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Heating, Ventilation and Air-Conditioning (HVAC) Systems


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.

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.

Special Note about Cleaning Condensate Lines

This is something that is usually not mentioned; It is the prevention of water dripping from the A/C pan under the A/C unit into the building. Most of the time the dripping is caused by the condensate line being clogged. The condensate line is usually a small PVC pipe that discharges the moisture that is captured by the A/C inside coil and is discharged to the outside of the building. To unclog the condensate line, it can be done easily by using a wet-dry shop vac. Simple take a small piece of cloth and wet it. Then place the vac hose over the discharge of the condensate line, and wrap the wet cloth around the vac and the condensate line. Turn on the vac and it will suck out the debris and slime from the condensate line. This needs to be done regularly, because, besides debris and slime in the condensate line, at times you may find frogs, lizards and other small creatures that crawl into the condensate pipe and clog it. This is something that is easy to do and can save you money. This is usually a less than 5 minute project.

Codes and Standards

The national consensus standard for outside air ventilation is ASHRAE Standard 62.1-200, Ventilation for Acceptable Indoor Air Quality.

Many state codes also specify minimum energy efficiency requirements, ventilation controls, pipe and duct insulation and sealing, and system sizing, among other factors. In addition, some states and localities have established ventilation and/or other indoor air quality related requirements that must also be followed.

  1. Design in accordance with ASHRAE standards Design systems to provide outdoor air ventilation in accord with ASHRAE Standard 62.1-2007 (available at www.ashrae.org) and thermal comfort in accord with ASHRAE Standard 551992 (with 1995 Addenda) Thermal Environmental Conditions for Human Occupancy.
  2. Ensure familiarity with, and adherence to, all state and local building codes and standards.
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 indoor air quality 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).

  1. 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.
  2. If operable windows will be used to supplement the HVAC system, ensure that:
    1. openings for outdoor air are located between 3-6 feet from the floor (head height);
    2. the windows are adjustable and can close tightly and securely;
    3. the 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, indoor air quality, and cost.

  1. Where feasible, use central HVAC air handling units (AHUs) that serve multiple rooms in lieu of unit ventilators or individual heat pumps.
    1. 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:
      1. Quieter, and therefore more likely to be turned on or left on by teachers and staff;
      2. Less drafty due to multiple supplies and a return that is away from occupants;
      3. Better at controlling humidity and condensed moisture drainage;
      4. Easier to maintain due to reduced number of components and few units to access;
      5. More space around units and can be accessed without interfering with class activities;
      6. Space for higher efficiency air filters, and more surface area;
      7. Made of heavier duty components;
      8. Less likely to have quantity of outdoor air supply inadvertently reduced.
  2. Specify the following features for all air handling units:
    1. Double-sloped drain pan - A double-sloped pan prevents water from standing and stagnating in the pan.
    2. Non-corroding drain pan - Made from stainless steel or plastic. Prevents corrosion that would cause water to leak inside the AHU.
    3. 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.
    4. Double wall cabinet - The inner wall protects the insulation from moisture and mechanical damage, increases sound dampening, and is easier to clean.
    5. 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.
    6. 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.
    7. 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.
    8. 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.
    9. Air filter assemblies (racks & 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.
    10. 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.
    11. Corrosion resistant dampers & 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.

Example of Building A/C System:

AC Plan

Commercial Building A/C Plan



AC Plan

Commercial Building A/C Calculations

AC Plan

Typical Duct - Register Detail


AC Plan

Heating and Cooling:

Cooling is the process of extracting humidity from the air, while Heating adds heat to the air molecules. All A/C Heating and Cooling design is performed by computing either heat gain or heat loss.

The design of an A/C (Heating and Cooling) system is a combination of computations relating to the number of persons in a building, the type of glass in the windows, the building components of the walls, roof, floors, lighting and any equipment or other components in the building that will either loss heat or gain heat thru the building envelope.

The basic concept of A/C Cooling design is to remove any heat gained by the building envelope or other things that produce heat within the design space.

The basic concept of A/C Heating design is to add heat to the building more quickly than it can be lost thru the building envelope.

The unit of measurement used to determine heat gain or heat loss is the British Thermo Unit (BTU).

As we all know when we purchase an A/C unit; we ask, "how many tons or how many BTU is that A/C unit?"

For A/C units, a ton is the same as 12000 BTU, thus a 3 ton unit is a 36000 BTU unit (3 ton x 12000 BTU).
Eventhough, the process of computing the number of BTU's required for a specific building is straight forward, it is still time consuming.
For this reason computers have been a great help to the Engineer in speeding-up the design process, it also produces more accurate calculations.

In the State of Florida, as part of the A/C consideration, all buildings must have an Energy Calculation performed on the building. The Energy Code takes into account the various building components, such as, how the walls, roof and floors are constructed and the insulation value used. Other things taken into account are lighting, building orientation, heat gain thru the glass, as well, as the type of glass. It is also necessary to identify the type and efficiency of the water heater, as well as, the size and efficiency of the A/C units.

Once the Energy Code calculations are done, the results are compared to a set of design standards established by the Department of Community Affairs to determine if the building to be constructed will be efficient, in it's use of energy, both for heating and cooling.

Therefore, a well designed building will pass the Energy Code, and will provide the future owner with the most economical building to operate. A well designed building will save the building owner money, by reducing electric consumption.

Most states now implement some form of energy calculation, check with your local building department.

See the Building Code Section for your State Building Department.