According to Home Energy Magazine May/June 2000, “good ventilation system should:
Provide a controlled amount of unpolluted outdoor air for both comfort and dilution
Have at least a 15 year life
Be acceptable to operate by occupants (low noise, low cost)
Not detract from the safety and durability of the house.”
Four basic approaches (more effective and easier to control if they are used in conjunction with airtight construction)
1. Exhaust systems: bathroom exhaust fan, kitchen range hood, expel polluted, or moisture laden air from the house. Clothes dryers and central vacuums can be exhaust devices, if they are exhausted to the outdoors, like they should be. Furnaces, fireplaces, and water heaters can also be thought of as exhaust devices. If you simply blow air out of an airtight house, you will tend to depressurize the whole building and the exhaust devices won’t work very well. Several manufacturers produce small grilles that can be placed in the exterior walls to provide specific pathways for air to enter. A designer may choose to use central exhaust ventilation equipment, it has low initial cost but incoming air is cold in winter or hot in summer. Filtering and distributing the incoming air can also be difficult because you would need a separate filter on each of the intake grilles.
2. Supply Systems: blows air into a building. This results in slight pressurization. …radon is kept out and backdrafting of chimneys is not a problem. However, if a house isn't constructed in an airtight manner, pressurization can force moisture from the living space into wall cavities, resulting in possible mold growth and rot. Small grilles can be placed in the exterior walls. One way some houses are currently being ventilated with a supply system is in conjunction with a forced air heating or cooling system. A disadvantage of coupling the ventilation system to a heating/cooling system is that it will only supply fresh air when the furnace fan is running. Some furnace fans can be set to run continuously, but because they have fairly large motors, this can mean a high operating cost. Supply ventilation has a relatively low installation cost but incoming air can be cold in the winter or very hot in the summer. This can contribute to a high operating cost.
3. Balanced Systems: Balanced systems that operate independently of the furnace system have a definite advantage. They are more costly to install, but offer more occupant control.
4. Heat Recovery Ventilation: …heat recovery ventilators (HRVs) -- air to heat exchangers or energy recovery ventilators (ERVs). Most balanced systems currently on the market are HRVs; 70% or more of the heat in the warm exhaust air is transferred to the cold incoming air. In very cold climates, they can pay for themselves in just a few years.
How Much Ventilation Is Enough: A figure of 15 cfm per person seems to work well, established in 1989 by ASHRAE – an alternative, 0.35 air changes per hour, the larger of the two figures be used as a minimum ventilating rate. These rates are enough to remove occupant generated pollutants, including moisture, body odor, bad breath, etc. (when using exhaust fans, central vacuums, or dryers, it is a good idea to crack a window to prevent depressurization). (Filtration Equipment: CRSI #600H Pure Air Systems)
Extended Surface Filters:
These have lower initial cost than an electrostatic precipitator and will require less regular maintenance. There is no electrical expense, but there is a cost associated with the periodic replacement.
Filtration Strategies: if the indoor air is filtered, less ventilation is needed, however, filtration cannot provide the oxygen that is depleted as we breathe.
Particulates include mold spores, pollen, asbestos, house dust, dead insect parts, etc. Gases include combustion gases, formaldehyde, and the hundreds of other VOCs floating in the air. Tobacco and wood smoke are composed of both gases and particulates...need two different types of filters.
Particulate Filters: The standard 1" thick furnace filter falls into this category. Unfortunately, it doesn't do a very good job, its primary purpose is to help protect the fan motor in the furnace.
Gaseous Filtration
Totally different approach: adsorption -- gas molecules adhere to a solid surface. Once the adsorption filter has captured all the gases it can hold, it must be replaced. Activated carbon is one of the most common adsorption materials; it does an excellent job, unfortunately, it is less effective at capturing gases with low molecular weights (e.g. formaldehyde) (special filters can be used). In a relatively unpolluted home, such a filter can be used to "polish" the air. (otherwise it is expensive).
HEPA Filters
…high efficiency particulate accumulator, sometimes called "absolute filters" because they are typically 95-99% efficient, they require more powerful fan motor, they do nothing to remove gases.
More on remedies
The best situation is to have a mechanical ventilation system in a tight house…correct amount of air exchange at all times -- air to and exhausting air from the right parts of the house. Larger quantities of pollutants are generated by specific activities, cooking, laundering, or hobbies, these are often best handled by local ventilation near the pollution source. Unfortunately, well-designed mechanical ventilation is not a common feature in houses built today and the concept is often poorly understood. Filtration is probably even less well understood. Ozone generators are currently being marketed, these are potentially dangerous.
House plants: Spider plants in particular have been shown to remove formaldehyde but not significantly. Having plants raises indoor humidity, resulting in increased populations of mold or dust mites. (Other houseplants that may remove toxins: Aloe vera, Elephant ear philodendron, English ivy, ficus, Golden pothos, Janet Craig, Peace lilly)
Electrostatic Precipitators: …fit into the furnace duct work and operate by placing an electrical charge on any particles of dust that pass through…cling to metal plates…must be removed and cleaned…work fairly well, although they lose their efficiency as the plates fill with dust. There is one primary disadvantage, small amounts of ozone are produced ( some people are bothered by this).
NightBreeze ventilation system, developed by Davis Energy Group, circulates cool filtered air through the home at night automated, smart system.
The Design Criteria:
The system should operate independently of the heating and air conditioning systems.
The capacity should be large enough to provide at least 15 dfm of outdoor air per person without running continuously.
Should provide for air circulation throughout the house, including closets.
Filtration system capable of removing pollen and mold spores, and occasional wood smoke from the incoming air.
Should also contain a moderate amount of adsorption media.
Should have a recirculation mode whereby the indoor air can be cleaned even if the outdoor air supply is turned off.
Should be energy efficient
Should be local ventilation -- bathroom fans and kitchen range fan.
