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Whole-House Ventilation System Designs
The decision to use whole-house ventilation is typically motivated by concern that natural ventilation is not providing adequate air quality, even with source control by spot ventilation. Whole-house ventilation systems are usually classified as:
v exhaust ventilation if the mechanical system forces inside air out of the home,
v supply ventilation if the mechanical system forces outside air into the home,
v balanced ventilation if the mechanical system forces equal quantities of air into and out of the home.
Exhaust ventilation systems work by depressurizing the building. By reducing the inside air pressure below the outdoor air pressure, they extract indoor air from a house while make-up air infiltrates through leaks in the building shell and through intentional, passive vents.
Exhaust ventilation systems are relatively simple and inexpensive to install. Typically, an exhaust ventilation system is composed of a single fan connected to a centrally located, single exhaust point in the house. A preferable design option is to connect the fan to ducts from several rooms (preferably rooms where pollutants tend to be generated, such as bathrooms). Adjustable, passive vents through windows or walls can be installed in other rooms to introduce fresh air rather than rely on leaks in the building envelope. However, their use may be ineffective because larger pressure differences than those induced by the ventilation fan may be needed for them to work properly. Spot ventilation exhaust fans installed in the bathroom but operated continuously can represent an exhaust ventilation system in its simplest form.
Exhaust ventilation systems are most applicable in cold climates. In climates with warm humid summers, depressurization can draw moist air into building wall cavities, where it may condense and cause moisture damage.
One concern with exhaust ventilation systems is that they may draw pollutants, along with fresh air, into the house. For example, in addition to drawing in fresh outdoor air, they may draw in radon and molds from a crawlspace, dust from an attic, fumes from an attached garage, or flue gases from a fireplace or fossil-fuel-fired water heater and furnace. This can especially be of concern when bath fans, range fans, and clothes dryers (which also depressurize the home while they operate) are run when an exhaust ventilation system is also operating. Also, exhaust ventilation systems can contribute to higher heating and cooling costs compared with heat-recovery systems because exhaust systems do not temper or remove moisture from the make-up air before it enters the house.
Supply ventilation systems work by pressurizing the building. They use a fan to force outside air into the building while air leaks out of the building through holes in the shell, bath and range fan ducts, and intentional vents (if any exist).
As with exhaust ventilation systems, supply ventilation systems are relatively simple and inexpensive to install. A typical supply ventilation system has a fan and duct system that introduces fresh air into usually one, but preferably several rooms of the home that residents occupy most often (e.g., bedrooms, living room), perhaps with adjustable window or wall vents in other rooms. Supply ventilation systems allow better control of the air that enters the house than do exhaust ventilation systems. By pressurizing the house, supply ventilation systems discourage the entry of pollutants from outside the living space and avoid backdrafting of combustion gases from fireplaces and appliances. Supply ventilation also allows outdoor air introduced into the house to be filtered to remove pollen and dust or dehumidified to provide humidity control.
Supply ventilation systems are most applicable in hot or mixed climates. Because they pressurize the house, supply ventilation systems have the potential to cause moisture problems in cold climates. In winter, the supply ventilation system causes warm interior air to leak through random openings in the exterior wall and ceiling. If the interior air is humid enough, some moisture may condense in the attic or cold outer parts of the exterior wall where it can promote mold, mildew, and decay.
Like exhaust ventilation systems, supply ventilation systems do not temper or remove moisture from the make-up air before it enters the house. Thus, they may contribute to higherheating and cooling costs compared with heat-recovery systems. Because air is introduced in the house at discrete locations, outdoor air may need to be mixed with indoor air before delivery to avoid cold air drafts in the winter. An in-line duct heater is another option, but it will increase operating costs.
Balanced ventilation systems neither pressurize nor depressurize a house if properly designed and installed. Rather, they introduce and exhaust approximately equal quantities of fresh outside air and polluted inside air, respectively. Balanced ventilation systems are appropriate for all climates.
A balanced ventilation system usually has two fans and two duct systems and facilitates good distribution of fresh air by placing supply and exhaust vents in appropriate places. Fresh air supply and exhaust vents can be installed in every room, but a typical balanced ventilation system is designed to supply fresh air to bedrooms and living rooms where people spend the most time,
and exhaust air from rooms where moisture and pollutants are most often generated (kitchen, bathrooms, and perhaps the laundry room). Some designs may use a single-point exhaust. Because they directly supply outside air, balanced systems allow the use of filters to remove dust and pollen from outside air before introducing it into the house.
Balanced systems are usually more expensive to install and operate than supply or exhaust systems because they require two duct and fan systems. Like these other systems, balanced ventilation systems do not temper or remove moisture from the make-up air before it enters the house and thus may contribute to higher heating and cooling costs compared with heat-recovery systems. Like supply ventilation systems, outdoor air may need to be mixed with indoor air before delivery to avoid cold air drafts in the winter.
Balanced, Heat-recovery Ventilation Systems A special type of balanced ventilation system adds a heat-recovery unit to the basic design. A heat-recovery unit reduces the heating and cooling costs of ventilation by transferring heat from the warm inside air being exhausted to the fresh but cold outside air in the winter, and vice-versa in the summer. Comfort is also improved because the supply air is tempered before delivery, reducing drafts. Some heat-recovery systems also transfer moisture—an advantage in warm, humid climates in the summer and cold climates in the winter.
Balanced ventilation systems with heat recovery are more costly to install than balanced systems without heat recovery because heat-recovery systems require more powerful fans that use more energy to overcome the air resistance of the heat exchanger.
Balanced, heat-recovery units are most cost effective in climates with extreme winters or summers, and where fuel costs are high. In mild climates, the cost of the additional electricity consumed by the fans may exceed the energy savings from not having to heat and cool the air introduced by the ventilation system.
Heat-recovery systems require more maintenance than other whole-house ventilation systems. They need to be cleaned regularly to prevent deterioration of ventilation rates and heat recovery, and to prevent growth of mold and bacteria on heat exchange surfaces. When warm, moist air is cooled, condensate forms on cool surfaces and must be drained from the heat-recovery system. In cold climates, very cold air brought into a heat-recovery system can cause frost formation in the heat exchanger. Because frost buildup reduces ventilation effectiveness and can damage the heat exchanger, heat-recovery systems must have devices to deal with frost.