomington, IN 47408.
Oikos/Green Construction Source (Features REDI 96, an online directory of products, including ventilation fans, devices, and controls.) Iris Communications, P.O. Box 5920, Eugene, OR, 97405-0911, Tel:(541)484-9353, World Wide Web address: http://irisinc.com/oikos
AIRBASE (database of over 7,000 abstracts of international papers on infiltration and ventilation), Air Infiltration and Ventilation Centre, Sovereign Court, University of Warwick Science Park, Sir William Lyons Road, Coventry, CV4 7EZ, U.K. Tel:44-203-692050, Fax:44-203-416306.
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