Just as there are different requirements for ventilation in different homes, locales and seasons of the year, the types of home ventilation vary as well. Moving stale air out of an enclosed area and letting fresh air in was probably the earliest home comfort strategy. Before central air conditioning and furnaces, people learned that keeping air in motion was critical to maintaining a comfortable and healthy indoor environment. That’s even more the case today, as houses are now built to much higher standards of airtightness than in the past in order to achieve maximum energy efficiency.
While homes of previous eras included substantial air exchange with the outdoors through structural gaps and cracks, today’s residential construction methods create a tightly sealed air envelope. The good news is that this ensures that expensive cooled and heated air stays inside and reduces operating costs. The bad news? It means indoor pollutants and humidity also remain indoors, often accumulating to unnaturally high levels due to the lack of fresh air exchange. The goal is to move stagnant air out while also introducing fresh air, without sacrificing energy efficiency and comfort due to loss of cooling and heating. Different types of home ventilation can help accomplish this with different degrees of success.
What’s In The Air?
Stagnant indoor air often includes pollutants and irritants you’d rather not be breathing. In addition, because the majority of breaths you take every day are inside your home, that means you’re getting a daily, concentrated dose of whatever’s in your household air. Some of the undesirable elements present in stagnant air include:
- Inorganic dust and lint.
- Mold spores emitted by indoor sources.
- Accumulation of airborne pollen from outdoors.
- Chemical fumes out-gassed from building materials and furniture.
- Trace amounts of carbon monoxide and other combustion byproducts.
- Microorganisms like airborne bacteria and viruses.
- The consequences of living in poorly ventilated enclosed spaces range from simple annoyance and discomfort all the way to chronic asthma and allergic reactions, skin and eye irritation and communicable respiratory illness.
Natural ventilation includes the leaky construction of many older homes that allows a free-flowing air exchange with the outdoors. But it also extends to simple, common methods such as opening doors and windows to establish cross-ventilation and keep air flowing through the house. The main drawbacks to these types of home ventilation are that it’s difficult to control the volume of air entering or leaving the house, as well as the fact that it makes indoor temperatures hard to regulate. On muggy or wet days, leaving the windows open also can add to indoor discomfort.
Spot ventilation addresses limited ventilation issues in smaller, enclosed spaces. It’s normally used to quickly remove airborne irritants such as smoke or water vapor in kitchens and bathrooms. Most spot ventilation is handled by dedicated exhaust fans that move air in one direction only – out of the house – usually venting all the way to the roof or an exterior wall. Most spot ventilation is on/off, and it’s one of the types of home ventilation that runs only on demand, not continuously. Many bathroom exhaust fans, for example, run on timers and shut off automatically after a few minutes. Another variety of spot ventilation are the fans in range hoods above stoves that exhaust smoke, odors and water vapor from cooking.
Unlike other types of home ventilation, whole-house ventilation moves air on a large scale throughout the structure. The intake and/or output of whole-house ventilators is usually controlled to maintain critical indoor air balance. These ventilators may be exhaust-only, in which the volume of outgoing air is controlled; supply-only, which limits the incoming volume or balanced; or balanced in which both supply and exhaust are controlled to achieve an equilibrium of fresh air entering the home and stagnant air leaving.
The Right Size
Exhaust fans need to be properly sized according to the amount of air volume they remove from the home in a specified period of time. Sizing is important not only to ensure that the fan is large enough to perform the ventilation function adequately, but also to guard against exhausting too much air and inducing negative air pressure in the room or entire home. A depressurized structure draws unconditioned outside air into the home through cracks and gaps and may even cause vented gas appliances to backdraft combustion fumes back into the structure. Sizing is based on the recommended number of air changes in a given room per hour (ACH). Generally speaking, the following sizing recommendations apply to residential exhaust fans:
- Bathrooms: eight air changes per hour
- Kitchens: 15 air changes per hour
- All other living spaces: six air changes per hour
All exhaust fans should be vented to the exterior of the home, not into an unconditioned zone like the attic or a crawl space. Exhaust fan ductwork in the kitchen should be specially fabricated to resist the accumulation of grease, and the route of ductwork to the exterior vent should be as short as possible.
Exhaust-only ventilation on a whole-house scale is best exemplified by attic fans. Usually mounted in the ceiling of an upper floor, these large ventilation fans draw air from the entire house and exhaust it into the attic, where it flows out of the attic vents at the soffits, gable and roofline. Exhaust fans require open windows or doors at the opposite end of the home for a passive supply of fresh intake air. These fans are primarily used to flush the house with cool fresh night air for a limited operating time. Because they draw large amounts of air volume into the home, exhaust-only fans may also induct excessive outdoor humidity.
Supply-only ventilation systems piggyback on the home’s HVAC system and utilize existing home ductwork. This system includes an external vent on the roof fitted with a duct that incorporates a fan and connects to the return ductwork of the HVAC system. When the fan is energized, a controlled amount of fresh outdoor air is drawn into the home through the roof vent and added to air circulating through the HVAC return duct. All incoming air is filtered by the conventional system air filter. Because they induct a measured volume of outdoor air, supply-only systems cause only a slight positive pressure within the home. In addition, because unconditioned outdoor air is added to the HVAC airflow before the furnace and air conditioner air handler, this air will be heated or cooled by the HVAC system before it’s dispersed through the house in the ductwork.
Balanced whole-house systems represent the state-of-the-art among types of home ventilation systems.
- Heat recovery ventilators (HRV) consist of dedicated, small-diameter ductwork that conveys fresh outdoor air into living spaces like bedrooms and family rooms while a separate span of ductwork removes stale air from other rooms like kitchens and utility rooms. The amount of air drawn in is balanced to equal the amount of air exhausted by a controller at a central unit that incorporates the intake and exhaust blower fans as well as a heat exchanger. In summer, the heat exchanger extracts heat from incoming fresh air and adds it to the exhausting air to preserve the cool indoor temperatures generated by the air conditioner. In winter, the process reverses so heat is extracted from indoor air as it’s exhausted and transferred to cold incoming air to warm it before being dispersed through the ductwork to living spaces. The benefit of an HRV is a continuous supply of fresh outdoor air balanced with exhausting stale indoor air with no impact on temperature comfort control inside the house.
- An energy recovery ventilator (ERV) performs the same function as an HRV but incorporates additional technology that also controls humidity. An ERV uses a process called enthalpy transfer to move humidity from the airstream that is moister to the drier airstream. In summer, that usually means that water vapor is extracted from incoming humid outdoor air and transferred to drier outgoing indoor air as it’s exhausted. During winter, the indoor air is usually more humid (but still relatively dry) so water vapor moves from the exhaust air and is added to incoming fresh air, which is typically dry in winter. With an ERV system, not only is fresh air continuously added at a temperature consistent with the indoor comfort, but also at a controlled humidity level.
For more information on the various types of home ventilation to clear the air in your Highland Park or Glencoe area home, please contact the professionals at Roberts Heating & Air Conditioning.
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