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Ventilation and Air Cleaning
Energy Efficiency, Comfort, and Health | Components | Ventilation | Air Infiltration | Natural Ventilation | Mechanical Ventilation | Basic Furnace Filter | Media Air Cleaners | HEPA Air Cleaners | Electrostatic Air Filters | Electronic Air CleanersQuick Facts
- Ventilation exchanges stale, and sometimes damp inside air with fresh outside air
- New building standards specify that air leaks and windows alone do not provide sufficient ventilation in most cases due to tighter building practices
- Air to air heat exchangers ( HRV and ERV ) provide ventilation by bringing in fresh air without losing heating or cooling energy
- Air cleaning devices can not be relied upon alone to clean air in a well sealed home and must be combined with adequate ventilation to provide quality indoor air
Energy Efficiency, Comfort, and Health
Providing adequate ventilation is part of many building codes and maintains a healthy home by expelling extra moisture which can lead to mold and mildew growth as well as by expelling common household allergens and pollutants.It can be combined with highly effective air cleaners to assure that air is clean enough for highly sensitive people including those with allergies and asthma, in addition to people at higher risk for respiratory diseases such as children and the elderly.Energy saving mechanical ventilation methods, such as air to air heat exchangers, provide fresh air ventilation without bringing in heat or cold from the outside, thereby maintaining comfortable temperatures without paying a large energy penalty.
Components and Options
Ventilation brings fresh air into the home and expels stale air and some indoor pollutants through dilution, while air cleaning removes airborne pollutants from indoor air.
Ventilation
There are three ways of bringing air into the home: infiltration, natural ventilation, and mechanical ventilation. However, only natural and mechanical ventilation bring in fresh air.
Air Infiltration | Natural Ventilation | Mechanical Ventilation
Air Infiltration
In a process known as infiltration, outdoor air flows into the house through openings, joints, and cracks in walls, floors, and ceilings, and around windows and doors.While it used to be a major source of ventilation for homes built before the energy crisis of the 1970s, air infiltration is no longer permitted as the principal source of ventilation.Since it allows pollutants and unconditioned air to enter the home and since it cannot be controlled, infiltration is neither an energy efficient nor a healthy form of ventilation.
About 30% of air inside a typical home comes from the crawlspace or basement, due to air leaks in the floors and ducts.Wind, as well as unbalanced ventilation or warm temperatures inside the house, cause the house to be negatively pressurized.Driven by the “stack effect”, this negative pressure draws air in through the lowest part of the house (such as the crawlspace or basement) and pushes indoor air out of the highest part of the house (such as the attic).
Natural Ventilation
In natural ventilation, air moves through opened windows and doors. Air movement associated with infiltration and natural ventilation is caused by air temperature differences between indoors and outdoors and by wind.Until recent new building standards (ASHRAE 62.2), opened windows were considered to provide sufficient ventilation; now mechanical ventilation is required to provide about half of the ventilation in the vast majority of homes.
Since they allow pollutants and unconditioned air to enter the home and since people don’t open windows as often as they should, open windows and doors often do not provide sufficient ventilation and are less healthy than mechanical ventilation.Except when providing passive cooling (cooling a home to outside temperatures by opening windows, as opposed to using air conditioning), natural ventilation is generally less energy efficient than mechanical ventilation since it wastes energy used to heat or cool inside air.
Mechanical Ventilation
While infiltration and natural ventilation rely on air temperature differences between indoors and outdoors and on wind to supply ventilation to a building, mechanical ventilation systems use fans and duct work to continuously remove indoor air and/or distribute filtered and conditioned outdoor air to strategic points throughout the house.
- Exhaust fans: Usually found in areas exposed to high levels of moisture and fumes (such as bathrooms and kitchens), exhaust fans can provide quick and focused ventilation to certain regions of a homes but are generally not sufficient to provide overall ventilation to the whole house.
- Air to air heat exchange: Also known as heat recovery ventilation, air to air heat exchange is a ventilation system that employs a counter-flow heat exchanger between the inbound and outbound air flow.
- Heat Recovery Ventilator: provides fresh air and improved climate control, while also saving energy by reducing the heating (or cooling) requirements
- Energy Recovery Ventilator: closely related to HRV, this type of mechanical ventilation also transfers the humidity level of the exhaust air to the intake air
Examples include:
Air Cleaning
Air cleaning uses either filtration or electric charge to remove particles from the air.
Basic Furnace Filter | Media Air Cleaners | HEPA Air Cleaners | Electrostatic Air Filters | Electronic Air Cleaners
Basic Furnace Filter
Basic furnace filters are meant to protect air handler fan blades and coils from dust and damage from debris.They need to be changed regularly to assure proper air flow for HVAC system efficiency, but do not remove very small particles from the air.These filters are small, typically only 1” deep.
Media Air Cleaners
These units are generally integrated into the furnace or air handler, in contrast to stand alone units (such as the most common HEPA filters).Media air cleaners use high efficiency pleated media to remove larger particles with over 99% efficiency, including many allergens. For irritants in the spore and pollen size range, they are as effective as HEPA filters. Where they differ is in their capability to filter out the super small particulates such as bacteria, viruses and respirable dust.
Media air cleaners are cost effective compared to HEPA filters because the media is usually less expensive and generally needs to be replaced only once every year. Media filters are typically less expensive to operate as well because they don’t restrict airflow the way HEPA filters do.As a result media filters don’t need a supplemental fan to overcome the very high levels of air resistance that HEPA filters exhibit.
Examples include:
HEPA Air Cleaners
HEPA (High Efficiency Particulate Air) cleaners use high efficiency pleated media to remove particles. To be designated a HEPA, an air cleaner must remove 99.97% of all particles 0.3 microns (dust and mold spores) in size. Due to high cost, operational complications and other problems, HEPA units are sometimes seen most often in homes as one-room, portable units.
Most HEPA filters that are integrated into the furnace ducting only filter a portion of the return air at a time, since such filters cannot keep up with the airflow of the furnace. When media in these units needs to be replaced, it’s often relatively expensive to do so. Some require charcoal filters that need to be cleaned frequently. Warranties for HEPA cleaners are normally one to three years.
Examples include:
Electrostatic Air Filters
Based on heating and air conditioning industry standards, electrostatic air filters are not recognized as true high efficiency air cleaners. However, they are generally recognized as being more effective than the standard one-inch throw-away filters. Electrostatic air filters depend on the movement of the air through the filter to give particles a weak electronic charge. Usually, these models are less than 20% efficient, with some models having efficiencies of less than 5%. They need to be cleaned often to maintain air flow, sometimes weekly. Electrostatic air filters have warranties ranging from one year to lifetime.
Electronic Air Cleaners
There are two types of electronic air cleaners; both electrically charge particles and attract them to a collection material. The standard electronic air cleaner will collect charged particles on a “plate” designed to attract those particles. Most electronic cleaners can obtain 95% efficiency or more on various particles when the collection plates and ionizing wires are clean, but they can lose some efficiency as they collect dirt.
A newer technology in electronic air cleaners is called electronically enhanced media. It combines elements of both electronic and media air cleaners. Particles are electrically charged and then collected by the massive air cleaning media of a traditional high efficiency cleaner. The replacement of the media is simple and there are no plates to clean, efficiency is maintained throughout the media’s life. Electronically enhanced media air cleaners are 99% effective in the removal of numerous particle categories. Electronic air cleaners generally have warranties of one to five years.
Examples include: