Heating-Ventilation-Air-Conditioning (HVAC) Systems free estimates. The main purposes of a Heating-Ventilation and Air-Conditioning (HVAC) system are to help maintain good indoor air quality through adequate ventilation with filtration and provide thermal comfort. HVAC systems are among the largest energy consumers in schools. The choice and design of the HVAC system can also affect many other high performance goals, including water consumption (water cooled air conditioning equipment) and acoustics.
The following actions detail how engineers can design a quality system that is cost-competitive with traditional ventilation designs, while successfully providing an appropriate quantity and quality of outdoor air, lower energy costs, and easier maintenance.
Central, ‘all-air’ air conditioning systems (or package systems) with a combined outdoor condenser/evaporator unit are often installed in modern residences, offices, and public buildings, but are difficult to retrofit (install in a building that was not designed to receive it) because of the bulky air ducts required.
An alternative to heating-ventilation-air-conditioning central systems is the use of separate indoor and outdoor coils in split systems. These HVAC systems, although most often seen in residential applications, are gaining popularity in small commercial buildings. The evaporator coil is connected to a remote condenser unit using refrigerant piping between an indoor and outdoor unit instead of ducting air directly from the outdoor unit. Indoor units with directional vents mount onto walls, suspended from ceilings, or fit into the ceiling. Other indoor units mount inside the ceiling cavity, so that short lengths of duct handle air from the indoor unit to vents or diffusers around the rooms.
Dehumidification (air drying) in an air conditioning system is provided by the evaporator. Since the evaporator operates at a temperature below dew point, moisture in the air condenses on the evaporator coil tubes. This moisture is collected at the bottom of the evaporator in a pan and removed by piping to a central drain or onto the ground outside. A dehumidifier is an air-conditioner-like device that controls the humidity of a room or building. It is often employed in basements which have a higher relative humidity because of their lower temperature (and propensity for damp floors and walls). In food retailing establishments, large open chiller cabinets are highly effective at dehumidifying the internal air. Conversely, a humidifier increases the humidity of a building.
Selection of HVAC Equipment
In most parts of the country, climatic conditions require that outdoor air must be heated and cooled to provide acceptable Heating-Ventilation-Air-Conditioning thermal comfort for building occupants, requiring the addition of HVAC systems. The selection of equipment for heating, cooling and ventilating the school building is a complex design decision that must balance a great many factors, including heating and cooling needs, energy efficiency, humidity control, potential for natural ventilation, adherence to codes and standards, outdoor air quantity and quality, indoor air quality, and cost.
- Where feasible, use central HVAC air handling units (AHUs) that serve multiple rooms in lieu of unit ventilators or individual heat pumps. Although there are many different types of air handling units, for general IAQ implications in schools, air handling units can be divided into two groups: unit ventilators and individual heat pump units that serve a single room without ducts; and central air handling units that serve several rooms via duct work. Unit ventilators and heat pumps have the advantage of reduced floor space requirements, and they do not recirculate air between rooms. However, it is more difficult to assure proper maintenance of multiple units over time, and they present additional opportunities for moisture problems through the wall penetration and from drain pan and discharge problems. Central air handling units have a number of advantages as compared to unit ventilators and heat pumps serving individual rooms. They are:
- Quieter, and therefore more likely to be turned on or left on by teachers and staff;
- Less drafty due to multiple supplies and a return that is away from occupants;
- Better at controlling humidity and condensed moisture drainage;
- Easier to maintain due to reduced number of components and few units to access;
- More space around units and can be accessed without interfering with class activities;
- Space for higher efficiency air filters, and more surface area;
- Made of heavier duty components;
- Less likely to have quantity of outdoor air supply inadvertently reduced.
Air Filtration for Heating-Ventilation-Air-Conditioning
In addition to “atmospheric dust,” airborne particulates can include pollen, mold (fungal) spores, animal dander, insect proteins, pesticides, lead, and infectious bacteria and viruses. Designers can integrate features into the ventilation system that will provide benefits for the school occupants as well as the efficiency and longevity of the HVAC system. In addition, these features can reduce the need for expensive cleaning of the duct work and air handling units.
- Air filters should have a dust-spot rating between 35% and 80% or a Minimum Efficiency Rating Value (MERV) of between 8 and 13. The higher the rating, the better the protection for the equipment and the occupants. It has been estimated that a 30% increase in static pressure across a coil results in a $200 per 10,000 cfm of air movement (at 7 cents per KWH). This does not include the added cost of cleaning dirty heating or cooling oils, drain pans, or air ducts. Designers should consider specifying a low efficiency (~10%) pre-filter upstream of the main filters. The pre-filters are generally easy and inexpensive to change, and will capture a significant amount of the particulate mass in the air thereby extending the useful life of the more expensive main filters. See ASHRAE Standard 52.2-1999 Method of Testing General Ventilation Air Cleaning Devices for Removal Efficiency by Particle Size available at www.ashrae.org
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The main purposes of a Heating-Ventilation-Air-Conditioning (HVAC) system are to help maintain good indoor air quality.