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Heating, Ventilation and Air-Conditioning Systems

There are a number of factors that contribute to needing a furnace replacement. How old is your furnace? If it’s over 10 – 15 years old, it’s likely time for a furnace replacement—simply because this will improve your energy efficiency. Other factors include: how well it’s heating your home, how often it requires repairs, how much it costs to heat your home, etc.

Potential for Natural Ventilation and Operable Windows

In some parts of the country, where temperature and humidity levels permit, natural ventilation through operable windows can be an effective and energy-efficient way to supplement HVAC systems to provide outside air ventilation, cooling and thermal comfort when conditions permit (e.g., temperature, humidity, outdoor air pollution levels, precipitation). Windows that open and close can enhance occupants’ sense of well-being and feeling of control over their environment. They can also provide supplemental exhaust ventilation during renovation activities that may introduce pollutants into the space.

However, sealed buildings with appropriately designed and operated HVAC systems can often provide better IAQ than a building with operable windows. Uncontrolled ventilation with outdoor air can allow outdoor air contaminants to bypass filters, potentially disrupt the balance of the mechanical ventilation equipment and permit the introduction of excess moisture if access is not controlled.

Strategies using natural ventilation include wind driven cross-ventilation and stack ventilation that employs the difference in air densities to provide air movement across a space. Both types of natural ventilation require careful engineering to ensure convective flows. The proper sizing and placement of openings is critical and the flow of air from entry to exit must not be obstructed (e.g., by closed perimeter rooms).

  • Designers should consider the use of natural ventilation and operable windows to supplement mechanical ventilation. Consider outdoor sources of pollutants (including building exhausts and vehicle traffic) and noise when determining if and where to provide operable windows.
  • If operable windows will be used to supplement the HVAC system, ensure that:
    • Openings for outdoor air are located between 3-6 feet from the floor (head height);
    • Windows are adjustable and can close tightly and securely; and
    • Windows are placed to take maximum advantage of wind direction, with openings on opposite sides of the building to maximize cross-ventilation.

Selection of HVAC Equipment

In most parts of the country, climatic conditions require that outdoor air must be heated and cooled to provide acceptable 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;
  • IAQ; 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, including:

  • 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; and
  • Less likely to have quantity of outdoor air supply inadvertently reduced.

Specify the following features for all air handling units.

Double-Sloped Drain Pan and Drain Trap Depth

  • Double-sloped drain pan – A double-sloped pan prevents water from standing and stagnating in the pan.
  • Non-corroding drain pan – Made from stainless steel or plastic. Prevents corrosion that would cause water to leak inside the AHU.
  • Easy access doors – All access doors are hinged and use quick release latches that do not require tools to open. Easy access to filters, drain pans and cooling coils is imperative.
  • Double wall cabinet – The inner wall protects the insulation from moisture and mechanical damage, increases sound dampening and is easier to clean.
  • Tightly sealed cabinet – Small yet continuous air leaks in and out of the AHU cabinet can affect IAQ and energy. The greatest pressure differentials driving leaks occur at the AHU.
  • Double wall doors with gaskets – Double wall doors provide better thermal and acoustic insulation and will remain flatter, allowing a better seal against door frame gaskets
  • Minimum 2 inch thick filter slots – For better protection of the indoor environment, as well as the equipment and ducts, the filters slots should be able to accommodate 2 in. or thicker filters.
  • Extended surface area filter bank – To reduce the frequency of filter maintenance and the cost of fan energy, the bank is designed to allow more filter area, such as the deep V approach or bags.
  • Air filter assemblies (racks and housings) designed for minimum leakage – The filter bank should have gaskets and sealants at all points where air could easily bypass the air filters, such as between the filter rack and the access door. Use properly gasketed manufacturer supplied filter rack spacers.
  • Air filter monitor – A differential pressure gauge to indicate the static pressure drop across the filter bank. This feature could easily be installed as an option in the field.
  • Corrosion resistant dampers and links – All moving parts such as pivot pins, damper actuators and linkages are able to withstand weather and moisture-induced corrosion for the full life of the system.
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