HVACR416 - Design IAQ / Ventilation / Pathways

Ventilation Most air handling units distribute a mix of outdoor air and re-circulated indoor air. Some HVAC designs may have units that introduce 100% outdoor air into a building. Uncontrolled quantities of outdoor air may enter a building through windows, doors, and gaps in the structure. This is called infiltration.

Ventilation Thermal comfort and ventilation are needs are met by supplying conditioned air. Conditioned air is a blend of outdoor and re-circulated air that has been filtered, heated, cooled and sometimes humidified or dehumidified. Large buildings may often have interior (“core”) spaces in which constant cooling is required to compensate for heat generated by occupants, equipment, and lighting. The perimeter rooms may require heating or cooling depending on outdoor conditions.

Common HVAC Systems Constant volume systems provide a constant flow of air and vary the temperature of this air to meet the heating and cooling needs. Variable volume systems supply air at a constant temperature and vary the volume of the air supplied to maintain the heating and cooling needs Early VAV systems did not provide a way to maintain outdoor air requirements.

Ventilation The amount of outdoor air that is required has varied over time. The current guidelines issued by ASHRAE is ASHRAE standard The local and state building code that was in force at the time the building was built may require lower levels of ventilation. Ventilation is measured in Cubic Feet of outdoor air per minute per person.

Control of Odors and Contaminants One method for controlling odors and contaminants is to dilute them with outdoor air. One such approach says that “the solution for pollution is dilution.” Dilution can only work if there is a consistent and appropriate flow of supply air that mixes effectively with room air. In this case the ventilation must be constant.

Control of Odors and Contaminants “Ventilation efficiency” is the term that is used to describe the ability of the ventilation system to distribute supply air and remove internally generated pollutants. Another technique for isolating odors and contaminants is to design and operate the HVAC system so that pressure relationships between rooms are controlled. This control is accomplished by adjusting the air quantities that are supplied to and removed from each room.

Control of Odors and Contaminants If more air is supplied to the room than is removed from the room the room is said to be under a positive pressure. If less air is supplied than is exhausted, air is pulled into the space and the room is said to be under a negative pressure. The control of pressure in spaces is very important in buildings with mixed and special use areas.

Control of Odors and Contaminants Lobbies and buildings in general often are designed to operate under a positive pressure. o Prevents and minimizes the infiltration of unconditioned air. o Eliminates draft. o Prevents dust and dirt. o Helps prevent thermal discomfort.

Control of Odors and Contaminants The third method to control odors and contaminants is that of local exhaust. Local exhaust can be linked to a specific piece of equipment, such as a kitchen range or used to treat an entire room (such as a smoking lounge or custodial closet). It is important that air be exhausted from the building to the outdoors and not re-circulated.

Control of Odors and Contaminants Areas that use local exhaust would be: o Copy rooms o Kitchens o Bathrooms o Locker Rooms o Beauty Salons

Control of Odors and Contaminants Areas that use local exhaust must also have makeup air and the local exhaust must function along with the rest of the ventilation system. Sometimes it is acceptable to transfer air from “clean” parts of the building to the “dirty” parts of the building for makeup air. This transfer can achieve significant energy savings.

Control of Odors and Contaminants Additional control of contaminants and odors can be achieved by using filtration and purifying equipment in the HVAC equipment. The effectiveness of the filtration and purifying equipment depends on proper equipment selection, installation, operation and maintenance. Use caution in evaluating new equipment and technology.

Pollutant Pathways and Driving Forces Airflow patterns in buildings result from the combined action of mechanical ventilation systems, human activity, and natural forces. Pressure differentials created by these forces move airborne contaminants from areas of relatively high pressure to areas of lower pressure through any available openings.

Pollutant Pathways and Driving Forces

The HVAC system is generally the predominant pathway and driving force for all air movement in a building. However all of a buildings component: walls, ceilings, floors, penetrations, HVAC equipment, and occupants interact to affect the distribution of contaminants. One example of this is the movement of people – doors open and close; elevators move between floors; and peoples shoes track dirt and contaminants.

Pollutant Pathways and Driving Forces Natural forces also create movement and force between areas. The buildings Stack Effect is created as warmer air rises and the building acts like a chimney. o Cold air in the bottom around the foundation o Warm air out the roof exhausts and vents o Contaminants move from basement all the way through the building.

Building Stack Effect

Pollutant Pathways and Driving Forces The buildings stack effect can transport contaminants between floors by way of stairwells, elevator shafts, utility chases or other openings.

Building Stack Effect

Pollutant Pathways and Driving Forces Wind also plays an important art in building air patterns. No matter how tight a building is the wind on one side of a building will create drafts and air currents through a building. Depending upon the leakage openings in the building exterior, wind can affect the pressure relationships within and between rooms.

Wind Effect

Pollutant Pathways and Driving Forces Air will always move from an area of higher pressure to areas of lower pressure through any available openings. A small crack or hole can admit significant amounts of air if the pressure differential is high enough. Event if the building as a whole is maintained under positive pressure there is always some location that is under negative pressure relative to outdoors.

Pollutant Pathways and Driving Forces The interaction between pollutant pathways and intermittent or variable driving forces can lead to a single source causing IAQ complaints in areas of the building that are distant from each other and from the source.

Building Occupants The term building occupants is used to describe people who spend an entire workday in a commercial building. The term building occupants may be used to describe people who live in a residential building. Clients and visitors are also building occupants but they have different expectations from those who live in or spend their entire workday in a building and these occupants may be more sensitive to odors.

Building Occupants Groups that may be particularly susceptible to effects of indoor air contaminants include but are not limited to: Allergic or asthmatic individuals People with respiratory disease People with suppressed immune systems Contact lens wearers

Building Occupants Some additional groups to keep in mind are: People with heart disease are more affected by exposure to low levels of carbon monoxide. Children exposed to environmental tobacco smoke have show to be at higher risks of respiratory illness. Those exposed to nitrogen dioxide have been shown to be at higher risk from respiratory infections.

Building Occupants Different people have different sensitivity to IAQ problems. This means that one person may sense or feel a discomfort that another neighboring person may not be able to sense. A single indoor air pollutant or problem can trigger different reactions in different people. Some may not be affected at all.