An introduction to ventilation

The presentation is a brief introduction of ventilation, both natural and mechanical.

1. An introduction to Ventilation By: Mohammed Abdul Ameed

2. What is Ventilation… The air exchange of outdoor air with the air already in a building is called ventilation. As per ASHRAE the ventilation is intentional introduction of air from the outdoors into a building. The outdoor air that flows through the building is often used to dilute and remove indoor air containments. Why Ventilation… Ventilation air is used to provide acceptable indoor air quality. It can be transfer air, suitably treated recirculated air or an appropriate combination of both. Th e allowable means of providing ventilation air varies in standards and guidelines. Building ventilation is achieved by natural or mechanical means, or by a combination of both.`

3. Natural ventilation : As per ASHRAE the natural ventilation is the flow of outdoor air inside the building caused by wind and thermal pressure through intentional openings in the building shell. In natural ventilation the flow of air is through open windows, doors, grills, roof ventilators, stacks and specially designed inlet or outlet openings.

4. Temperature control by natural ventilation is often the only means of providing cooling when mechanical air conditioning is not available. Under some circumstances, it can effectively control both temperature and containments in mild climates, but it is not considered practical in hot and humid climate or in cold climate. The arrangement, location, and control of ventilation openings should combine the driving forces of wind and temperature to achieve a desired ventilation rate and good distribution of ventilation air through the building. However, the intentional openings cannot guarantee adequate temperature and humidity control or indoor air quality because of dependence on natural air flow. In buildings that rely on natural ventilation for cooling the floor to ceiling heights are often to increase well beyond the normal level(2.5 to 3.2m). Higher ceilings, as seen in buildings constructed before air condition was available, allows warm air and containments to rise above the occupied portion of rooms then exhausted from ceiling openings, and cooler air is provided near the floors.

5. Mechanical ventilation:  As per ASHRAE the mechanical ventilation or forced ventilation is the intentional movement of air into and out of the building using fans and intake and exhaust vents.. Most commercial buildings use mechanical ventilation, which is more controllable and responsive than natural ventilation in providing adequate indoor air quality.

6. Supply Only Supply ventilation systems involve the use of a fan to bring outside air into the space while air leaks out of the space through fan ducts and / or intentional vents. A typical supply ventilation system introduces fresh air into usually one or more spaces within the space that residents occupy most often (e.g., bedrooms, living room).

7. Exhaust Only An exhaust only system removes stale and moisture-laden air from the space. It works by extracting indoor air from a space while make-up air infiltrates through the intentional openings in the space.  Typically exhaust-only solutions remove air from rooms where moisture and pollutants is most often generated (kitchen, bathrooms and perhaps the laundry room).

8. Balanced Systems Balanced ventilation systems, if properly designed and installed, neither pressurize nor depressurize a home. Rather, they introduce and exhaust approximately equal quantities of fresh outside air and polluted inside air, respectively. A balanced ventilation system works within the building’s HVAC system to facilitate proper distribution of fresh air throughout the living environment.

9. The figure below shows a simple air handling unit(AHU) or air handler that conditions air for a building. Air brought back to air handler from the conditioned space is return air(RA). The return air is either discharge to the environment (exhaust air, EA) or is reused ( recirculated air, CA ). Air brought in intentionally is from the environment is outdoor / outside air(OA) or. Because outdoor air may need treatment to be acceptable for use in a building it should not be called fresh air(FA). Outside and recirculated air are combined to form mixed air(MA), which is then conditioned and delivered to thermal zone as supply air(SA). Any portion of the mixed air that intentionally or unintentionally circumvents condition space is bypass air(BA).

10. Terminologies used in ventilation. Infiltration / Air leakage is the flow of outdoor air into a building through cracks and other unintentional openings. Exfiltration is leakage of indoor air out of a building through cracks or unintentional openings. Transfer air is air that moves from one interior space to an other, either intentionally or not.

11. Air exchange rate: The air exchange rate is comparison of air flow to volume of space, and express as I = Q / V Where I = change rate Q = Volumetric flow rate of air into space, m³ / s V = interior volume of space, m³ The air exchange rate has unit of 1/ time, usually express in hours¯¹. When the time unit is hours, the air exchange rate is also called as air changes per hour(ACH). The below chart shows some of recommended air changes/hour for different areas.

12. Ventilation with Energy Efficiency Energy recovery ventilation systems provide a controlled way of ventilating a home while minimizing energy loss. They reduce the costs of heating ventilated air in the winter by transferring heat and/or humidity from the warm inside air being exhausted to the fresh (but cold) supply air. In the summer, the inside air cools the warmer, incoming supply air to reduce ventilation cooling costs. There are two types of energy-recovery systems: heat-recovery ventilators (HRV) and energy-recovery ventilators (ERV). HRV systems draw fresh air into a home while simultaneously exhausting stale air from the home. During this exchange, the two airstreams pass through a core within the unit where a portion of heat from one air stream is transferred into the other (usually the indoor air in winter and the outdoor air in summer). An ERV works similarly to an HRV, except an ERV allows a portion of the moisture in the more humid air stream (usually the indoor air in winter and the outdoor air in summer) to be transferred to the dryer air stream.

13. Building Pressurization Building pressurization refers to the air pressure relationships that exist between the inside of a building relative to the outside of a building across the building envelope; it also refers to the pressure relationships that exist within different parts of a building relative to each other. Building pressurization is used to limit infiltration, which can lead to indoor air quality problems because air that enters a building via infiltration bypasses the air handling systems and can introduce contaminants into a building and contribute to moisture problems. Pressurization also can be used to control the movement of air contaminants within a building. Whether a building can be pressurized depends upon the building’s geometry, HVAC system design, and envelope tightness as well as weather conditions. Building pressurization requires that the HVAC system be able to deliver more air to the occupied space than is being exhausted and lost due to exfiltration. It may not be possible to pressurize a leaky building without first addressing envelope leakage. 

14. Maintaining positive air pressure in one zone of a building relative to another can limit the distribution of an aerosolized biological agent released within that building by means of airflows created by pressure relationships that exist within different parts of the building relative to each other. Such an approach can be used to isolate special-use spaces such as lobbies, parking garages, and mail rooms that may be more vulnerable to an internal release by maintaining them at negative pressure relative to adjacent parts of the building.

15.   The indoor air quality The air within your home can be up to 10 times more polluted than the air outside.  Microbial pollutants like mold, pet dander and plant pollen along with chemicals such as radon and volatile organic compounds (VOCs) create a toxic environment in your home. The build-up of pollutants such as these is shown to lead to allergies, asthma and other health concerns. To combat the issue of poor ventilation, many studies from Building Science organizations, government and independent building science experts indicate that today’s homes need balanced ventilation throughout the day to maintain a healthy indoor environment . ASHRAE standards are commonly used as baselines for building codes and programs. The ASHRAE 62.2 standard was developed in response to concerns of increasing levels of indoor contaminants and mold growth in residential buildings. Years of studies and input by building science experts and health professionals indicated a need for mechanical ventilation in homes to protect the health of occupants and the value of the building .

16. The two main requirements of ASHRAE 62.2 are Whole building mechanical ventilation to maintain acceptable air quality. Local exhaust fans in each kitchen and bathroom to reduce the levels of contaminants and moisture in these spaces. As per above, local exhaust in kitchens and bathrooms is required by ASHRAE 62.2 to remove high volume/intensity pollutants produced in those areas before they spread to the rest of the home. The humidity, cooking by-products and odors produced in these spaces adversely affect the overall indoor air quality of the home. Ventilation fans for these spaces must have verified performance to make sure they are capable of accomplishing their task. For bathrooms the fan must be capable of delivering a minimum of 50 CFM of exhaust ventilation when installed and for kitchens ventilation it is 100 CFM. ASHRAE provides a number of options to determine proper operation. The first is using the rated performance of the fan at .25 static pressure (Ps) tested per the standards of the Home Ventilating Institute (HVI).

17. Ventilation in LEED . The indoor air quality is largest category to focus on human health being. The below chart shows some of the credit with focus on indoor air quality.

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